# Charge Cutoff Voltage



## Sunking (Aug 10, 2009)

OK gang another question. As some of you know I am on the Bottom Balance side of management. For recharging going to use one-step Constant Current until the first cell (weakest cell in bottom balanced application) reaches some voltage. I want the end result so the weakest cell reaches about 90% SOC. I will be charging at roughly C/8 to C/6.

So question is what voltage do I want to stop charging? At the moment 3.7 volts sounds about right.


----------



## twright (Aug 20, 2013)

Most batteries want you to set the cutoff voltage to 3.65 volts. But any voltage above about 3.5 volts doesn't really affect your range much. The voltage of the higher cell is climbing pretty fast at that point and you don't put much more charge into the pack.


----------



## glyndwr1998 (Apr 27, 2013)

I have my lifepo4 set to 3.5v charge cut off.

I got my zivan charger set to a max voltage so that when the whole pack reaches close to target the amps ramp down in stages, at max charger puts out about 14 amps, then as the pack gets close to max, charger ramps down to around 9 amps then to about 5 amps until the first cell hits 3.5v, then charger cuts off.

It did take quite a while making very small adjustments to the voltage pot on the charger to get it to the stage now where it is working quite well.

I also noted that when my max cell voltage was set to 3.65v for a few charges, it was a matter of less that 2 minutes for the cell to go from 3.5v to 3.65v, so I took the opinion for the sake of the extra 2 minutes of charge wasn't worth the risk of loss of life at high volts, so I reduced the charge voltage to 3.5v as in reality there wasn't a grat deal of energy going I after this point anyway.

Been running the pack daily now for about 18 months and haven't seen a real change in battery capacity loss, or haven't lost any cells. I do have 4 cells that are slightly lower capacity than the rest and these are the ones that tend to stop the charger, but I know which ones they are and am monitoring them, but they haven't changed at all since installed.

My pack was purchased as a used pack and in general I am very pleased with them.


----------



## onegreenev (May 18, 2012)

Sounds like you have it dialed in with your charger. Excellent.


----------



## Sunking (Aug 10, 2009)

glyndwr1998 said:


> I have my lifepo4 set to 3.5v charge cut off.


Are you doing this at pack voltage or cell voltage level? In other words monitoring the cells and cut off when the first cell reaches 3.5 volts? Or say 157.5 volts on a 45S 144 volt battery pack?


----------



## glyndwr1998 (Apr 27, 2013)

Yes I took a while making small turns on the potentiometer but it is working rally well now, it does give piece of mind that at the top of the charge there is little current being output from the charger I have read it as low as 2 amps with a dc clamp meter jst before the relay trips it out, so max pack voltage when balanced is very close to charger max set point.

The charger can output 19 amps but my cable was getting warm at 16 amps so I turned down the current pot to around 14 amps which is not far from the lowest setting.
Really pleased with the way it works now.

The zivan is a 3kw charger but as I have ramped down the amps potentiometer it is operating around 2.2kw. Not a problem for me as I'm home most of the time and in no particular rush in most cases.



onegreenev said:


> Sounds like you have it dialed in with your charger. Excellent.


----------



## glyndwr1998 (Apr 27, 2013)

HI Sunking,

The charger gets cut at cell level, so if any cell hits 3.5v the cell logger alarm output triggers and in turn this triggers a signal to an opto coupler that shuts down a relay in the charging circuit.

The charger has been adjusted so that its output is very close to th pack voltage at max charge, if the charge relay should fail and the cells go alittle higher pack voltage would equal charger set max voltage and no current would flow into the pack (in theory anyway), I am hoping I never have to test that theory either lol...
I I should really have another fail safe relay in the line should the one that is fitted fail, it's on the to do list.
When I fit the additional relay I shall also make use of the jld 404 output, and set the jld404 to max pack voltage so when the pack voltage gets to set point on the jld404 this triggers a signal to cut the charge, another fail safe and its on the to do list too, when I get some time to tidy up and finish the job.

I have been holding off hoping that I come across a leaf pack, I can then use that and take out the existing lifepo4 cells and reutilise them In a hybrid solar pv system that I'm currently working on, all to do....... If only there were 48 hours in a day.....




Sunking said:


> Are you doing this at pack voltage or cell voltage level? In other words monitoring the cells and cut off when the first cell reaches 3.5 volts? Or say 157.5 volts on a 45S 144 volt battery pack?


----------



## glyndwr1998 (Apr 27, 2013)

I have 76 cells in series, they are not in perfect balance but close enough and I knkw the whole pack is governed by the 4 Cells I know are the weakest.

Ok, 76 cells at 3.5v, charger set voltage is 266v.

Most of the cells after charge rest and settle around 3.3v and the pack voltage at this time is 250v.


----------



## dougingraham (Jul 26, 2011)

Sunking said:


> So question is what voltage do I want to stop charging? At the moment 3.7 volts sounds about right.


