# You are all destroying your LiFePO4 cells!



## bgeery (Oct 17, 2011)

OK, so I'm exaggerating a little bit.  But you are going to get fewer life cycles than specified by the manufacture if used under that standard conditions.

A fully 100% charged battery has a resting voltage of 3.33 volts per cell at 25 Celsius.

The only way to insure we never overcharge the battery is to charge at 3.33 volts until it tapers down to 0.00 amps. That's a fully 100% fully charged cell. For those that want to charge to less than 100%, just discontinue that charge before current drops to 0.00 amps.

The problem with this is that it takes longer to charge this way; so the manufacture gives us a formula of at 25 Celsius, apply .5C, until the cell reaches 3.6 volts, then disconnect the charge. This is 100% charged.

I see allot of people charging at 3.5-3.8 volts per cell until 0 amps. This is overcharging the cells to some degree. Yes, they may accept some additional charge above 100%, but it's past the manufacture's definition of the 100% charged point.

Either charge at 3.33 volts until it tapers down to 0.00 amps, or go with the manufacture's formula of .5C charge rate to 3.6 volts and disconnect. Either one is the only known safe and accurate charging procedures to reach 100%.

How's that for a first post? 

PS: Other charging formulas could be developed by the manufacture, if they choose to do so. We could also, if we knew the exact conditions they use to determine and define 100% state of charge.


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## frodus (Apr 12, 2008)

..... except you won't cause any current to flow if the battery is at 3.33V and the charger/power supply is at 3.33V. There has to be a voltage difference between the two.... period.

The larger that differential between supply and load (battery) the larger the amps you'll draw from the supply. Thats how you cause current to flow. Most chargers we use are CC/CV and they current limit to XXAmps, and voltage limit to YYVolts.

As long as you don't A) go over the manufacturer max charge current B) Manufacturer max temperature or C) manufacturer max voltage, you're going to be within the limits of the cell. That's how the manufacturer actually rates cell life-cycle, using those specs.

We're not overcharging them at all. It's actually the only way you can charge a battery. You have to pull the battery voltage above it's nominal voltage and cause current to flow into it.


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## bgeery (Oct 17, 2011)

frodus said:


> ..... except you won't cause any current to flow if the battery is at 3.33V and the charger/power supply is at 3.33V. There has to be a voltage difference between the two.... period.
> 
> The larger that differential between supply and load (battery) the larger the amps you'll draw from the supply. Thats how you cause current to flow. Most chargers we use are CC/CV and they current limit to XXAmps, and voltage limit to YYVolts.


 Agreed. But if the cell is at 3.33 volts resting voltage, it's already at 100% state of charge, and any further charging is an overcharge. You don't want further current flow.

A battery under 100% SoC will rest at something less than 3.33 volts until it's fully charged. The charge will be slower and slower as it approaches 3.33 volts, but the current will flow, until the cell reaches the true 3.33 resting voltage equilibrium with the charger and is at 100% SoC, and current flow will cease.

Using higher than resting voltage on the cell does nothing but increase the rate you can charge; avoiding the long trickle charge that would occur as the cell approached 3.3 volts and less and less current flowed. That's a valid practical compromise, but it doesn't get a 100% charged cell. It gets a 100.2% or 99.6% charged cell.



frodus said:


> As long as you don't A) go over the manufacturer max charge current B) Manufacturer max temperature or C) manufacturer max voltage, you're going to be within the limits of the cell. That's how the manufacturer actually rates cell life-cycle, using those specs.
> 
> We're not overcharging them at all. It's actually the only way you can charge a battery. You have to pull the battery voltage above it's nominal voltage and cause current to flow into it.


The manufacture gives us exatly *one* charge curve that they rate the battery at, and if you are not charging in exactly the same conditions, you can't tell your state of charge with true accuracy. You can come close enough for practical use, but you still don't really know exactly.

For example, how many people are charging at 3.5-3.8 volts until the charge tapers to 0.0 amps? I see top balance supporters mentioning this all the time here. Where does that extra power go between the 3.8 volts and the 100% charged resting voltage of 3.33 volts? It's not a whole lot of energy, but it is an overcharge of the battery. My guess is that it's oxidizing some of the plates and diminishing some minor amount of cell capacity.


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## DavidDymaxion (Dec 1, 2008)

Great first post. I throw this out as a discussion point, and not as hard science. Jack mentioned this in his videos. (All hail Jackton! All hail Jackton!)

The voltage for a cell is Vcell = V0 + I*R

So suppose you have a cell with 5 milliOhms of resistance, you want to charge at 10 Amps.

Vcell = 3.33V + 10 Amps * 0.005 Ohms = 3.38V

Suppose you are regenning at 100A (or have a super powerful charger):

Vcell = 3.33V + 100 Amps * 0.005 Ohms = 3.83V

Anyway, it's a thought that if the cells are cool enough and the currents are low enough, perhaps this isn't hurting them at all?


bgeery said:


> OK, so I'm exaggerating a little bit.  But you are going to get fewer life cycles than specified by the manufacture if used under that standard conditions.
> 
> A fully 100% charged battery has a resting voltage of 3.33 volts per cell at 25 Celsius.
> 
> ...


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## piotrsko (Dec 9, 2007)

my $.02:
It is a CHEMICAL reaction. the current is what is forcing the chemicals to physically revert / change to some sort of different state. When the charge current goes to zero, then theoretically all the chemicals have been changed, all the electron holes in the collector plates are empty again. Not exactly true, but close enough for this argument. As you overcharge you may even apply a slight coating made from the collector plates to the materials in the electrolytic thus altering the characteristics, possibly forever. In one of the battery books I have read, as little as 1 percent chemistry change destroys the cell. heat does this process also.
ymmv


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## frodus (Apr 12, 2008)

Stop for a minute and really try to stop stating these things as facts....



bgeery said:


> Agreed. But if the cell is at 3.33 volts resting voltage, it's already at 100% state of charge, and any further charging is an overcharge. You don't want further current flow.


As soon as you start charging it, the voltage will rapidly increase, and within a very short time it will match the voltage of the charger and the current will drop to zero. You're not overcharging it. This happens almost immediately.



> A battery under 100% SoC will rest at something less than 3.33 volts until it's fully charged. The charge will be slower and slower as it approaches 3.33 volts, but the current will flow, until the cell reaches the true 3.33 resting voltage equilibrium with the charger and is at 100% SoC, and current flow will cease.


Right.... but hardly any current will initially flow into the cell if you try charging a lifepo4 cell at 3.33V and when it does get to 3.33V, no current will flow. The system would then be in equilibrium. The cell wouldn't take long to get to 3.33V, even if it's at 2.7V. This is exactly how I recondition low cells. You HAVE to pull the voltage above nominal. This is how current flows.

The manufacturer specifies that the charge voltage is to be no more than 3.6-4.0Volts (depending on your cell). As long as you're at, or below that, you'll never overcharge the cell.



> Using higher than resting voltage on the cell does nothing but increase the rate you can charge; avoiding the long trickle charge that would occur as the cell approached 3.3 volts and less and less current flowed. That's a valid practical compromise, but it doesn't get a 100% charged cell. It gets a 100.2% or 99.6% charged cell.


The cell is 100% charged when current stops flowing, but the cell has to be pulled higher than it's nominal voltage, even if it's just a tad. Higher voltages just cause it to charge faster, but if you're only at nominal voltage, you're not going to charge. 



> The manufacture gives us exatly *one* charge curve that they rate the battery at, and if you are not charging in exactly the same conditions, you can't tell your state of charge with true accuracy. You can come close enough for practical use, but you still don't really know exactly.


Have you even looked at a charge curve from a manufacturer? It shows voltage and all the one's I've seen are above way 3.3V.
http://liionbms.com/pdf/thundersky/TS-LFP100.pdf
http://liionbms.com/pdf/zhejiangheadway/HW38120LS.pdf
http://liionbms.com/pdf/psi/PC40138F1.pdf
http://liionbms.com/pdf/shandong/200ah.pdfhttp://liionbms.com/pdf/goldpeak/GP18EVLF.pdf
http://liionbms.com/pdf/huanyu/HYP-3.2V-100Ah.pdf

That charge curve is at a given C-rate. The lifecycle is at that c-rate. As long as the cell never goes above the manufacturer's recommended charge voltage, you're going to charge to 100%. Most of the charge curves I've seen are at 1C. True, you could charge at 3.5V, which many do, but 3.5V > 3.33V, so charge flows.



> For example, how many people are charging at 3.5-3.8 volts until the charge tapers to 0.0 amps? I see top balance supporters mentioning this all the time here. Where does that extra power go between the 3.8 volts and the 100% charged resting voltage of 3.33 volts? It's not a whole lot of energy, but it is an overcharge of the battery. My guess is that it's oxidizing some of the plates and diminishing some minor amount of cell capacity.


Thats how I charge, every time. My charger is set to a certain max voltage, and a certain max current. Set it and forget it. It turns off when current goes to 0. 

It's not power we're looking at... it's energy. What you aren't looking at is that Energy = Wh = the area under the curve. 

Lets say you have a 10Ah 3.2V nominal cell. Situation 1: Charge voltage is set to 3.5V. Charge current is 1C, or 10A. Situation 2: Charge voltage is set to 3.7V. Charge current is the same.

The slope of the charge curve is steeper with situation 2, so it gets to 3.7V faster. Then it holds that voltage until current drops to 0. The area under the curve will be XXWh.

The slow of the charge curve is less with situation 1. Not only does it take longer to rise to the 3.5V charge level, it takes longer for the current to drop to zero.... so it takes longer to charge. The area under the curves is essentially the same (some is blown off due to IR of the cell).


