# Lithium Voltage Sag



## MUTHSEV (Jun 9, 2011)

Our 48 Thundersky LiFePO4 100Ah batteries have a top charge of 162 but sag while conservatively driving to around 124 or so. Is this acceptable, or harmful to the pack?


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## dladd (Jun 1, 2011)

lots of unknowns here, (what current are you pulling? are they balanced?), but I'll start with a quick observation that 162v is not charged for 48 cells. You should be bringing them up to more like 168v or so. 162 is 3.38 volts per cell which is still in the flat part of the curve.


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## dtbaker (Jan 5, 2008)

MUTHSEV said:


> Our 48 Thundersky LiFePO4 100Ah batteries have a top charge of 162 but sag while conservatively driving to around 124 or so. Is this acceptable, or harmful to the pack?


if you are only charging to 162v (3.375 vpc) you have no idea how full your cells are as you re stopping 'somewhere' in the very flat part of the curve. The big sag you are seeing is a bad sign that you are probably near 'empty' and run the risk of killing cells.

Your end-of-charge could/should be around 175v with 48 cells (3.65 vpc) with a top balanced pack.


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## tomofreno (Mar 3, 2009)

After a full charge my pack of 36 180Ah CALB cells usually is at around 120V at rest, and sags to about 115V at moderate currents of about 100A or less. At 500A it will sag to about 108 - 110V. So it sounds like yours is sagging more than it should, though hard to say since you don't give any details such as current magnitude, cell capacity, whether the voltage you give is a rest voltage after charge or the end voltage while charging... Have you checked for loose connections?


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## MUTHSEV (Jun 9, 2011)

Sorry for being so vague! The system is at rest at around 162 volts usually, it finishes charge at 174V. Typically, the current draw is 100 amps or less, but on occasion, it can pull up to 700+ amps(not typical). With the low voltage today of 127 volts, the current was 412 amps. The cells are about 7 months old, have not been balanced, and usually the car is driven no more than 20 miles at one time.
We are waiting on our BMS system, and will probably find that there are a cell or two that are damaged(hope not). We may need to top balance before we install the BMS.
Thanks


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## PTCruisin (Nov 19, 2009)

The amount of sag is highly dependent on temperature as well, so the colder it is the more sag you will experience. I have 38 cells and the lowest voltage that I have measured is 100V under approx 500A load during the winter, with a nominal running voltage of 120V. That would be around 2.63vpc.

Do you have a way of measuring individual cell voltages during the charge cycle? I would be concerned that you may be overcharging the lower capacity cells if the pack has not been balanced.


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## dtbaker (Jan 5, 2008)

MUTHSEV said:


> Sorry for being so vague! The system is at rest at around 162 volts usually, it finishes charge at 174V.


oh, that's more 'normal' then



MUTHSEV said:


> The cells are about 7 months old, have not been balanced, and usually the car is driven no more than 20 miles at one time.
> We are waiting on our BMS system, and will probably find that there are a cell or two that are damaged(hope not). We may need to top balance before we install the BMS.
> Thanks


that was a huge mistake. running a normal charge curve, without a BMS, and without top-balance is just asking to fry any cells that were higher.


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## Rational (Nov 26, 2011)

MUTHSEV said:


> The system is at rest at around 162 volts usually
> With the low voltage today of 127 volts, the current was 412 amps.


The total internal cell resistance is then (162v-127v)/412a = 85 milliohms. Is this within manufacturer's specs?


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## MUTHSEV (Jun 9, 2011)

Thanks guys. Any suggestions on how to top balance our 7 month old pack?(Yes, I know there is a search feature) We do have a Mastech HY1803D power supply if each cell needs to be individually charged. Also, we need to check each cell, what should we use for a load tester? When the BMS is installed, it will monitor all cells as they are being charged on a GUI, so we can evaluate the state of charge and internal resistances etc.


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## cruisin (Jun 3, 2009)

MUTHSEV said:


> Thanks guys. Any suggestions on how to top balance our 7 month old pack?(Yes, I know there is a search feature) We do have a Mastech HY1803D power supply if each cell needs to be individually charged. Also, we need to check each cell, what should we use for a load tester? When the BMS is installed, it will monitor all cells as they are being charged on a GUI, so we can evaluate the state of charge and internal resistances etc.


I you are purchasing Mini BMS I can help you with your top balance and what to expect for a sag.


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## EVfun (Mar 14, 2010)

174 at the end of charge and 124 for the lows under load look about right for 48 cells, that is 3.625 volts at the top and 2.583 volts minimum. If these cells are well matched in capacity and state of charge then there should be no harm in these numbers (though some panic and feel lifespan will be shortened below about 2.7 volts under load.)

You are looking at about 16 milliohms for 100 amp hour cells which seems a little high as my 60 amp hour TS cells have about 15 milliohms of resistance. This could easily be caused by winter cold (which my pack doesn't see) so I wouldn't be to concerned, except to keep the current well under 700 amps to prevent excess sag. I try to keep mine above 2.5 vpc. 