The first cell that pops up over 3.5 volts is where I stop. But I am charging at about 6 amps which is just over C/20 so my cells are a little more charged than yours will be. But then you are wanting to charge only to 90%. I would try stopping when that first cell just starts to climb rapidly. Probably about 3.45 volts. And even at that you might be closer to 95% SOC on that weakest cell.


----------



## Sunking (Aug 10, 2009)

dougingraham said:


> TProbably about 3.45 volts. And even at that you might be closer to 95% SOC on that weakest cell.


THX yeah I know stopping at 90% is a bit of a challenge. So far have not found any graphs or tables that reference SOC for LFP accurately.


----------



## dougingraham (Jul 26, 2011)

Sunking said:


> THX yeah I know stopping at 90% is a bit of a challenge. So far have not found any graphs or tables that reference SOC for LFP accurately.


This is tough to determine. The charge rate and termination voltage will interact. The higher termination voltage will of course impart a higher SOC while a lower current will also impart a higher SOC. And the batteries from different vendors and different capacities from the same vendor can behave differently. I would suggest trying a single cell out at the charge rate you will see at near full and try different termination voltages to get to the SOC you want. Start low and work your way up in voltage. Maybe 3.40 volts to get a baseline number.

I would have to go back and look at it but Jack Rickard shows a 3C charge of a 60AH CALB CA cell where he stopped the charge at 3.6 volts and found the cell was something like 95% charged. This was a 180 amp charge to 3.6 volts. My GBS cells would not be nearly so charged if treated that way. Made me want to switch to the CALB CA cells right then.


----------



## Sunking (Aug 10, 2009)

dougingraham said:


> This is tough to determine. The charge rate and termination voltage will interact. The higher termination voltage will of course impart a higher SOC while a lower current will also impart a higher SOC. And the batteries from different vendors and different capacities from the same vendor can behave differently. I would suggest trying a single cell out at the charge rate you will see at near full and try different termination voltages to get to the SOC you want. Start low and work your way up in voltage. Maybe 3.40 volts to get a baseline number.
> 
> I would have to go back and look at it but Jack Rickard shows a 3C charge of a 60AH CALB CA cell where he stopped the charge at 3.6 volts and found the cell was something like 95% charged. This was a 180 amp charge to 3.6 volts. My GBS cells would not be nearly so charged if treated that way. Made me want to switch to the CALB CA cells right then.


I will be charging at C/8 so I suspect a bit lower on the curve like 3.4.

What I can do is I am also an RC pilot and have a really good hobby charger capable of 1000 watts and can easily charge a single cell 12 to 13 amps. Hum just thought of something. I can discharge a cell with my hobby charger. then charge it at C/8 until I pump in 90 AH and log the voltage when I hit 90 to 95 AH on a 100 AH cell should answer that question.


----------



## JonasFilipe9 (Jan 9, 2015)

Hi guys, 

have you noticed what is the voltage of the cells after they reach 3.6V and left resting for more than 10h?

I'm searching for some signal of damaged cells... Do anyone have and ideia what behaviour may be of a damaged cell?
I charged 8 cells and some of them had different voltages after left resting... some got 3.479V others 3.5V... with 2 consecutive charges I had cells with 3.55V but the voltage drops day by day. I had these values 2 days ago, I'll check them againg tomorrow... I'll post here the table with the values but the conditions of charging where not exactly the same.
This is what I thinkg might be a procedure to find damaged cells: Discharge them one by one to 2.6 or 2.8V and then charge one by one again to 3.6V and them discharge them as a series pack until one reaches 2.8V. This way I'll identify which one has the lowest capacity ans consequently is the weakest... Am I thinking right?


----------



## glyndwr1998 (Apr 27, 2013)

Hi jonas,

From what I have read about lifepo4 cells, a fully charged lifepo4 cell is considered fully charged if resting at 3.4v.

My experience with my own cells is I charge to 3.5v, and most of the cells rest at 3.33v.

Hope this helps.


----------



## JonasFilipe9 (Jan 9, 2015)

glyndwr1998 said:


> Hi jonas,
> 
> From what I have read about lifepo4 cells, a fully charged lifepo4 cell is considered fully charged if resting at 3.4v.
> 
> ...


Thanks a lot. I hope my cells aren't damaged...


----------



## glyndwr1998 (Apr 27, 2013)

also from what i have read, that due to the current manufacturing process, getting the capacity of the cells all the same is nigh on impossible, so the manufacturers will quote cells as having say 100ah capacity, and they will have a minimum of 100ah capacity, but some will have 102ah, some 105ah etc... so what you are probably seeing is some cells having less capacity and being a higher resting voltage as they are indeed "full" and the rest are nearly full,

hope i have explained it ok as best i can




JonasFilipe9 said:


> Thanks a lot. I hope my cells aren't damaged...