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## bgeery (Oct 17, 2011)

DavidDymaxion said:


> Great first post. I throw this out as a discussion point, and not as hard science. Jack mentioned this in his videos. (All hail Jackton! All hail Jackton!)
> 
> The voltage for a cell is Vcell = V0 + I*R
> 
> ...


Anyway, it's a thought that if the cells are cool enough and the currents are low enough, perhaps this isn't hurting them at all?[/QUOTE]

Very interesting. So that might be the "formula" for coming up with a recipe for charging in excess of 3.3V and insuring exactly 100% SoC is reached but not exceeded.
In your example then, if I wanted to charge at 100 amps, I'd apply 100 amps, and charge would be complete to moment the cell reached 3.83 volts. Cease charging and you have a 100% SoC cell. Of course, in a string of cells that all have different resistances, I'm not sure how that would interact.

My main issue is with those holding cells at 3.5-3.8 volts and forcing more and more current into the cell, until reaching 0.0 charging current.



> Anyway, it's a thought that if the cells are cool enough and the currents are low enough, perhaps this isn't hurting them at all?


In theory, if the current is flowing into the cell after 100% SoC, it has to be going somewhere. In a lead acid battery, the forced energy electrolyze the water in the electrolyte creating hydrogen and oxygen, and I believe also can warp the plates. Not a big deal to add water. Maybe in LiFePo4 it's just converted to heat, but I don't think so. Jack would know.


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## EVfun (Mar 14, 2010)

> For example, how many people are charging at 3.5-3.8 volts until the charge tapers to 0.0 amps? I see top balance supporters mentioning this all the time here.


I have heard of this being used by a few people to top balance a pack in parallel but I have never heard of this being a regular charging routine. It is a drastic way of sharply finding a full charge, similar to those who bottom balance by getting the resting voltage down to 2.5 to 2.8 volts to find a matched empty. 

I did my top balance by charging to 3.65 volts until the current tapered down to about 0.5 amps (60 amp hour cells.) It was mostly the time that it took my to get them all the same. That might cause some wear, but it is a single cycle. My charging routine is 12 amps until the voltage reaches 3.50 volts, then hold 3.5 volts for 40 minutes (around 2.5 amps ending current.) The next morning the cells consistently read 3.34 volts each and after 24 hours they consistently read 3.33 volts each. 

Here is the Thunder Sky charging documentation, from "Thunder Sky LiFeYPO4 Power Battery Performance Test Instructions":



> 5.2.4 Charging At 20°C±5°C temperature, the cell is discharged at a current of C3 till voltage of the cell reach 2.8V, and then start to perform constant current charge at a current of C3 till voltage of the cell reach 4.0V.
> 
> 5.2.4.1 Low temperature charging At -18°C±5°C temperature, the cell is discharged at a current of C3 till voltage of the cell reach 2.2V, and then start to perform constant current charge at a current of C3 till voltage of the cell reach 4.2V.


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## dougingraham (Jul 26, 2011)

bgeery said:


> A fully 100% charged battery has a resting voltage of 3.33 volts per cell at 25 Celsius.
> 
> The only way to insure we never overcharge the battery is to charge at 3.33 volts until it tapers down to 0.00 amps. That's a fully 100% fully charged cell. For those that want to charge to less than 100%, just discontinue that charge before current drops to 0.00 amps.
> 
> ...


Where did you get the 3.33 volt number? I have not seen that on any of the manufacturers data sheets. The only reference I have found to a float voltage is 3.45V found on the A123 systems data sheets that comes with the 26650 cells. And since those are plain Jane LiFePo4 it seems like a very reasonable number that could be used on other brands as well. I have floated the A123 cells at 3.45V for several days at that voltage with no ill effects.


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## bgeery (Oct 17, 2011)

* @ frodus:
**I find it hard to write a reply without nested quotes. sigh.

**As soon as you start charging it, the voltage will rapidly increase, and within a very short time it will match the voltage of the charger and the current will drop to zero. You're not overcharging it. This happens almost immediately.*
*
*Yes, I said this is a slower method of charging, but is guaranteed to find and stop at exactly 100% SoC. The cell voltage only rises in response to the charge current. Taper the current and the cell voltage will drop again and current flows, this happens until the cell is actually at 100SoC and does not fall.*

**The manufacturer specifies that the charge voltage is to be no more than 3.6-4.0Volts (depending on your cell). As long as you're at, or below that, you'll never overcharge the cell.*

*The cell is 100% charged when current stops flowing, but the cell has to be pulled higher than it's nominal voltage, even if it's just a tad. Higher voltages just cause it to charge faster, but if you're only at nominal voltage, you're not going to charge. *

Yes, but they don't say hold 4.0 volts all the way to 0.0 amps. And a discharged cell's *static* voltage is not 3.3 volts, thus will accept current regulated at 3.3 volts, at a slower and slower rate, until the cells' *static* voltage reaches 3.3 volts. This is 100 SoC.

Lets make it clear. I'm not saying charging this way is practical for everyday practice, as it may or may not take too long to reach 100% SoC. But, I am saying it's the only way to know you are actually *at* a perfect 100% SoC.

*Have you even looked at a charge curve from a manufacturer? It shows voltage and all the one's I've seen are above way 3.3V.
http://liionbms.com/pdf/thundersky/TS-LFP100.pdf
http://liionbms.com/pdf/zhejiangheadway/HW38120LS.pdf
http://liionbms.com/pdf/psi/PC40138F1.pdf
http://liionbms.com/pdf/shandong/200ah.pdfhttp://liionbms.com/pdf/goldpeak/GP18EVLF.pdf
http://liionbms.com/pdf/huanyu/HYP-3.2V-100Ah.pdf

That charge curve is at a given C-rate. The lifecycle is at that c-rate. As long as the cell never goes above the manufacturer's recommended charge voltage, you're going to charge to 100%. Most of the charge curves I've seen are at 1C. True, you could charge at 3.5V, which many do, but 3.5V > 3.33V, so charge flows.*


no, you charge a >3.33 so the charge flows faster. But in charging in excess of 3.33 volts, you are forcing the electrons into the cell, instead of alloing them to flow in naturally. That means it's up to you to discontinue the charge a some time befor you have tapered down to 0.0 amps.

Looking at your very first link for the LFP 100 clearly shows that the cell reaches 100% SoC before you taper the current down to zero. It looks to me the current should taper to 4 amps or so and stop. The battery is 100% charged. Continuing to 0.0 amps at *any* voltage in excess of 3.3 is an overcharge of the cell. How much does this damage the cells each cycle? That's a good question that I don't have an answer for.

*Thats how I charge, every time. My charger is set to a certain max voltage, and a certain max current. Set it and forget it. It turns off when current goes to 0. 

*If that certain voltage is >3.3 volts, then you are slightly overcharging the cell each cycle.*

It's not power we're looking at... it's energy. What you aren't looking at is that Energy = Wh = the area under the curve. 

Lets say you have a 10Ah 3.2V nominal cell. Situation 1: Charge voltage is set to 3.5V. Charge current is 1C, or 10A. Situation 2: Charge voltage is set to 3.7V. Charge current is the same.

The slope of the charge curve is steeper with situation 2, so it gets to 3.7V faster. Then it holds that voltage until current drops to 0. The area under the curve will be XXWh.

The slow of the charge curve is less with situation 1. Not only does it take longer to rise to the 3.5V charge level, it takes longer for the current to drop to zero.... so it takes longer to charge. The area under the curves is essentially the same (some is blown off due to IR of the cell). *

I get what you are saying, and you have a point about the resistance of the cell requiring something greater than 3.33 to allow a charge. So lets call it 3.331 volts then-- or whatever small amount is need to overcome the couple of milliohms of resistance in these cells.

I get the point that charging at higher voltages get the charge dome quicker. The point is, these fast charges (and that's what they are as far as the cell is concerned) need to terminate at some point before the current tapers to 0.0 amps. The higher above 3.3 volts you are charging at, the higher point in the current curve you need to terminate the charge. And in any case, with these fast charges, you never will reach 100% charge, and will always overshoot or undershoot by some percentage. The better you picked your termination current point, the closer you will be to 100%, but it will never be exactly right.

One alternative might be to fast charge to say 99% and then slow charge at 3.331 volts to 0.0 Amps. That will get you to 100% SoC much quicker, and insure you don't overshoot and overcharge.*
*


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## tomofreno (Mar 3, 2009)

> The voltage for a cell is Vcell = V0 + I*R
> 
> So suppose you have a cell with 5 milliOhms of resistance, you want to charge at 10 Amps.
> 
> Vcell = 3.33V + 10 Amps * 0.005 Ohms = 3.38V...


I remember Jack talking about this. Seems to me he also demonstrated charging a cell a 100A to higher voltage than the charging spec claiming no ill effects based on this reasoning. Makes sense. Doesn't mean it is correct though.

Some with shunt balancers might hold several cells at 3.5V at 0.5A or so charge current for a half hour to balance. According to the above formula those higher voltage cells should only be at 3.3325V. They are not because they are on the exponential part of the curve where cell resistance is higher. I've often wondered if that does any harm to them. If the voltage drop across the electrolyte is high enough it will break it down, resulting in plating on the electrodes. According to the CMU prof that requires about 4.3 to 4.4V terminal voltage. I think the voltage starts rising exponentially as the intercalation sites start becoming scarce and it gets more difficult for ions to work their way in, requiring larger electric field, E, and force, qE, so higher voltage, to pull them in. But that is just a guess. If correct, I wouldn't think any harm would be done as long as the voltage is not high enough to start breaking down the electrolyte. Though it may stress the electrode material more, which could lead to somewhat lower cycle life.