I would highly recommend you find out where your pack balance is at -- like *next* cycle! It is easy to do if you check each cell for voltage while charging but near the end of the cycle. See how far they vary, hopefully they are all grouped pretty close together and none are over 4 volts. Check all the cell voltages after a typical drive and make sure they are all grouped pretty close together at the bottom. Lithium cells do not like being overcharged or over-discharged (or overheated, or charged when they are below freezing) so as long as you avoid these things they should live happily. A BMS is optional -- it's just one way to avoid taking a cell to high or to low.


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## dtbaker (Jan 5, 2008)

MUTHSEV said:


> Thanks guys. Any suggestions on how to top balance our 7 month old pack?(Yes, I know there is a search feature) We do have a Mastech HY1803D power supply if each cell needs to be individually charged. Also, we need to check each cell, what should we use for a load tester? When the BMS is installed, it will monitor all cells as they are being charged on a GUI, so we can evaluate the state of charge and internal resistances etc.


Sounds like your first step is to do an initial top balance as soon as possible to get things 'close'. If you install a shunting BMS and they are too far out,the Shunts are usually not set up to handle more than an amp or two.

If your BMS system is a cell level system that can stop the charge when the first cell hits limit, you'll be protected, but not have full capacity becaue all cells won't be 'full'.

you COULD charge each cell individually with everything in place to a set voltage (like 3.80 or somewhere up the knee) with your power supply set to constant voltage (CV) and just leave it on the cell until amps drop to near 0 before moving on to the next cell, and get things pretty close... then do some final balancing after a charge cycle in series.


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## MUTHSEV (Jun 9, 2011)

We recently purchased an Orion BMS and are waiting for it to arrive. Do we need to top balance the cells individually or balance the pack in parallel? 
Since we have been driving the car, with reckless abandon, without a BMS for 7 months, we need to determine the health of our pack and determine if we have any bad cells or not. What is considered a "bad" cell? Our cells have varied voltages ranging from 3.417v to 3.902v under charge, at the end of the charge cycle(174.5v). However, after being driven a few hundred feet to knock down the static voltage, the cells return to a 3.3v-3.4v range.


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## dtbaker (Jan 5, 2008)

MUTHSEV said:


> We recently purchased an Orion BMS and are waiting for it to arrive. Do we need to top balance the cells individually or balance the pack in parallel?
> Since we have been driving the car, with reckless abandon, without a BMS for 7 months, we need to determine the health of our pack and determine if we have any bad cells or not. What is considered a "bad" cell? Our cells have varied voltages ranging from 3.417v to 3.902v under charge, at the end of the charge cycle(174.5v). However, after being driven a few hundred feet to knock down the static voltage, the cells return to a 3.3v-3.4v range.


a cell would be 'bad' if it doesn't take a charge, or has vastly reduced capacity and starts dropping voltage way before the others.

The first issue you'll find out when you attempt to top balance. If they all accept a charge to something like 3.80, and amps drop to zero or close to it, then they are not 'terrible' and should fine for at least while.

The second part.... you won't really know if the cells that have been hitting 3.9+ every charge cycle have been damaged or have lower capacity from internal 'plating' until the voltage starts dropping quicker than the others. you can probably not worry about it unless you intend to push the range way deep; then you'd better have a low-voltage bms in pace, or keep a close eye on pack voltage if it starts taking a dive.

you will get 'better' balancing if the pack is re-wired in parallel and set to target voltage up the knee, and just left to sit until voltages equalize and amps drop to near zero.... but you can get pretty close if you work your way thru cells individually at end of charge so you know which are high, and knock them down a little with a resistor, then run another charge cycle to spot the next-highest.


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## dladd (Jun 1, 2011)

I'm currently in the process of slowly balancing my pack a bit better. At the end of a charge cycle (in my case I terminate my CALB cells at ~3.5v and have a BMS that shunts at that point), I'm measuring the voltages, finding a lowish cell, and putting it on an individual 3.5v power supply till it comes up to the point of shunting. My goal over time is to have all the cells light up the BMS at the same time (more or less). You could alternately knock down a cells voltage with resistors as dtbaker suggested above instead of charging low cells. Or do a combination of both.

Once I have all the cells closer in top balance, I'm going to disconnect the DC/DC and heater loads, and drive down to a deep discharge until I get a single cell to a low voltage condition (with my MiniBMS, the buzzer will remain in a constant on state when a cell is at 2.7v continuous). I can then read the AH used from my controller and determine the capacity (and identity!) of my weakest cell.

A re-measurement of all the voltages at that point should also give me an indication of the relative consistency of all the cells with regards to internal resistance/capacity differences.

I'm a ways away from completing this, as I'm only really boosting one cell at a time when I have the opportunity, but eventually I'll have a better understanding of my battery pack!


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## MUTHSEV (Jun 9, 2011)

Thanks everyone for all of your helpful comments. I have two more questions, how do you determine the charge "knee" of a cell/pack and what is a good way to test the pack under a load(headlight)?