----------



## Sunking (Aug 10, 2009)

glyndwr1998 said:


> also from what i have read, that due to the current manufacturing process, getting the capacity of the cells all the same is nigh on impossible, so the manufacturers will quote cells as having say 100ah capacity, and they will have a minimum of 100ah capacity, but some will have 102ah, some 105ah etc... so what you are probably seeing is some cells having less capacity and being a higher resting voltage as they are indeed "full" and the rest are nearly full,
> 
> hope i have explained it ok as best i can


When you bottom balance the vell voltages will only be equal at 0% SOC. Only the weakest cell will ever approach 100% SOC voltage. The rest will be slightly lower, but all cells will have the same AH capacity at any point. So if you weakest cell is say 101 AH and all the rest are 110 AH, none will ever have more than 101 AH.


----------



## JonasFilipe9 (Jan 9, 2015)

I think you're both right for what I've studied. This video think that ilustrates simply the two precesses: https://www.youtube.com/watch?v=U7-WHUTLbHY

This is a post I should post on the Top vs Bottom balancing thread but anyway... I won't discuss with is better in this thread.


----------



## mizlplix (May 1, 2011)

I gave up on bottom balancing because in order for it to work you needed to predict your driving miles and be near a charge station or home at that point. (I have found that 3.10 Volts per cell is pretty much exhausted and there is about 2 miles or so on the pack at that point)

My BMS top balances, so I got used to that. I just drove the car once to 3.10 volts per cell and remembered the total miles. I now zero my speedo at charge time and I am good to go. I just know my total safe miles and live within that.

Once per year, I use a small RC cell charger and max out each cell at 3.58 per cell. Over night they settle down to 3.4 per cell and are ready for another year. (I also retorque the straps and other DC connections.)

Miz


----------



## JonasFilipe9 (Jan 9, 2015)

Thanks for your contribution miz. That's interesting.

Did you stayed walk ever? 

So you basically let the batteries go just to 3.1V and charge them again at that point. What is the BMS you are using and what bypass voltage do you use to perform the charging?

I left my cells resting connected in series during the weekend. Tomorrow I'll see what are the voltages they stabilized...


----------



## mizlplix (May 1, 2011)

J: It is best to leave the cells in parallel. I did for a week. Yes, it got them really close. But after I placed them in series and put a full charge cycle on them, they were .027/volt split. (Not close enough for a good pack)

I bought a small 2 amp RC lithium charger and while I worked on the rest of the car, charged each one separately. Then installed a "Mini BMS" from 

http://www.cleanpowerauto.com/MiniBMS.html

I chose the distributed system. It is the cheapest/best system I found at that time. Since then there are others.

It has a cell board on each cell and a headboard under the dashboard of the car. There is a single wire loop from the headboard and thru all of the cell boards then back to the headboard again. (The alarm loop)

I choose the cell boards that had a bleed down resistor to give a tiny top end balance each charge cycle. It helps keep the cells in better balance.

The cell cut off point is built unto the cell boards and is not adjustable on my model. The alarm loop also monitors the low end voltage and has an audible alarm when any cell gets toward "empty" when driving.

It has one last feature, it can be set to cut off the charger too. Some require a relay, mine did not (an Elcon). I set my Elcon to charge one cell count larger than my pack is and allow the BMS to shut it off. Even if the BMS fails, the Elcon will still shut off (and not be a lethal overcharge).

Set thus, my system shuts off at 3.58 volts per cell and is 3.4 volts per cell after setting a while. (There is not much power below 3.10 or above 3.40 VPC)

I have never been stranded...so far. Just as you routinely know how far your car goes on a tank of fuel....I know how far on a charge. Exactly the same thing. (People freak out for nothing)

NOTE: I drove the car for a year at first with no BMS. My range kept getting shorter and required me to manually balance the odd cells every 6 months......(a real boring deal)

Miz


----------



## JonasFilipe9 (Jan 9, 2015)

mizlplix said:


> J: It is best to leave the cells in parallel. I did for a week. Yes, it got them really close. But after I placed them in series and put a full charge cycle on them, they were .027/volt split. (Not close enough for a good pack)
> 
> I bought a small 2 amp RC lithium charger and while I worked on the rest of the car, charged each one separately. Then installed a "Mini BMS"
> Miz


Oh, ok, firstly you charged them in series without a BMS to make the balancing... This split you're saying was the resting voltage after charging? I think I can go over this because the BMS makes top balancing...

I tried to charge 4 cells in parallel







, but the final voltage was not 3.6V don't know why, so I decided to charge them one by one. 

I've charged them separatly with a power source to 3.6V. The current on the power source reaches 0.05A at the final stage.

After this, the cells had different resting voltages varying from 3.464 to 3.55V.
Do you suggest to connect them in parallel, let them have the same voltage, assembly the pack and perform some charge/discharge cycles? (1)
Or I may assembly the pack, even with different voltages, install the BMS Boards on and set the by-pass voltage to 3.45V for example. This way all the cells will have 3.45V before performing a discharge as a pack? (2)

The only thing I'm worried with is that the weakest cell will discharge much faster than the others (due to less capacity)(this might occur because I think one or two cells had voltages lower than 2.5V for some time).
How do I identify eventually the weakest or damaged cell?