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## frodus (Apr 12, 2008)

I think I might have misunderstood you when you were saying 3.33V. I thought you were saying that was nominal. My mistake. I have to read in spurts at work sometimes..... 


Most people aren't damaging them with voltage.... people damage them by putting a charger on them with too high of a current limit on it. The lower the current for discharge and charge the better you're gonna be. Lowering the charge voltage does that by nature. 

So lets say your cell's nominal voltage is 3.2V for instance, setting the charger to it's 100% SOC voltage of 3.33V as you said.... it would take days to charge a cell even from 50% DOD. Really really slow, but safe for the cell. 

Now same cell, set to 3.4V, wait for the current to fall below an amp, as shown in some of the graphs. It's just fine. As long as you're not putting too much current into the cell, getting the cell too hot or getting the voltage too high, you're fine.


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## EVfun (Mar 14, 2010)

Looking at the specs you provided some of the manufacturers are calling out quite high termination voltage levels, but finishing current level was less clear.

Several seemed to agree on the idea of 3.65 volts with the ending current of 0.02 to 0.05C (ending amp level 2% to 5% of the amp hour capacity.) I think most of the non-BMS Lithium users are stopping well before that point. I'm stopping at about 3.5 volts and 0.04C. I have wondered how long it would take to charge if I held the cells to around 3.4 volts.


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## bgeery (Oct 17, 2011)

frodus said:


> I think I might have misunderstood you when you were saying 3.33V. I thought you were saying that was nominal. My mistake. I have to read in spurts at work sometimes.....
> 
> 
> Most people aren't damaging them with voltage.... people damage them by putting a charger on them with too high of a current limit on it. The lower the current for discharge and charge the better you're gonna be. Lowering the charge voltage does that by nature.
> ...


I think we are mainly in agreement here. Any time we charge above the natural final static voltage of the cell (plus a variable amount to account for IR) we should remove the current before it reaches 0.0 amps. The further above 3.33 volts, the sooner we want to remove the current.

We also agree, charging at 3.3 volts would probably take too long in practical terms, so it's more an academic argument. Interesting none the less.


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## bgeery (Oct 17, 2011)

EVfun said:


> Looking at the specs you provided some of the manufacturers are calling out quite high termination voltage levels, but finishing current level was less clear.
> 
> Several seemed to agree on the idea of 3.65 volts with the ending current of 0.02 to 0.05C (ending amp level 2% to 5% of the amp hour capacity.) I think most of the non-BMS Lithium users are stopping well before that point. I'm stopping at about 3.5 volts and 0.04C. I have wondered how long it would take to charge if I held the cells to around 3.4 volts.


Do you have an user adjustable charging source? I'd be interested to know myself. I know that the lower your finishing voltage, the lower you can adjust your final cutoff current. So maybe 3.4 volts to 0.02 amps?


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## EVfun (Mar 14, 2010)

bgeery said:


> Do you have an user adjustable charging source? I'd be interested to know myself. I know that the lower your finishing voltage, the lower you can adjust your final cutoff current. So maybe 3.4 volts to 0.02 amps?


My charger is very adjustable, but also very analog. I can set my charging current, charger voltage, and end of charge timer. The charger will run at the set current until the target voltage is reached, then hold the voltage for the selected amount of time. Amperage readings during the timer stage is me reading a 50 amp shunt with a Fluke DMM. 

The trick is determining a full charge. It seems you only know the next day. Since the charger is timer based instead of finish current based a charge cycle is not perfectly repeatable. Perhaps I will charge the pack at 3.4 volts per cell and pick some lower current level. 

When charging to 3.50 volts I use (about) 0.04C (2.4 amps ending current with 60 amp hour cells.) With 3.40 I could try 0.01C (0.6 amps.) I will have to determine the appropriate timer setting by timing the first such charge.

_Edit: checking notes the end of charge is about 3.6 amps (0.06C)_


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## ElectriCar (Jun 15, 2008)

I stop my charge around 3.45V/cell. Using the CC/CV method, my charger ramps up to about 30A initially and steadily increases voltage in order to maintain 30A as the batteries SOC increases until the voltage reaches approximately 3.45VPC. At that point it maintains the voltage level allowing the current to taper until it reaches 0.0A. 

I have 200AH cells so I'm well under the .5C charge current. This method as frodus said is very common, efficient, doesn't over current anything and charges the pack quickly so you're in business to drive again quickly. In a pack with AH as high as mine, I fail to see how charging to 3.33VPC would be of any benefit to me whatsoever. It would take forever to charge as well. With a 5kw charger it already takes about 6-7 hours if I'm nearly empty to recharge.

I just checked mine which is charging now and with 25Ah to go or about 7/8 charged, I'm at 3.418VPC. If I lowered it to 3.33 or so I'd be forever charging with a pack my size. 

I see NO BENEFIT to charge to 3.5VPC or higher. There just isn't any capacity up there plus you're risking a cell spiking and going into over voltage if you're not running a BMS which I don't. So if there's no capacity up there why bother to go there and in the case of a BMS, waste energy draining it off the high cells? 

After draining some of the charge ALL MY CELLS are at the same voltage to within 1/1000V. It's only when you get close to completely charged that they start to diverge in voltage, and that's where you're getting onto thin ice.

Just checked it again FWIW, ALL 50 CELLS charging at full throttle are reading 3.430V, every single one with 9.5Ah to finish. Soon they will start to diverge. I'm going to try and find that voltage. I may adjust my Max V downward after a few more voltage checks, depending on at what VPC they start to diverge and how far from the finish line the pack is.

Edit - Checked again: 3.466VPC (average) Cell voltages noticeably diverged. 0.5Ah remaining and charger is cutting back quickly, now @7.5A


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## ev4me (May 10, 2011)

ElectriCar said:


> I see NO BENEFIT to charge to 3.5VPC or higher. There just isn't any capacity up there plus you're risking a cell spiking and going into over voltage if you're not running a BMS which I don't. So if there's no capacity up there why bother to go there and in the case of a BMS, waste energy draining it off the high cells?


The idea is that you charge at a higher voltage, but stop the charge somewhere before reaching zero current.

For example, imagine these two charging configurations:
-Constant current at 30 A until voltage reaches 3.33 V, then constant voltage until current reaches 0.0 A
-Constant current at 30 A until voltage reaches 3.65 V, then constant voltage until current reaches 0.5 C

Both situations should push approximately the same amount of charge (amp-hours) through the battery, but the second configuration will do it faster. After removal from the charger the cell voltage will be around 3.33 V in either case (not 3.65 V as you might think in the second case, since some current was still flowing at that voltage).

The disadvantage here is that the cell voltage is held at 3.65 V for a few hours during the charge. I have not seen any sort of consensus on how this affects the battery or how high one can safely go. If temporary high voltages were okay, one could theoretically charge at constant current all the way to 100% SOC. Unfortunately, anything above 4.3 V or so decidedly destroys the cell. But what sort of effect does a voltage between 3.33 V and 4.0 V have? We do 3.65 V, but I've seen others who do anything from 3.45 V to 4.0 V.

In conclusion, there is a difference between "charging to 3.65 V" and "charging at 3.65 V," so long as current is not zero.


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## EVfun (Mar 14, 2010)

I decided to pour over some of the data in the garage. I'm adding 7.2 amp hours (20% of the total capacity) between hitting 3.44 vpc (110 pack volts) and the end of charge at 3.50 volts. That is 12 amps for 14 minutes, from 110 volts to 112 volts, and then tapering down to 3.6 amps over the next 40 minutes. This is a pretty good end of charge for my Thunder Sky pack because 24 hours later the cells are resting at 3.33 volts. 

If I held 110 volts (3.44 vpc) I could get to the same point. I just have no good idea how long it would take.


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## GizmoEV (Nov 28, 2009)

bgeery said:


> Yes, but they don't say hold 4.0 volts all the way to 0.0 amps. And a discharged cell's *static* voltage is not 3.3 volts, thus will accept current regulated at 3.3 volts, at a slower and slower rate, until the cells' *static* voltage reaches 3.3 volts. This is 100 SoC.
> 
> Lets make it clear. I'm not saying charging this way is practical for everyday practice, as it may or may not take too long to reach 100% SoC. But, I am saying it's the only way to know you are actually *at* a perfect 100% SoC.


When I got in my car to leave work today the pack was sitting at 66.6V. I have 20 cells so that is 3.33vpc. According to your statement above my pack was at 100%SOC. So, did I charge at work or not?

If you said I charged at work you would be wrong. My 200Ah pack was sitting at 79%SOC. It hasn't been charged for 4 days and just over 42Ah had been extracted from the pack. This is not just a one time occurrence. It happens every discharge cycle. 3.33V is not 100%SOC. Therefore, it is not a way to determine 100%SOC.


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## bgeery (Oct 17, 2011)

GizmoEV said:


> When I got in my car to leave work today the pack was sitting at 66.6V. I have 20 cells so that is 3.33vpc. According to your statement above my pack was at 100%SOC. So, did I charge at work or not?
> 
> If you said I charged at work you would be wrong. My 200Ah pack was sitting at 79%SOC. It hasn't been charged for 4 days and just over 42Ah had been extracted from the pack. This is not just a one time occurrence. It happens every discharge cycle. 3.33V is not 100%SOC. Therefore, it is not a way to determine 100%SOC.


I don't know what to tell you. If you take energy out of the pack, the static resting voltage must decrease. LiFePo4 cells have a shallow voltage curve, but not flat. Jack R even did an episode one time demonstrating this voltage drop with every 10% drop in SoC. Perhaps measuring to only the hundredths is not enough precision to detect the slope? We talk about 3.33 being 100% SoC, but is that 3.330? I don't know. Perhaps it's 3.333 or even 3.335. Jack has stated more than once that a full battery is 3.4, but he has also refered to 3.33, but I don't think 3.330. I'm confused.


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## GizmoEV (Nov 28, 2009)

bgeery said:


> I don't know what to tell you. If you take energy out of the pack, the static resting voltage must decrease. LiFePo4 cells have a shallow voltage curve, but not flat. Jack R even did an episode one time demonstrating this voltage drop with every 10% drop in SoC. Perhaps measuring to only the hundredths is not enough precision to detect the slope? We talk about 3.33 being 100% SoC, but is that 3.330? I don't know. Perhaps it's 3.333 or even 3.335. Jack has stated more than once that a full battery is 3.4, but he has also refered to 3.33, but I don't think 3.330. I'm confused.


The CALB spec sheet lists 3.4V as the "float" voltage. Someone else mentioned 3.45V for A123 cells. I take this to mean that if the terminal voltage is held at 3.4V indefinitely the cell will not overcharge. I charge to 3.465vpc (69.3V for 20 cells). If it didn't cost me nearly $100 to have my charger reprogrammed I'd get it set to shut off when the current dropped below 1A. Only a fraction of an Ah gets into the pack after that point.

Remember that in Jack's demonstration he didn't let the cell sit for several hours between the discharge and the subsequent voltage reading. When I got to work this morning the pack voltage read about 65V, IIRC, just before turning the key off. This was with just a few second rest at under a 2A load. 8 hours later it was up to 66.6V. While I haven't been able to repeat the test Jack did, I don't have an extra battery to play with, I'm sure his results would have been different if he had waited a couple of hours between discharge and voltage measurement. Also, he was measuring out to the ten-thousandths place so to the nearest 0.0001V.


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## Guest (Nov 10, 2011)

The voltage was active discharge voltage and that is what you want to see. The resting voltage after discharging can be deceiving. It will rise back up after sitting but the cell is still empty. It has voltage but not capacity. The capacity has been used up for that charge/discharge cycle. If you let your cell sit for hours then go out and begin discharging again it will drop to that old discharge voltage very very fast. I have tested that. When its empty its empty and its best seen during active discharging. 

Pete


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## ElectriCar (Jun 15, 2008)

David didn't you mean 0.001V? That is 1/1000 of a volt.  That's what my Fluke meter measures down to.


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## GizmoEV (Nov 28, 2009)

ElectriCar said:


> David didn't you mean 0.001V? That is 1/1000 of a volt.  That's what my Fluke meter measures down to.


I meant ten-thousandths. I went back and fixed it. He used an Agilent(sp?) bench top meter. Unless I'm confusing that test with the one where he was comparing 2.5 year old cell with a new one.


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## Guest (Nov 10, 2011)

bgeery said:


> I don't know what to tell you. If you take energy out of the pack, the static resting voltage must decrease. LiFePo4 cells have a shallow voltage curve, but not flat. Jack R even did an episode one time demonstrating this voltage drop with every 10% drop in SoC. Perhaps measuring to only the hundredths is not enough precision to detect the slope? We talk about 3.33 being 100% SoC, but is that 3.330? I don't know. Perhaps it's 3.333 or even 3.335. Jack has stated more than once that a full battery is 3.4, but he has also refered to 3.33, but I don't think 3.330. I'm confused.


Jack refers to 3.33 volts as HIS charged 100% SOC. The cell could in fact hold more, but why bother because the amount you could stuff in would be used in about a mile or less of actual driving. He has charged and discharged enough times to know this. I concur. 3.33 is pretty much right on the money for resting voltage after about 24 hours of resting after a charge. My cells actually rest at a lower voltage than Jacks but mine are close. I charge to an average of 3.65 per cell and wait until the amperage is at about 2 amps. I can add in an extra cell or change the hold voltage as I have 10 to pick from. I have room to play. The manufacturer says 3.8 volts end charge voltage for my cells and 2 volts end discharge for my cells. I end my discharge well above the 2 volt range. 

Pete 

During active charging right near the end of the charge cycle I never see a cell above 3.8 volts. Some are close but those are the low capacity ones in the pack. Some are less but hold more. When the charge is done all the cells receive the same amount of AH's.


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## spdas (Nov 28, 2009)

My sweet spot for charging is at 3.477V per cell. After I top balance, it allows for a "rogue" cell to go up to 3.55 and still be safe and a lagging cell to be as low as 3.40 which fully charged and into the top anyway where there is not much amps stored. When I charged at 3.49V each cell, I saw considerably more variance in the "no amperage" zone at the top. I will be lowering my charging voltage to 3.455, but it will take longer to charge at this rate.

Francis


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## ElectriCar (Jun 15, 2008)

spdas said:


> My sweet spot for charging is at 3.477V per cell. After I top balance, it allows for a "rogue" cell to go up to 3.55 and still be safe and a lagging cell to be as low as 3.40 which fully charged and into the top anyway where there is not much amps stored. When I charged at 3.49V each cell, I saw considerably more variance in the "no amperage" zone at the top. I will be lowering my charging voltage to 3.455, but it will take longer to charge at this rate.
> 
> Francis


I doubt you'll notice any difference unless you don't charge at the normal CC/CV mode.


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## EVfun (Mar 14, 2010)

I played with the charge cycle tonight. I charged to 3.44 vpc (normally I charge to 3.50 volts) and held that until the current dropped to 1.2 amps (0.02C for my 60 amp hour cells.) Then I turned the charger voltage back up to 3.50 vpc. The current initially surged up to my 12 amp setting but as quick as I could get to the shunt (opposite side of the car from the charger) the current was already down to 2.5 amps and within minutes it was back down to 1.2 amps. As near as I can tell, I put in about 1/4 amp hour. 

I bled the surface charge off with the headlights and then readjusted the charger down to 110.1 volts at 200 milliamps. I watched it stay at that voltage as the current dropped to 100 milliamps over perhaps another 15 minutes. I will try this lower charging voltage out for a while. If I'm only missing a small fraction of one amp hour and only adding about half an hour to the end of charge I don't see any reason to charge higher.


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## tomofreno (Mar 3, 2009)

> If I'm only missing a small fraction of one amp hour and only adding about half an hour to the end of charge I don't see any reason to charge higher.


 I've done similar tests, just watching my Ah counter during charging, and concluded the same, there isn't much charge above that point. Others have done similar tests with similar results, so I think that is well-established now.


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## ev4me (May 10, 2011)

tomofreno said:


> I've done similar tests, just watching my Ah counter during charging, and concluded the same, there isn't much charge above that point. Others have done similar tests with similar results, so I think that is well-established now.


Has anyone tested the effect of charging voltage on charging _speed_? That's where I think the tradeoff may be. Setting a higher charging voltage allows you to stay in CC mode longer.


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## GizmoEV (Nov 28, 2009)

ev4me said:


> Has anyone tested the effect of charging voltage on charging _speed_? That's where I think the tradeoff may be. Setting a higher charging voltage allows you to stay in CC mode longer.


Not much. When I set a higher ending voltage it shortens the last part of charge by 10-15min. In that few minutes only about an Ah gets put into my 200Ah pack. If you are in a hurry just unplug and go. Remember, however, that in my case I'm charging at 15A into a 200Ah pack so full throttle isn't that fast. I haven't tried different voltages with my 40A charger but the ramp up in voltage after 3.4Vpc is so fast it won't make as much difference as you might think.


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## JRP3 (Mar 7, 2008)

bgeery said:


> How's that for a first post?


Not that good


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## EVfun (Mar 14, 2010)

ev4me said:


> Has anyone tested the effect of charging voltage on charging _speed_? That's where I think the tradeoff may be. Setting a higher charging voltage allows you to stay in CC mode longer.


It isn't much. That was actually my goal, but the time it took my to adjust my charger set point to 110 volts swamped any initial measurements. On my way to 110 volts (3.44 volts per cell) I did time each 1/2 volt change in pack voltage.

3.38 vpc to 3.39 vpc (108v to 108.5v) 32 minutes
3.39 vpc to 3.41 vpc (108.5v to 109v) 30 minutes
3.41 vpc to 3.42 vpc (109v to 109.5v) 18 minutes
3.42 vpc to 3.44 vpc (109.5v to 110v) 7 minutes

The voltage climb starts accelerating at about 3.4 volts. Trying to hold the voltage above that point causes the current to fall pretty quickly but some of my numbers don't quite agree on how quick. I plan some more tests over the next few days.


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## ElectriCar (Jun 15, 2008)

FWIW, I adjusted my charger down tonight from about 3.46VPC to 3.44 and found the charge only about .5Ah less. Taking it back to 3.45 added about 0.2Ah to the pack. This VPC voltage was the average based on the voltage reading on a 50 cell pack.


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## sickness1 (Mar 7, 2011)

If you look at this datasheet (I pick this one for example purpose):
http://liionbms.com/pdf/thundersky/TS-LFP100.pdf

You will see that the batteries should be charged above 3.33 Volts, in fact the manufacturer tells clearly that, to charge the batteries is mandatory to charge them (in the case of this datasheet) to 4.25V. The charge has 2 phases in this case:
-1phase, current limited charge:
In the graph you can see that charge is current limited at aprox 50Amps and is maintained till the battery reaches 4.2V (the maximum voltage recommended by manufacturer)
-2phase, voltage limited charge:
As you should not go through the voltage of 4.2, when this voltage is reached you limit voltage to this point and wait till the current drops till practically 0Amps.
Doing this charge you are not overcharging the battery nor breaking it, all the manufacturers recommend this kind of recharging (you can see for example yuasa batteries datasheet, to see another kind of battery charging method). 
And if you look the discharging of the battery, the beginning voltage is not 3.3V, is 4.2V. The thing is that this kind of batteries have their nominal voltage in 3.2 - 3.3V but that doesn’t mean that they are not able to work at 4.25 or 2.7V. If this would be real then you wouldn't be able to discharge batteries below 3.2 because you would be overdischarging it. So your operative range for the battery would be between 3.2 and 3.3V (the 90% of the time batteries will be between those values at a 30ºC temperate) but you would be loosing between 5 and 10% of battery capacity.