----------



## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> The only thing I'm worried with is that the weakest cell will discharge much faster than the others (due to less capacity)(this might occur because I think one or two cells had voltages lower than 2.5V for some time). How do I identify eventually the weakest or damaged cell?


The pack AH capacity is dictated by the weakest cell. If you have a 80 AH cell in a string of 100 AH cells, you only have a 80 AH cell. If you top balance the chances are very high you will destroy the 80 AH cell by over discharging it and driving it into reverse polarity. 

If you top balance you find the weakest or bad cell during the discharge cycle as it will have the lowest voltage once you get near the bottom. It will be the cell(s) that force your BMS to disengage the drive system, and flag an alarm. 

In a bottom balanced pack the weakest cell is the first one that gets fully charged in a short period of time as compared to the others.


----------



## JonasFilipe9 (Jan 9, 2015)

I see. The behavior chances depending on the type of balancing... 

But when a BMS balances the cells so all have 3,41V for example, may this be considered a top balancing? I'm a little confused because regarding the video I've posted http://youtu.be/U7-WHUTLbHY
This is not a top balancing. Top balancing should perform a charge so all cells reach 3.6V.

Although he's representing SOC with the cups, we have to admit SOC and voltages are related.


----------



## mizlplix (May 1, 2011)

1- on receipt of cell shipment, I lined them up on the floor and tied them in PARALLEL. Left them for a week. All cells were then around 3.33 volts.
(Yes, there was some balancing going on as they were closer together than when I got them out of the box.)

2-After I installed them in the car in SERIES, I gave them their first charge.
Low to high, they were .027 volts apart. 

3- I then used a small charger to charge them separately, one at a time.
(Getting them really close)

4- Drove the car for 6 months. Checked the pack and they had drifted apart again. Charged the pack then brought the low cells up to match. 

5- After two times of that nonsense, I installed a BMS and 2 years later am a happy driver.

Please note: If all of your cells are factory tagged and the capacities are really close together, top balancing works fine. I am 3.5 years down that road without any complaints. If you buy cells used or from dodgy dealers, you will not be happy as they might be from mixed lots, or he picked all the uniform ones and you got the mavericks.......

Miz


----------



## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> Although he's representing SOC with the cups, we have to admit SOC and voltages are related.


Cups, sticks, or whatever floats your boat. SOC or State of Charge is a voltage representing the percentage of charge the battery has. It does not tell you anything about the capacity of the cell. It is merely a reference point. Top Balance is made at 100%, and Bottom Balance is at or near 0%. When balanced either at the top or bottom the cells are only equal voltage at their respected reference point at either 0% or 100%.

At 100% SOC voltage we have no idea of what the actual capacity really is. A 100 AH cell maybe as little as 100 AH or as much as 120 AH. All we know is the weakest cell dictates the capacity of the whole battery system. You can throw a 80 AH cell into a string of 100 AH cells and the pack capacity will be only 80 AH. 

However at bottom balance we know exactly what the capacity is of each cell of being 0% SOC. Going back to the example of a 80 AH cell in a 100 AH pack, we now charge until the 80 AH records 100% SOC and terminate the charge. The 80 AH cell reads 100% SOC of say 3.5 volts at rest, but all the other 100 AH cells only read 80% SOC of say 3.35 volts. That is exactly what we expect. However all cells in the pack have the exact same 80 AH in them. As we discharge as we approach 0% SOC is when the voltages will equalize because that was the reference point. 

If you topped balanced all cells would be at 100% SOC voltages. The danger is as you discharge there is a very good chance of taking the weak 80 AH cell to 0%, while all the other cells still have 20% SOC and will drive the weak cell into reversal and destroy it. If you bottom balance you eliminate that risk. 

Bottom line for me is this. LFP batteries are very expensive. I want as many cycles as I can get out of them. To do that I only charge to 90% on the weakest cell, and disconnect at 10%. That should give me roughly 50% more cycle life and never risk destroying a cell from over charging or over discharging. Bottom Balance is less expensive and less complicated to implement with fewer failure points. To me that is SMART MONEY. Companies and salesman may not like that approach, but I do not care what they like or not. As an engineer I can sleep at night with more money in my pocket and knowing my batteries are OK.


----------



## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> However at bottom balance we know exactly what the capacity is of each cell of being 0% SOC. Going back to the example of a 80 AH cell in a 100 AH pack, we now charge until the 80 AH records 100% SOC and terminate the charge.
> 
> If you topped balanced all cells would be at 100% SOC voltages. The danger is as you discharge there is a very good chance of taking the weak 80 AH cell to 0%, while all the other cells still have 20% SOC and will drive the weak cell into reversal and destroy it. If you bottom balance you eliminate that risk.