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## JRP3 (Mar 7, 2008)

That's an out of date data sheet, you do not need to charge to 4.25, or 4.20, and I think TS now recommends 3.8 as max charge. I'm pretty sure some people damage their cells by thinking they had to charge to 4.25 each time, when in fact that will shorten their life by increasing the speed with which the electrolyte breaks down.


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## Jan (Oct 5, 2009)

Charge as slow and low as you can afford, this will prolong their life. 

But I don't think there are any commercial charger that give you both of these choises. A amp throttle is the maximum you can get. But a %SOC throttle is imho a must too.


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## JRP3 (Mar 7, 2008)

You can limit SOC with your finishing voltage and charge current to some degree. I don't think there is any point in going below 80% SOC anyway. 3000 cycles on a 50 mile pack is 150,000 miles. At 10K miles per year that's 15 years. I hope to have better cells available long before that.


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## Jan (Oct 5, 2009)

JRP3 said:


> You can limit SOC with your finishing voltage and charge current to some degree. I don't think there is any point in going below 80% SOC anyway. 3000 cycles on a 50 mile pack is 150,000 miles. At 10K miles per year that's 15 years. I hope to have better cells available long before that.


Yes, you're probably right. But charging 80% SOC normaly and once in a while 100% is not an easy option with most commercial chargers.


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## sickness1 (Mar 7, 2011)

JRP3 said:


> That's an out of date data sheet, you do not need to charge to 4.25, or 4.20, and I think TS now recommends 3.8 as max charge. I'm pretty sure some people damage their cells by thinking they had to charge to 4.25 each time, when in fact that will shorten their life by increasing the speed with which the electrolyte breaks down.


As I told it was a datasheet with example purpose, another thing is that thunder-sky changed the maximum voltage of the battery because they noticed that it wasn't correct. 

Normally there will be no problem charging at the maximum current that our chargers can afford, if we have a 18Kwh pack and we charge at 3Kwh we will need 6hours to charge, so we will be charging at 1/6C, battery isn't going to suffer at this current, is worst when we are driving and even worst when we let the battery capacity drop below the 10% of its nominal capacity. If you look at the battery life cycles you will notice that thundersky indicates 3000 cycles with 80% DOD and 5000 cycles with 70% DOD. I don't think many people will reach 3000 cycles since there are a lot of factors that will influence in this cycles, factors like temperature, current, DOD...


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## ev4me (May 10, 2011)

JRP3 said:


> That's an out of date data sheet, you do not need to charge to 4.25, or 4.20, and I think TS now recommends 3.8 as max charge. I'm pretty sure some people damage their cells by thinking they had to charge to 4.25 each time, when in fact that will shorten their life by increasing the speed with which the electrolyte breaks down.


Would you happen to know of an official statement from the manufacturer that the old datasheet is obsolete? I know we aren't supposed to charge to 4.25 V, but I still hear a lot of people within my own organization citing the old datasheet. It's hard to argue with something that came from the manufacturer.


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## JRP3 (Mar 7, 2008)

The Winston battery page is showing 4.0 max, http://en.winston-battery.com/index.php/products/power-battery Sinopoly is showing 3.8
http://www.sinopolybattery.com/html/products_batteries.php
Additionally we know that keeping the cells at lower voltage prolongs their life, regardless of what manufacturers show for maximum voltage. You don't need to fully charge LiFePO4 unless you need the full capacity.


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## bgeery (Oct 17, 2011)

In my original post I was under the false assumption that 3.33V resting was a full cell. I've since learned the real value is 3.400V. This also clearly makes sense, as the manufactures states the float voltage as 3.4V. They know regardless of how long you let it trickle, current will cease and hold at 100% charged.

This still makes me caution those that are holding >3.5V all the way to 0 current. What really frightens me is those that are top balancing by connecting all their cells together in parallel and then charging to 3.6V or even higher, and holding that voltage "for days". I will bet a set of new batteries they are harming the cells to some unknown degree and plating the cells by this practice. This demonstrates another advantage of bottom balancing, where connecting all their cells together in parallel and then holding the voltage at some low level "for days" does no possible harm to the cells. I do understand that some folks can't adjust the CV setpoint of their charger so a bottom balance isn't practical, but please minimize your time above 3.4V if you want to maximize cell life.


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## dtbaker (Jan 5, 2008)

in the average charge cycle I doubt the cells are above 3.4 for more than a few minutes... they climb to 3.65vpc rapidly, switch to CV, finish, and start dropping all within minutes from what I've seen.


The original initial top balance you hope to catch within an hour or two, not weeks. The best guess when starting initial charge is that the cells are about 1/2 capacity, so just figure aproximate charge time based on your power supply and total 'missing' amp-hours.

no worries.
d


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## tdnevmoDK (Jul 21, 2011)

When the PSU at 3.3 sees the battery cell at 3.3 og 3.2 it will say to it self: "Why on earth should i pump current into something that has got the same, or almost the same voltage as my self? Thats stupid" Because the current always flows the easiest way, so why would it bother flowing through the cell, when it it easier to stay inside the PSU?  That is why you have to give more volts than the nominal voltage. A 12 volt car battery gets 14.4 from the generator, because if it just got 12 volts, it would not charge up enough Ah. Less Ah = Less available current to draw. This will give you problems, like the starter won't turn quick enough to start the engine. The voltage simply drops too much, when battery is low on capacity 
When battery is 12 volt and charger is 12 volts, the charger charges the battery as much as the battery charges the charger = 0.0 Amp draw.


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## frodus (Apr 12, 2008)

Yup, Current only flows when there's a voltage differential. No differential, no current flow.


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## PStechPaul (May 1, 2012)

There are some very good discussions of batteries, charging, SOC, and other important issues here:
http://www.mpoweruk.com/chargers.htm


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## russatt (Aug 30, 2013)

Hey Guys. I know this thread is old, but if any of you are still around I have a question.

Can anyone assist with some technical knowledge.
I have 220v AC power. Can I build a simple battery charger using a voltage regulator on the AC input, run it through a large rectifier to get DC, clean it up with a few capacitors.

Batteries to charge would be 160v bank of LiFePo.

I understand the constant current, then switch to constant voltage.
I could build something to monitor and shut it off at the correct point.
Lets say for now I will monitor it manually. Turn the voltage slowly until batteries are drawing the desired current, when batteries reach near peak voltage turn input voltage down to correct peak voltage, and cut when correct current draw is reached.

Lets say I manually manage the constant current - constant voltage correctly withing the LiFePo safe charging specs.

Will it work??
Can it damage the batteries(Frequency?? some other tech word), if volts and amps stay within batt spec ?
What might its efficiency be?


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## steven4601 (Nov 11, 2010)

Hi,

Working on EV's and driving them is very exiting. 
While the technology itself is safe, working with the required voltage is not safe if performed by unexperienced persons or equipment not designed for the task. 

If you are asking how to charge a battery from 220Vac, No disrespect, but you´re best of buying a ready made charger for the application. It will be safer & cheaper eventually comparing to starting from scratch.


Don´t underestimate the importance of the charging facility of an EV .


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## russatt (Aug 30, 2013)

steven4601 said:


> Hi,
> 
> Working on EV's and driving them is very exiting.
> While the technology itself is safe, working with the required voltage is not safe if performed by unexperienced persons or equipment not designed for the task.
> ...


Hi
Thanks for reply and as always with me, safety is first, that is why I am asking. But maybe to ask in a different way.

If I have a variable voltage AC Supply and can convert it to DC with a simple rectifier for a few Dollars. 

Other than the Logic in the processor of a ready made charger. What make it soo expensive.


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## PStechPaul (May 1, 2012)

There are "bad boy" chargers that work like you describe and can be built very cheaply. But I think the expense comes with features like the following:

1. *Isolation*. This safety consideration involves either a large, heavy mains transformer (about 5 kVA and 150 pounds or so, and maybe $500), or high frequency switching circuitry, which is much lighter but requires special ferrite or powdered iron transformers and/or chokes, and associated circuitry.

2. *Programmability*. This allows the same circuit to be used for different packs and can be set to an optimum charging profile.

3. *Safety*. Continuous monitoring of individual cells (BMS), overvoltage and overcurrent detection, physical containment of components which can overheat, ignite, or explode.

4. *Efficiency*. Power factor correction (PFC), minimization of losses, maximum output from a current-limited supply.


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## JRP3 (Mar 7, 2008)

On the other hand there are plenty of us using non isolated chargers with no BMS without any issues for many years.


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## russatt (Aug 30, 2013)

PStechPaul said:


> There are "bad boy" chargers that work like you describe and can be built very cheaply. But I think the expense comes with features like the following:
> 
> 1. *Isolation*. This safety consideration involves either a large, heavy mains transformer (about 5 kVA and 150 pounds or so, and maybe $500), or high frequency switching circuitry, which is much lighter but requires special ferrite or powdered iron transformers and/or chokes, and associated circuitry.
> 
> ...