Isn't it basically the same in terms of control for both cases? To know when the weakest cell is fully charged, you need to monitor the voltage of each cell to know when it reaches the maximum voltage.
Same thing happens for Top balancing but the difference is you need to monitor when it reaches minimum voltage...

If you need to drive after the batteries reach minimum levels, with bottom balancing you will burn all cells, with top balancing you would burn probably just one or two cells. I see an advantage there to top balancing.


----------



## JonasFilipe9 (Jan 9, 2015)

mizlplix said:


> 3- I then used a small charger to charge them separately, one at a time.
> (Getting them really close)
> Miz


My cells are not very close after the individual charging. But I'll check them tomorrow because I measured the voltages of some cells just one day after the charging. They should be more uniform now after some days.

If the cells still have different voltages should I connect them in parallel before discharging, then right?


----------



## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> Isn't it basically the same in terms of control for both cases? To know when the weakest cell is fully charged, you need to monitor the voltage of each cell to know when it reaches the maximum voltage.
> Same thing happens for Top balancing but the difference is you need to monitor when it reaches minimum voltage...
> 
> If you need to drive after the batteries reach minimum levels, with bottom balancing you will burn all cells, with top balancing you would burn probably just one or two cells. I see an advantage there to top balancing.


Ho do you properly Top Balance? Most likely you just said with Vampire Boards right? How many cells? I use 16 for 48 volts, and I would need 16 Vampire Boards or one for each cell. I have a constant 20 amp charger. If I use the Vampire boards as soon as the first cell reaches 3.6 volts I would have to change the 20 amp control to match the Vampire Board bypass current of 1 amp or less. If I fail to do that I am now over charging the crap out of a fully charged cell. Second cell now reaches 3.6 and now he is being over charge. The Vampire Boards can only bypass 1 amp or less, but the other 19 amps are still flowing through a fully charged cells and heating up fast. It will continue to do so until the last cell reaches 100%. So now I have 15 over charged cells and one happy cell. 

Why on earth would I want to do that? My weak cell is 90 AH and the strongest is 120 AH. I can only use 90 AH. If for some reason I go beyond 90 AH, I start destroying cells. The other 15 cells don't know what is going on.

That is not how I want to operate. I want to reference 0% so all cells have equal capacity, not equal voltage at the TOP. The only place they will be equal is at 2.5 volts or 0% capacity remaining. That gives me 3 fail safes. 

First line of defense is the LVD in the cell monitor. When any one cell reaches 2.5 volts for more then 15 seconds, th eLVD operates, lights go out.

Second line of defense if the LVD circuit fails is the motor controller detects 40 volts and shuts down. If I bottom balanced is 2.5 vpc on a 16S pack.

Last line of defense is the batteries being bottom balanced they all arrive at 2.5 volts at the same time and cannot drive any cell into reverse polarity. The cart just comes to a stop and will not go any more. 

To charge I just pump 20 amps in until the first cell reaches 3.5 volts and the charger turns off. No Vampire Boards to suck power and cause an imbalance. No cell gets over charged and every cell has the same capacity. Much cheaper and simpler to implement and a lot less eggs in the basket to crack like 16 unnecessary Vampire Boards sucking the life out of my expensive batteries. Vampire Boards have two failure modes, neither one is pretty.

If it fails shorted, you destroy the cell. If it fails open you risk over charging the crap out of that cell and possible every other cell because the charger never shuts off. 

So for me I like the KISS approach. It saves me a lot of money and extends the battery life.


----------



## mizlplix (May 1, 2011)

My only real world bitch about bottom balancing is that how do you know exactly when you are going to be at 3.10 VPC? My car had 2 miles in it at that point. I was 4 miles from home. I had to walk, get a friend and a rope to tow it to home shop. (Where all of my tools were.)

Then evened out each cell with a load tester, Then put a full 8Hr charge.
Know what? They looked the same as if I had top balanced the pack.

Like I said, to each his own.

Miz


----------



## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> The Vampire Boards can only bypass 1 amp or less, but the other 19 amps are still flowing through a fully charged cells and heating up fast. It will continue to do so until the last cell reaches 100%. So now I have 15 over charged cells and one happy cell.


Yes, Vampire Boards. 8cells. Oh gosh this is so obvious... This is what makes it impossible to balance voltage cells to 3.5V for example with this boards of 1A by-pass current. I was wondering why I could just do the balance to 3.41 or 3.42V with the vampire boards. Didn't think of the effect that the remaining current will have on the charge cell.



Sunking said:


> That is not how I want to operate. I want to reference 0% so all cells have equal capacity, not equal voltage at the TOP. The only place they will be equal is at 2.5 volts or 0% capacity remaining. That gives me 3 fail safes.
> 
> First line of defense is the LVD in the cell monitor. When any one cell reaches 2.5 volts for more then 15 seconds, th eLVD operates, lights go out.
> 
> ...