Thanks for the info.
I am thinking of using a 
220v PWM voltage regulater on the AC side, through a 50A single phase rectifier,
I will get some help on smoothing out the DC power with capacitors or maybe somethig higher grade.
Then with an Arduino board, a shunt, some resisters, a relay, and some temp sensors I can manage the cut off's. Will probably add an LCD display.

For the BMS I have been thinking of either using some zeener diodes and resister across each cell, to drain off any current over a 3.6v, to top balance the cells.
I am also considering another arduino to monitor the cells voltage, but to do it properly it will take more than 1 processor.

e.g if I monitor the cells in 5 groups of 10. I could read the 1st cell voltage, and then the combined voltage of the 10 cells, then if Cell 1 is 3v, the group should be 30v, if it is not, depending on my settings and logic, either cut charging, or if driving, create some sort of alert. Then at least I Know which cells to check.

But for starters I will equalize all the cells in parallel when they arrive, and probably cut my charge early, and keep driving range shorter.


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## russatt (Aug 30, 2013)

JRP3 said:


> On the other hand there are plenty of us using non isolated chargers with no BMS without any issues for many years.


What is most Off putting for me is that most of the chargers dont have programable cut-offs, and pack size. So if for e.g 1 cell in my bank of 50, turns out to be a "Lemon", if I have to remove the cell, I will probably need to have the charger reprogrammed.


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## steven4601 (Nov 11, 2010)

Not sure what you mean, but I would certainly not recommend to buy chargers with the charge termination voltages only programmable by the manufacturer.


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## russatt (Aug 30, 2013)

I have seen many online for sale where they ask you 30 Questions prior to purchase.
Voltage, Amperage, requirements, Cell Manufacturer and recommended charge etc etc.
I am sure I have read some posts on this forum of Guys having to have their chargers reprogrammed, and that there are many that are factory programmed.


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## dtbaker (Jan 5, 2008)

russatt said:


> Can anyone assist with some technical knowledge.
> I have 220v AC power. Can I build a simple battery charger using a voltage regulator on the AC input, run it through a large rectifier to get DC, clean it up with a few capacitors.
> 
> Batteries to charge would be 160v bank of LiFePo.



you COULD, but why would you want to do it yourself when good simple chargers are available off the shelf for fairly cheap? I'm guessing the parts to do it right will cost as much as a retail charger if you go with a simple fixed curve CC-CV unit. you have to be VERY accurate with the end voltage to prevent damaging LiFePO4 cells.

a cheap, small Elcon (TCCH) charger will only cost you $500-600 for a 1500 watt , or $950-$1050 for a 3000 watt.

I've got several of each available new in the box if you'd like. I keep meaning to put them up on the classifieds, but just having an issue with time and a new job....


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## PStechPaul (May 1, 2012)

I have built a simple voltage relay using a PIC that can be programmed for a wide range of total pack voltages, and it also incorporates a delay and hysteresis so that it can hold a safe top voltage for long enough to allow a deep charge, and then wait for the pack to drop to a lower voltage before re-initiating the charge. Or it can be set up so that it is disconnected once the peak voltage has been reached, requiring manual reset to initiate another charging cycle. It can be built for about $20 and I have posted the schematic, and I also can obtain PC boards and supply components and programming code as a kit, or a built and tested unit for about $100.

I am working on a shunt charge limiting BMS that can be built for about $5/cell (parts only), or perhaps $15-$20 each fully assembled if quantities allow automated assembly. I will be using the design for charging some LiFePO4 18650 cells for an 18V electric drill. They will incorporate a means to shut off a charge cycle after the entire pack (6 cells) reaches full charge, and will then monitor individual cells and disconnect from the drill motor if any cell goes too low. It will have a constant drain of about 50 uA (or optionally about 50 nA with PIC12LF1822), so the 1250 mAh cells I use will lose 1/2 charge in 12,500 hours (1250/.025) which is negligible (almost two years).

The system will also be capable of providing detailed voltage information on each cell by means of a daisy-chain communication system, and it may also be capable of performing a bottom-balancing procedure on a pack of cells. For higher capacity cells, some larger and more expensive components may be needed, but not much compared to the cost of the cells. It will also be possible to set up the entire string of BMS elements for any chemistry by means of a command that will propagate along the daisy chain, and may even be used for NiMH (3 cells in series per element) and NiZn (2 cells each). LiPo cells (3.7V) may also be accommodated, and there will be an option for 12V lead-acid.


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## dtbaker (Jan 5, 2008)

PStechPaul said:


> I have built a simple voltage relay... It can be built for about $20 and I have posted the schematic, and I also can obtain PC boards and supply components and programming code as a kit, or a built and tested unit for about $100.
> 
> 
> ... For higher capacity cells, some larger and more expensive components may be needed,



this is kindof the point. to build a high capacity charger (outputting 1500-3000 watts) for a 120v-160v nominal pack of LiFePO4 cells is not trivial, cheap, or very practical for the average DIY compared to buying off the shelf. Especially when expensive cells are at risk if you screw up.


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## PStechPaul (May 1, 2012)

The charger itself need not be hugely expensive, if you are willing to accept the possible hazards of non-isolation, and the reduced efficiency of lousy power factor which will also reduce the maximum charge you may be able to get from the usual 20-30 amp outlet. If the cells are top balanced, you can probably get by with a single maximum-voltage shut-down relay, which can be built for about $20 and purchased for about $100. By using two or more such relays on parts of the pack, you can more easily detect a single overcharging cell and shut down when that happens. The more voltage sensors you have, the better the sensitivity, and of course one unit per cell is ideal. Considering that a typical 100 Ah cell costs well over $100, adding a $5 to $20 shunt charge controller and monitor to each cell adds a rather small percentage to the overall cost, and will pay for itself if it protects one or two cells in a pack of 50.

I have looked into a "stupid simple" overcharging protection circuit using just zeners and resistors and even BJTs and MOSFETs with simple analog circuits, and they are just not accurate or stable enough to be effective, and by the time you add comparators with voltage references and other circuitry (as was proposed in another recent thread), you will be at the cost point where you can just as cheaply use a small PIC and have much better features and performance.


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## russatt (Aug 30, 2013)

dtbaker said:


> you COULD, but why would you want to do it yourself when good simple chargers are available off the shelf for fairly cheap? I'm guessing the parts to do it right will cost as much as a retail charger if you go with a simple fixed curve CC-CV unit. you have to be VERY accurate with the end voltage to prevent damaging LiFePO4 cells.
> 
> a cheap, small Elcon (TCCH) charger will only cost you $500-600 for a 1500 watt , or $950-$1050 for a 3000 watt.
> 
> I've got several of each available new in the box if you'd like. I keep meaning to put them up on the classifieds, but just having an issue with time and a new job....


$1000 is expensive for me as our exchange rate is almost R11/$1 + postage $80+ duties of about 14%


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## russatt (Aug 30, 2013)

dtbaker said:


> this is kindof the point. to build a high capacity charger (outputting 1500-3000 watts) for a 120v-160v nominal pack of LiFePO4 cells is not trivial, cheap, or very practical for the average DIY compared to buying off the shelf. Especially when expensive cells are at risk if you screw up.


I Guess we all have some experience in different areas. I am having my coupler made for me. I guy who works with Lathes and can weld would probably make his own.

I can buy the Ac regulator, rectifier, and capacitors for about $30. I have the Processor "Arduino", current shunt, voltage sensors, relays etc to control the charge logic. And I would enjoy building it.
My only concern is Safety and destroying the expensive batteries.


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## russatt (Aug 30, 2013)

PStechPaul said:


> The charger itself need not be hugely expensive, if you are willing to accept the possible hazards of non-isolation, and the reduced efficiency of lousy power factor which will also reduce the maximum charge you may be able to get from the usual 20-30 amp outlet. If the cells are top balanced, you can probably get by with a single maximum-voltage shut-down relay, which can be built for about $20 and purchased for about $100. By using two or more such relays on parts of the pack, you can more easily detect a single overcharging cell and shut down when that happens. The more voltage sensors you have, the better the sensitivity, and of course one unit per cell is ideal. Considering that a typical 100 Ah cell costs well over $100, adding a $5 to $20 shunt charge controller and monitor to each cell adds a rather small percentage to the overall cost, and will pay for itself if it protects one or two cells in a pack of 50.
> 
> I have looked into a "stupid simple" overcharging protection circuit using just zeners and resistors and even BJTs and MOSFETs with simple analog circuits, and they are just not accurate or stable enough to be effective, and by the time you add comparators with voltage references and other circuitry (as was proposed in another recent thread), you will be at the cost point where you can just as cheaply use a small PIC and have much better features and performance.


Thanks.
From what I gather from most of the posts, the biggest danger is overcharging.
Just as a thought. If I started off with top balanced cells, and lets say thet the Spec is max voltage of 3.65v/cell. If I switched to constant voltage at 3.55v per cell.
Isn't that more than 90% charged.

Also just a question to all.
If I hold a constant voltage slightly less that the peak voltage for a long period of time. Wont all the cells eventually equalize themsleves.(top balanced at that voltage)


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## mk4gti (May 6, 2011)

russatt said:


> Also just a question to all.
> If I hold a constant voltage slightly less that the peak voltage for a long period of time. Wont all the cells eventually equalize themsleves.(top balanced at that voltage)



no, the current will want to flow to the cells "above the 3.45v" knee, thus over charging the ones that are already full and not charging the less charged ones. There is a neat paper on the miniBMS product page called "How to perform initial LiFePo4 battery pack balancing" (note: I am not affiliated with minibms)


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## steelneck (Apr 19, 2013)

russatt said:


> Thanks.
> From what I gather from most of the posts, the biggest danger is overcharging.