Sunking, thanks a lot for this post, really I'll pay you a bounch of beers! You touch crucial points here and with your explanation (that I think is more than correct) I may solve a lot of problems I was dealing with! But I think the vampire boards are still a good feature to have, not to make the top balance cause that is what they don't do, but to make the bottom!  You set the by-pass current to 2.5V and the boards will dissipate the energy of the cells till that point and voilá! bottom balance anytime you want and performed by any user...

Bottom balance won one more fan 
I will perform it tomorrow morning rightaway!


----------



## JonasFilipe9 (Jan 9, 2015)

mizlplix said:


> My only real world bitch about bottom balancing is that how do you know exactly when you are going to be at 3.10 VPC?
> 
> Know what? They looked the same as if I had top balanced the pack.
> Miz


why do you want to know when the cells are 3.1V? Oh you want to switch off at that voltage because there is not much energy from 3.1 to 2.5V right?

Well, assuming what Sun say, if the voltages are all the same only at the bottom, when you charge the pack the voltages of each cell will be different.
But don't you have a way to switch of when the boards see a certain voltage in one cell? LVD should be programmable...


----------



## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> Yes, Vampire Boards. 8cells. Oh gosh this is so obvious... This is what makes it impossible to balance voltage cells to 3.5V for example with this boards of 1A by-pass current. I was wondering why I could just do the balance to 3.41 or 3.42V with the vampire boards. Didn't think of the effect that the remaining current will have on the charge cell.
> 
> 
> 
> Sunking, thanks a lot for this post, really I'll pay you a bounch of beers! You touch crucial points here and with your explanation (that I think is more than correct) I may solve a lot of problems I was dealing with! But I think the vampire boards are still a good feature to have, not to make the top balance cause that is what they don't do, but to make the bottom!  You set the by-pass current to 2.5V and the boards will dissipate the energy of the cells till that point and voilá! bottom balance anytime you want and performed by any user...


You are welcome, but if you Bottom Balance, the Vampire Boards are of no use to you. They cannot Bottom Balance as designed. They are designed to turn on at a specified volt like 3.5 or 3.6 volts, then turn off after the charger turns off and they bleed the cell down to a cut-off voltage. 

To Bottom Balance is only done once. It must be done with great precision. Easiest way is remove the cells and connect them in parallel. Then with a load device like a power reistor or something that can draw say 5 amps at 3 volts and bleed the cells down to 2.5 volts. Let them rest for an hours, recheck voltage, and repeat process until when at rest the batteries hold 2.4 to 2.5 volts. 

Then put them back in and from there you just need a BMS control unit that is capable of monitor cell voltages, and can send a signal to either a charger to shut off the charger when the first cell hits say 3.5 volts, and another signal to a LVD to operate when the first cell reaches say 2.5 volts for more than 15 seconds. 

Vampire boards are parasitic loads. Each draws a slightly different amount of idle current. Over time that is what causes cell voltages to vary. Get rid of them. At a bare minimum all you have to do is monitor cell voltages. Additionally monitoring temperatures is of some benefit. Personally I do not monitor cell temperatures, just the pack temperature. In my application, a NEV and Golf Cart I do not make any runs greater than a few minutes. Longest distance I may go is from the house to the golf clubhouse of about 2 miles or a 3 to 4 minute ride. I charge at C/3 so no real heating issues on the charge side which does not occur until over charge which I should never experience. 

Take what you like, leave the rest.


----------



## mizlplix (May 1, 2011)

At this point I think I will take my exit from this thread. 

You guys really crack me up. LOL Everything in the system is parasitic. The fuel gauge sensor, the charge monitor board, even the main motor for all that. The question is "Can you live with it?"

The fact remains that I (and many others) have a system that works. Calling them cute little names fails to change that fact.

Miz


----------



## JonasFilipe9 (Jan 9, 2015)

Yes I know, but the vampire boards active the bleeding resistor at the by pass current you set. If I for instance set to 2.5V aren't the boards precise enough to do this discharge to 2.5V? I know it would take a while but at least is easier. Do they unbalance the cells so much? We need the boards always to ensure a good lifetime...

Ok miz wait. You said you drove for 1 year and need to balance every 6 months that's not very good for the application I want... 
Do you do balancing every charging cycle? What is the by pass voltage you have?
With top balance you'll have a full cell, reaching 3.6V and the others are not... (If you set by pass voltage to 3.5 for example you'll never have all cells balanced at that voltage) 
Unless you set the by pass voltage to 3.41/3.42...

I have a little problem with different cell voltages at the low end.. They should be very similar at this point and with top balance that doesn't happen right?


----------



## JonasFilipe9 (Jan 9, 2015)

Hi guys,

These are the values after the individual cells charge:

Cell Nº	Initial voltage after individual charge (V)
1 3,478
2 3,528
3 3,498
4 3,523
5 3,511
6 3,51
7 3,527
8 3,519
9 3,515
10 3,469
11 3,504
12 3,522

I'm using a 24V bank from cell 5 to 12. So I have 4 backup cells.
I think I'm going to susbtitute cell nº10 by cell n2, and cell nº11 by cell nº4.

Then I'll perform a discharge with a 3.3Ohm resistor until I reach 2.53V in one cell. (0.03V of safety voltage).

But with top balancing we just charge one cell to the top...


----------



## JonasFilipe9 (Jan 9, 2015)

glyndwr1998 said:


> I have my lifepo4 set to 3.5v charge cut off.
> 
> 
> 
> ...


Hy glind,

So you set the cut off voltage o 3.5V. But you do charging until that voltage always? Or you charge to a by pass voltage ?

It is very difficult to make all Cells 3.45V for example because of what sunking said, in order to balance all the cells to that voltage the current going throw he pack should be like 1A what never happens... (With 9A going throw the pack and a 1A vampire board, there are still 8A going throw the cell) 
So top balance at a certain voltage never occurs, or am I thinking wrong?


----------



## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> Yes I know, but the vampire boards active the bleeding resistor at the by pass current you set. If I for instance set to 2.5V aren't the boards precise enough to do this discharge to 2.5V?


Unless you have or know of a product I am not aware of Vampire Boards are fixed to turn on at 3.5, and turns off when the charger is turned off. You cannot change that to stay on until they discharge to 2.5 volts as that defeats the purpose of them.

Secondly the discharge current or bypass current is also fixed. Units I have seen range from .2 to 1 amps. They turn on when the cell voltage reaches 3.5 to 3.6 volts that the manufacture has built into them. But here is the challenge with them. If you have say a 20 amp charger, the first Vampire Board turns of and diverts its .2 to 1 amp leaving 19 amps still flowing through the fully charged cell. You would be over charging the crap out of that cell, and any cells that the Vampire boards have turned on. You keep applying 20 amps until the last cell reaches 3.6 volts

To really make the system work the Vampire Boards would have to communicate with the charger, so that when the very first Vampire Board Turns On, signal the charger to limit the current to the level of .2 to 1 amp the Vampire Board can safely bypass the cell so that cell stops charging. If that does not happen every cell except 1, the last one that turns on, gets over charged. 

IMHO if you TOP BALANCE with Vampire boards and BMS you are paying a lot of money (BMS and Vampire Boards cost a lot of money) to destroy your batteries automatically and faster reliving you the chore of doing it yourself. 

I prefer the less expensive safe way using Bottom Balance one time only, and a cell monitor.


----------



## glyndwr1998 (Apr 27, 2013)

Hi Jonas,

My charger cuts when any cell hits 3.5v. I balance manually by either adding charge or removing energy from individual cells when they drift too far from what i want, so all cells are close together not perfect but close enough for my needs



JonasFilipe9 said:


> Hy glind,
> 
> So you set the cut off voltage o 3.5V. But you do charging until that voltage always? Or you charge to a by pass voltage ?
> 
> ...


----------



## Sunking (Aug 10, 2009)

glyndwr1998 said:


> Hi Jonas,
> 
> My charger cuts when any cell hits 3.5v. I balance manually by either adding charge or removing energy from individual cells when they drift too far from what i want,


Why are your cells drifting? They should not drift. Only way it can do that is if you have Parasitic Loads.


----------



## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> Unless you have or know of a product I am not aware of Vampire Boards are fixed to turn on at 3.5, and turns off when the charger is turned off. You cannot change that to stay on until they discharge to 2.5 volts as that defeats the purpose of them.
> 
> Secondly the discharge current or bypass current is also fixed. Units I have seen range from .2 to 1 amps. They turn on when the cell voltage reaches 3.5 to 3.6 volts that the manufacture has built into them. But here is the challenge with them. If you have say a 20 amp charger, the first Vampire Board turns of and diverts its .2 to 1 amp leaving 19 amps still flowing through the fully charged cell. You would be over charging the crap out of that cell, and any cells that the Vampire boards have turned on. You keep applying 20 amps until the last cell reaches 3.6 volts
> 
> ...



1) SUNKING, I can set the by-pass voltage I want with BMS123 😜

2) its by pass current is 1A so you're right. Conclusion: it is impossible to balance cells at 3.5V, unless that charger works as you described. An BMS123 uses a Elcon charger to charge the pack. This charger varies the current from 2 to 6A with the balancing algorithm. But It always happen: there is alway one cell that rises the voltage quickly when the current reaches 6A if you set bypass voltage to 3.5V.

But if I set the by pass voltage to 3.4 as the manufacturer advices the cells will all have 3.4V. This is top balance. But with top balance there is never one cell reaching Vmax= 3.6V, the chargers stops earlier.


----------



## JonasFilipe9 (Jan 9, 2015)

Btw, I thought there was a thread about top vs bottom balancing or did the admin delete it?


----------



## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> Why are your cells drifting? They should not drift. Only way it can do that is if you have Parasitic Loads.



He bought used cells... This may not help...