I think the other end of the scale is more dangerous, when cells are unbalanced at low SoC. When you drive the car and accelerate there will be a large voltage drop when cells are starting to get empty, even more so in winter and cold temperatures. As far as i know it can be expected to see single cell voltages under two volts at acceleration in the cold even at mid SoC. Now if the cells are unbalanced in the bottom, single cells can go to zero voltage under acceleration, while others have a much more healthy voltage. Remember now, cells in series have their positive terminal connected to the next cell negative. For that weak cell this will be like hooking up a charger with hundreds of amps, with wrong polarity! This is how cells can get their polarity reversed, and be destroyed. At the other end while charging, we are not talking about hundreds of amps, unless there is heavy generative braking, but creating the bad situation where strong cells can hurt the weak, is a matter of seconds while driving. This means that you must end discharge at a much higher total pack voltage, to not put weak cells in danger if your pack is top-balanced. The reverse if of course true with a bottom balanced pack, charge must stop at a much lower voltage. Energy-wise i do not there is much difference in kWh. I believe the bottom balanced pack will work a lot better in the cold, i do not think i will have any problems allowing temp. single cell acceleration sag voltages even dow to 1.5V, but that demands a bottom balanced pack or else strong cells will hurt the weak ones.


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## EVfun (Mar 14, 2010)

russatt said:


> Thanks.
> From what I gather from most of the posts, the biggest danger is overcharging.
> Just as a thought. If I started off with top balanced cells, and lets say thet the Spec is max voltage of 3.65v/cell. If I switched to constant voltage at 3.55v per cell.
> Isn't that more than 90% charged.
> ...


It depends on voltage and current. Charged may be defined as something like 3.60 volts at 0.05C. 3.6 volts at 0.1C would be something less than full according to the manufacturer. That said, there is no harm in undercharging LiFePO4 cells every cycle, just a little less available capacity. I take your idea a lot farther and it has been working well. I charge to 3.42 vpc and hold that voltage for 45 minutes. That is a little less than 0.05C for my cells. They are top balanced and seem to be staying in tight formation. 

If you hold Lithium cells at the peak voltage they will not self equalize. They will continue to decline in current and overcharge, while the continuing current decline pretty much keeps any that are behind from ever charging. You can actually get into a situation where the voltage spread between those cells at a higher SOC and lower SOC is getting larger as you hold the voltage high.


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## steelneck (Apr 19, 2013)

EVfun said:


> I charge to 3.42 vpc and hold that voltage for 45 minutes. That is a little less than 0.05C for my cells. They are top balanced and seem to be staying in tight formation.


I have a 2.75V bottom-balanced pack and my cells stay within +-0.01V up to 3.43V. If i go beyond that, with so little as 1.5kWh on my 78 cell 100Ah pack, my weak cells (3 of them) will _very_ quickly rush away even beyond 4 volts. So i will stop at 3.43V. But on the other hand i can allow them to go very deep in the other end.

When i did my bottom balancing of my TS/Winston 100aH cells, i saw the same very quick rush in the other direction as soon as cells went below 2.8V.


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## russatt (Aug 30, 2013)

EVfun said:


> It depends on voltage and current. Charged may be defined as something like 3.60 volts at 0.05C. 3.6 volts at 0.1C would be something less than full according to the manufacturer. That said, there is no harm in undercharging LiFePO4 cells every cycle, just a little less available capacity. I take your idea a lot farther and it has been working well. I charge to 3.42 vpc and hold that voltage for 45 minutes. That is a little less than 0.05C for my cells. They are top balanced and seem to be staying in tight formation.
> 
> If you hold Lithium cells at the peak voltage they will not self equalize. They will continue to decline in current and overcharge, while the continuing current decline pretty much keeps any that are behind from ever charging. You can actually get into a situation where the voltage spread between those cells at a higher SOC and lower SOC is getting larger as you hold the voltage high.


Thanks for the info
What AH are your cells and what current do you start charging at.


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## russatt (Aug 30, 2013)

steelneck said:


> I have a 2.75V bottom-balanced pack and my cells stay within +-0.01V up to 3.43V. If i go beyond that, with so little as 1.5kWh on my 78 cell 100Ah pack, my weak cells (3 of them) will _very_ quickly rush away even beyond 4 volts. So i will stop at 3.43V. But on the other hand i can allow them to go very deep in the other end.
> 
> When i did my bottom balancing of my TS/Winston 100aH cells, i saw the same very quick rush in the other direction as soon as cells went below 2.8V.


Thanks for the post.
How many cycles youd you say you have done on your pack. Any reason for the 3 bad cells.


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## steelneck (Apr 19, 2013)

russatt said:


> Thanks for the post.
> How many cycles youd you say you have done on your pack. Any reason for the 3 bad cells.


Two, or maybe three counting some small testing charges befor i did my bottom balancing. My Ev-conversion is not quite ready yet, even if the car is fully drive able. Have so far only made test drives, testing my 4500W heater system and so on. I was actually quite surprised that the cells stayed so very close to each other under almost the whole charge. And judging from the wall kWh meter, i get in more than 100Ah in my pack even if i stop at 3.43V. At my first full charge from the very bottom to a bit too much (i was not expecting the weak cells to rush that fast once over 3.45V), my little Elcon 1500W had drawn 34kWh from the wall.

I do not think there are any special reason for the weak cells, apart from manufacturing tolerances. This is why it is impossible to have a pack that is both top and bottom balanced.

(*Edit)* If one coses to the most power possible out of the pack, then it is no question about it. Top balance since that will allow for most voltage, though you will be sacrificing the very bottom. But if you live up north and want the most out of the pack in very cold climate at normal driving. Bottom balance to be able to allow much more voltage sag at low SoC. I think think this is a good summary regarding top vs. bottom balancing.


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## EVfun (Mar 14, 2010)

russatt said:


> Thanks for the info
> What AH are your cells and what current do you start charging at.


My pack is made up of 39, 60 amp hour Thundersky cells that where first installed in 2010. I start charging at 12 amps (0.2C.) Right now the controller is turned down because I'm breaking in new brushes, but typically I limit discharge current to 6C (360 amps) and set the controller to not allow the pack under 91 volts (2.33 vpc.)


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## electric_driver (Feb 4, 2017)

The intent of bgeery with this post has obviously been to try to improve battery maintenance so as to increase service life of the (LiFePo4) batteries.

It is clear from this post that keeping the LiFePo4's of never exceeding 40°C is very important to increase service life. In addition, the LiFePo4's should never be allowed to be discharged below 2,8V per cell, and they should be replaced when they have more than 20 mΩ of internal resistance per cell.

I would think that the latter 2 management jobs are probably allready done by most battery management systems.

However, the first one is not being done at all probably. To resolve this, the batteries can be placed in a wooden box, and an opening with either a fan or a Peltier cooler can be placed into one of the sides of the box. The fan or Peltier cooler can then be activated whenever the temperature in the box has exceeded say 25-30°C -while the batteries are being either charged or discharged-.

I think this solution would increase service life of the (LiFePo4) batteries somewhat too, and doesn't require using slower battery charge rates.


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## russatt (Aug 30, 2013)

Hi Guys

I have 2 bad LifePo4 Cells in my pack. They are right in the middle, and in a sealed battery box. I don't have the time at the moment to remove / replace the cells. My pack voltage is always greater than my maximum motor voltage. I cut throttle at 237v on the pack, and motor is limited by controller setting to 150v.

My question is how dangerous is it if I just leave the 2 bad cells in the pack to go to zero volts. Obviously it corrupts the pack voltage readings, but I use my BMS Highest and Lowest cell voltage for my charging and driving.

The Pack is 83 x 60ah cells. Other than range, how much power are these 2 cells pulling from the other cells when driving, and are they dangerous. As in heat, venting, fire ?

Any help will be appreciated.


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## kennybobby (Aug 10, 2012)

Does your BMS monitor each cell?

It could be very dangerous to charge cells that have been pulled too low.

There are too many unknowns to guess or risk your life and that of others.
For the cell to go to zero volts implies an internal short circuit, e.g. copper and aluminum foils in direct contact, possibly melted together. 

For example it is unknown the size of the internal contact patch, what is the resistance and how much current could it carry before it fuses open circuit, in which case you are stranded on the highway, or worse, the car quits just as you turn across traffic and somebody hits you.

From what i've read about cell chemistry, a fire would likely occur during a charging session after the degraded cells have been discharged so low that metal transfer occurred within the cells.


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## russatt (Aug 30, 2013)

Hi.

Yes the BMS monitors individual cells. Some test data to follow. Excuse the spacing.

Thanks for the info. I was hoping that the cell might short, like bridging the cell, but chemically its dead. so still safe.


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## dtbaker (Jan 5, 2008)

russatt said:


> Hi Guys
> 
> I have 2 bad LifePo4 Cells in my pack. They are right in the middle, and in a sealed battery box. I don't have the time at the moment to remove / replace the cells.
> 
> Any help will be appreciated.



you should jumper around the dead cells.... and adjust your charger end of charge voltage. Probably worth doing a manual check on balance of the remaining cells whether you are top or bottom balancing.


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## pm_dawn (Sep 14, 2009)

Hi russat !

I have had the same happen to Thundersky cells.
Make sure to bridge past them before driving or charging.
The cells can get really hot when puching current through them.
That can damage cells next to them.

REgards
/Per


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## russatt (Aug 30, 2013)

electric_driver said:


> The intent of bgeery with this post has obviously been to try to improve battery maintenance so as to increase service life of the (LiFePo4) batteries.
> 
> It is clear from this post that keeping the LiFePo4's of never exceeding 40°C is very important to increase service life. In addition, the LiFePo4's should never be allowed to be discharged below 2,8V per cell, and they should be replaced when they have more than 20 mΩ of internal resistance per cell.
> 
> ...


Hi. What would be the a rough measurement of mΩ on say a brand new cell fully charged, vs a cell that is about 1 year old. Say 100 cycles of use within specs.

I have 2 bad cells, where the voltage is below 2v, and I am going to remove them tonight. While the pack is open I would like check all the cells with a meter.


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## kennybobby (Aug 10, 2012)

Are you thinking to measure the internal resistance of the cells with a meter?


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## russatt (Aug 30, 2013)

Yes. I have a Digital Multimeter - Fluke. It cam read mOhms


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## Tony Bogs (Apr 12, 2014)

Monitoring the performance and health of the battery cells is certainly what I am going to do at regular service internals. Say twice a year.
With a custom made hand-held device based on a high precision "smart" ARM processor, a battery, a couple of mosfets and a small display for the human interface.
Method to be used: pulse charging and discharging. 

It is the same method that am going to use in my charger design for the entire pack.

BMS: got to keep an eye on the temperature of the cells. Extremely important, but that's about it. 
If a cell goes bad, its temperature characteristic will show the signs long before it bursts into flames.

If service life is what this thread is about, there are two major factors that contribute negatively: extreme temperatures and repeated high DOD (> 70 to 80%). 

And don't use the CV part of the CCCV cycle. Buy or build a better charger. 

Just my two pennies as an electrical engineer. My view is based on info from several scientific publications and sources.


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## russatt (Aug 30, 2013)

Hi Tony

What will you be measuring. Internal resistance ? If it is what values indicate the state of the cell


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## kennybobby (Aug 10, 2012)

russatt said:


> Yes. I have a Digital Multimeter - Fluke. It cam read mOhms


i think you will damage your meter if you try to do this. The internal resistance of a battery cell is a theoretical-concept. It is a calculated value based upon voltage and current measured under load and no-load conditions--it is not a true resistance which you could directly measure with a multimeter.


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## russatt (Aug 30, 2013)

pm_dawn said:


> Hi russat !
> 
> I have had the same happen to Thundersky cells.
> Make sure to bridge past them before driving or charging.
> ...


Hi. Im going to bridge past the cells. I need to make a longer buss bar. The one I made before was copper flat bar. 25mm wide and 3mm thick. I have lots of Aluminium flat bar. How does it conduct compared to copper.


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## pm_dawn (Sep 14, 2009)

Hi !

I would suggest you do a good flexible cable instead. 
That way you can bridge one cell much easier. You can probably buy a good starter motor cable in the car parts store with a fixed length and pre crimped terminal lugs.

Something like this.
http://www.ebay.com/itm/2-FT-2-AWG-...802577?hash=item3d376b9591:g:hK8AAOSwU8hY4xp1

Then you can have a flexible bridge.

Regards
/Per


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## Tony Bogs (Apr 12, 2014)

russatt said:


> What will you be measuring. Internal resistance ? If it is what values indicate the state of the cell


Impedance (including DC internal resistance), over-voltage (chemical interactions) and recovery time.

DC internal resistance is an important parameter, as mentioned in an earlier post, for the calculation of the maximum battery voltage at the terminals during charging with a constant current: 
V0 + I * Rinternal,DC. 

IMPORTANT: the internal resistance is not constant. It must be measured by the charger at the start of and during the charging session for safe charging at higher rates.
Actually, the charger measures the total series DC resistance (including cables, lugs etc.). The AC internal resistance is often lower than the DC value.

As kennybobby wrote, the internal resistances are virtual. You can not hook up a milli-ohm meter at the two ends.

Regular measurement of individual cells is used to determine the maximum deviation in the pack from nominal. 
A cell has to be replaced if it shows an excessive deviation. The maximum deviation is an input parameter to the charger.

So I guess the topic title is plausible.


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## Sunking (Aug 10, 2009)

Have not read all replies but measuring Internal Resistance (Ri) is valuable data and on paper is very easy to do. But it is pretty much useless if you do not have a base line to compare against. You need to know what the Ri was when when the battery is new. 

The most accurate way is using Delta Voltage Delta Current calculation. Sounds complicated but very easy to do. All you do is measure voltage and current at a Low Discharge Rate, and a High Discharge rate.

Example say on a 100 AH cell you apply a 1 amp load, does not have to be exactly 1 amp, What is important is you can accurately measure the battery voltage and load current. Record the Cell Voltage directly on the battery Term Post and Cell Load Current, Call the Voltage V1 and Current A1. Example let's say V1 = 3.3 volts, and A1 = 1.1 amps

Repeat the test at a much higher current level say 20 amps. and record the voltage and current. Call them V2 and A2. Example V2 = 3.27 and A2 = 21.3 amps.

Now get pencil and paper and calculate Ri. 

Ri = Dv/ Da
Dv = V1 - V2
Da = A2 - A1

Dv = 3.3 volts = 3.2 - 3.27 = .030 volts
Da = 21.3 - 1.1 = 20.2 amps
Ri = .030 / 20.2 amps = .00148 Ohms. 

To be useful and meaningful the test has to be done at a specific temperature and SOC. Example a cell temp at say 75 degrees and 50% SOC. If not done that way is meaningless because if the temp and SOC are not controlled, the impedance will change. Example a 40 degree cell will read much higher than when it is 75 degrees. 

As the battery ages the Ri will go up. Once it goes up 50% say from .001 phms to .0015, break out the check book as new batteries will be needed soon. Ri has another valuable piece of data, It will tell you the real C-Rate the battery can deliver without significant heating and power loss. Example 10% power loss or Voltage Sag is .32 volts / Ri. Using the above example the battery can deliver up to .32 volts / .00148 ohms = 216 Amps or C2. At 216 amps the cell is being heated with 35 watts. and delivering 713 watts of usable power.


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## Tony Bogs (Apr 12, 2014)

Yeah, the internal DC resistance is dependent on temperature, SoC and age. It is also dependent on the chemistry. 
A recent study of a different chemistry shows that the internal DC resistance stays pretty much constant during charging (increasing SoC) at 25C. 

If the temperature of the cells is below the reference for measurement,
pre-heating the pack must be part of the protocol.

I'm going to measure the internal resistance with a short (few seconds) charging pulse from the charger to the entire pack. 
Discharging a single cell can cause a SoC unbalance in the pack. It is also a waste of energy.
Pulse charging also eliminates the need for a second measurement at a different current. 
It does require the use of a digital scope or similar (logging) device.

And I'd like to emphasize:
In case of absence of a full blown BMS (for instance: only temperature sensors), the charger 
must measure the total series resistance of the pack for safe charging at high rates.


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## russatt (Aug 30, 2013)

Hi Guys. Thanks for all the Info. I am going to add the formula in Sunking's post to my BMS code as I know the constant resistance of my MBS shunts. I believe it to be Kirchhoff's Law.

I have a Digital Multimeter "Fluke" quite a fancy one. I don't know if it has a formula for battery resistance, and actually takes a reading, loads the cell and reads again, but it is all Automatic. It Measured about 2 mΩ for some cells, and 12 mΩ was the highest. It would only measure if I put the Positive Lead, on the Negative Battery terminal and Negative on Positive terminal. Pos to pos and neg to neg read Zero. Also the 2 cells that were dead, ie standing voltage that was less than 1V read > 240 mΩ and would also give a reading Positive lead to positive terminal and - to -, where the ok cells only read one way. I don't know how accurate the reading is but the higher internal resistance of the bad cells was very evident. I managed to bypass the cells by shifting the cells around and putting the 2 bad cells next to each other, and turned their terminals to be parallel, then I just bridged the buss bars across the 2 cells, but only paralleling one of the terminals. I will post a pic to help describe what I did. Quite simple. 

I bypassed the bad cells on the weekend. Quite a Job. Its belly pan off, bumper off, BMS Box out, Accessory Battery tray out, front top battery box out, then the front lower box out where the bad cells were. Front Lower Box weighs over 80kg. 10 minutes of swapping batteries and then assemble all again. Id did 2 test runs about 50km (31miles) each, and all the remaining cells were ok. There are 1 or 2 that seem to be the lowest each time, but they were holding their voltage above 3v so I am happy. 

Thanks again for all the input. It is great to be able to get help when you are making first time decisions. Especially where safety is concerned.


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## russatt (Aug 30, 2013)

Here are some pics of the event. As you can see I had one of my assistants supervising at all times.


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## russatt (Aug 30, 2013)

bgeery said:


> OK, so I'm exaggerating a little bit.  But you are going to get fewer life cycles than specified by the manufacture if used under that standard conditions.
> 
> A fully 100% charged battery has a resting voltage of 3.33 volts per cell at 25 Celsius.
> 
> ...


General Question. After driving your EV. Do you let the cells rest before charging, or just plug it straight in. Is there anything to gain by delaying the charge. ie Longer Battery Life, better balance.


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## dtbaker (Jan 5, 2008)

russatt said:


> General Question. After driving your EV. Do you let the cells rest before charging, or just plug it straight in. Is there anything to gain by delaying the charge. ie Longer Battery Life, better balance.


no resting required.


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## piotrsko (Dec 9, 2007)

Cooler internal battery temps if that matters.


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## EVfun (Mar 14, 2010)

russatt said:


> General Question. After driving your EV. Do you let the cells rest before charging, or just plug it straight in. Is there anything to gain by delaying the charge. ie Longer Battery Life, better balance.


When I have the time I wait 1 hour before charging. My battery pack is 3 rows of 13 cells all together. I designed for a 30 mile range so I can empty my pack in well under 1 hour of driving. I regularly run my LiFePO4 cells up to 5C peak current so the center ones could run warmer than the outside rows.


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