----------



## JonasFilipe9 (Jan 9, 2015)

Here is the charging current of the Elcon: https://www.dropbox.com/s/s2mnge61aicti2q/2014-09-30 17.05.09.mov?dl=0


----------



## glyndwr1998 (Apr 27, 2013)

Hi Sunking,

the cells are slightly drifting due to the monitoring system i have.

I use cell loggers to monitor my whole pack, i have 11 of them, each cell logger is monitoring 7 cells.
From what i have read, the cell logegrs have been known to unbalance packs, albeit very slowly, which is waht i am seeing.

It is a disadvantage this way, as i may have to spend maybe a few hours one afernoon every 3 months with a 50 watt resistor draining some of the higher cells a few amps.
I really dont mind doing this though, at least i can then contro how much i drain them and for how long.
Advantage, it is not auto balancing and id didnt cost a small fortune to install.
For a full blown bms for 76 cells i would have probably been looking at spending maybe $2000 on a bms system, something i didnt want to spend that kind of money on, so decided a manual approach and cell monitoring rather cell management.

As for the cells being used has nothing at all to do with it, the cells are in great shape to be honest and have been very relaible, in terms of cost these cells cost me less than $1100, for over 100 cells, thats about $10 per cell, at that price for lifepo4 it was a bargain, and after they have finished working in the car they will be put ot work recovering energy from my solar pv system.

When i purhased them, i tested 3 cells at ransom, and from 2.8v they all took 32ah energy, so were nigh on as good as new.

I am really happy with them.




Sunking said:


> Why are your cells drifting? They should not drift. Only way it can do that is if you have Parasitic Loads.


----------



## Sunking (Aug 10, 2009)

glyndwr1998 said:


> Hi Sunking,
> 
> the cells are slightly drifting due to the monitoring system i have.
> 
> ...


That is know as Parasitic loads. The result is each cell monitor point does not draw the exact same amount of current, and over time will cause some drift. Its normal and expected. 

FWIW if you want some precision and automation you might consider consider buying a RC Model hobby charger like:

ICharger 106B from Buddy RC 

Or

Cell Pro Multi4 available from Progressive RC

Both of these units can charge, discharge, cycle, measure capacity of any battery and any future battery. They are bidirectional meaning you can take a charge from one battery and put it into another battery within a voltage range up or down. You can top or bottom balance. 

I used mine, an larger model because I do have RC planes to initially Bottom Balance my pack on my NEV. I connected all the cells in parallel, set the discharge to 20 amps at 2.5 volts finish voltage. Had to do it twice. Second time used 2.4 volts finish voltage. After letting them rest overnight they settled on 2.46 volts.

I have only had the NEV in operation for a couple of months now and have not experienced any meaningful drift. I do use a cell monitor, Orion Jr, to control my CC charger to turn it off when the first cell reaches 3.45 volts, and another control to LVD relay is any cell reaches 2.5 volts for more than 2.5 volts. I have never had the LVD operate and I do let my pack get down fairly low before recharging about once a week or as needed. 

In the event I need to do a little Balance, I will use the hobby charger to either bleed/charge a high/low cell. 

Like you I do not see a need to do expensive Top Balancing with BMS and risk an over charge or cell reversal. I sleep better at night with extra money in my pocket and knowing my expensive batteries will not be harmed from abuse and/or failures in a Vampire Board.


----------



## glyndwr1998 (Apr 27, 2013)

Hi sunking,

You and me are on the same thought process, 

I already have an imax hobby charger, also purchased a meanwell 3.3v psu, as it was cheap on eBay works really well though to boost a charge into one cell at a time.

Yes, I also sleep abit better knowing that I haven't got an active bms shunting away at the top with the possibilty of overcharge as the shunt can't keep pace with the current injected into the cell.

I did weigh up a ful blown bms but decided against it due to the cost, plus spent a few hours reading and watching jack Rickard video blogs and the harm that some bms cause rather than save battery.
I know I have to spend some time manually balancing some cells at some time, but I can put up with that and I have the $2000 still in my pocket too


----------



## Sunking (Aug 10, 2009)

glyndwr1998 said:


> Hi sunking,
> 
> You and me are on the same thought process,
> 
> I already have an imax hobby charger, also purchased a meanwell 3.3v psu, as it was cheap on eBay works really well though to boost a charge into one cell at a time.


If you want that charger to be a little more functional you might consider getting a 12 to 24 volt power supply capable of say 1000 watts. Not familiar with your charger and what the input requirements are. 

In addition to being a professional EE I have a few hobbies which include ham radio operator so I have some larger 12 and 24 volt bench supplies I use. 

However for dirt cheap you can pick up a PC ATX power supply units on Ebay that have 24 and 12 volt rails that can be easily converted to power your charger. 

I use mine for everything like power tools of any voltage and battery type. I just cut the guts out of the tools base receiver, and put a 3.5 mm jack in to connect to my charger. Set the voltage and profile and charged in an hour. There is no battery it cannot charge, discharge, measure Ri, measure capacity, monitor, temperature, or cycle under 40 volts.


----------