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## dtbaker (Jan 5, 2008)

MUTHSEV said:


> Thanks everyone for all of your helpful comments. I have two more questions, how do you determine the charge "knee" of a cell/pack and what is a good way to test the pack under a load(headlight)?



the manufacturers publish voltage vs DOD for their cells.... you'll see that the knee at both ends is very steep, and the middle very flat, which is why you cannot use voltage to tell where you are with SOC. You have to start either at the top or the bottom where the voltage starts changing perceptably, and count ah via instrumentation like the CycleAnalyst, evDisplay, or other....

load will create some sag in the curve.... basically 'lowering the whole curve'. but hardly at all until you apply more than 1C loads.


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## Rational (Nov 26, 2011)

MUTHSEV said:


> how do you determine the charge "knee" of a cell/pack


I cannot find an answer to that question, for any graph with a knee. 

Measuring a specific voltage is convenient, but, basically, above the knee the voltage change with time [dV/dT] is small and below it is large.

You could use both endpoint voltages that are just outside of the strongly curved region but if no one else does this it's hard to make a comparison with what's on the market. 

You could also have a cutoff circuit that senses a significant increase in a baseline value of dV/dT and triggers on that but that may not be how they do it for consumer products. 

And, the shape and slope of the curves change with the discharge current level, and probably also with temperature.


I'd guess I'd ask battery makers for their battery cutoff decision protocol but they may not say, citing proprietary information concerns.

The trick is to base the cutoff on science and not opinion or a judgement call.


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## major (Apr 4, 2008)

MUTHSEV said:


> Thanks everyone for all of your helpful comments. I have two more questions, how do you determine the charge "knee" of a cell/pack and what is a good way to test the pack under a load(headlight)?


Hi MUT,

You can see the knee on a tested curve like this: 









The battery companies typically publish similar charts. And you can devise your own discharge test method or purchase such equipment. This thread shows some larger equipment I use. http://www.diyelectriccar.com/forums/showthread.php/lithium-battery-monitor-39708.html?highlight=kokam+discharge 

I notice looking at that chart that I made a mistake. Yeah, it happens  I should have used the voltage immediately after applying the 200A load to calculate the internal resistance. So it should read (4.1-3.8) = 0.0015 Ω.

Hope that helps.

major


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## dtbaker (Jan 5, 2008)

Rational said:


> I cannot find an answer to that question,
> I'd guess I'd ask battery makers for their battery cutoff decision protocol but they may not say, citing proprietary information concerns.



the battery mfg absolutely all publish the voltage vs capactiy curves as well as the effects of temp and load. They also recommend 'safe' high and low voltages to use in charge cycle and low-voltage warning...

this is not a mysterious thing, its published.


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## MUTHSEV (Jun 9, 2011)

Hey guys, our BMS will be arriving in about three weeks. Should we charge the pack in parallel at 3.8v? over the holiday break? This could be a good opportunity to top balance our cells before the BMS arrives.


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## dtbaker (Jan 5, 2008)

MUTHSEV said:


> Hey guys, our BMS will be arriving in about three weeks. Should we charge the pack in parallel at 3.8v? over the holiday break? This could be a good opportunity to top balance our cells before the BMS arrives.



thats what I would do! Just set your power supply to 3.75 or 3.80 and let them sit wired in parallel until amps are close to 0... turn off power supply and leave them wired in parallel for a couple more days.


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## dladd (Jun 1, 2011)

MUTHSEV said:


> Hey guys, our BMS will be arriving in about three weeks. Should we charge the pack in parallel at 3.8v? over the holiday break? This could be a good opportunity to top balance our cells before the BMS arrives.


maybe if someone can bring them home to keep an eye on them, but if you mean leaving your batteries on a charger, unattended at school for two weeks while you are on vacation... that sounds like a really bad idea to me!


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## dtbaker (Jan 5, 2008)

dladd said:


> maybe if someone can bring them home to keep an eye on them, but if you mean leaving your batteries on a charger, unattended at school for two weeks while you are on vacation... that sounds like a really bad idea to me!


it probably would be good to check on them every couple days, but *shouldn't* be a problem if the bench power supply is set to constant-voltage some conservative amount over the target vpc the charger will be shooting for.... say .10volt over the target average so that it is most likely the pack will hit the target 'balanced' (making the BMS redundant really). as the pack balances to a fixed voltage below the 'max', but at least part-way up the knee, the amps will drop to zero, so why are you concerned?


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## EVfun (Mar 14, 2010)

LiFePO4 cells should not be floated. Manufactures that allow that typically suggest about 3.40 to 3.45 volts. I would not leave them for an extended period of time at 3.8 volts. Rumor has it that repeated or extended mild overcharging results in increasing internal resistance. 

If the cells are pretty well matched it isn't hard to use the BMS to finish balancing them. That is what it will do in service. If you set the charger voltage to just over the regulation voltage times the number of cells you can watch the cells as they finish. Keep an eye on the voltage of any cells with the red bypass LED on and turn the charger down if needed (or insert some series resistance between it and the pack) to keep any cell from exceeding 3.90 volts. It shouldn't take long for all the cell regs to be lit and the cells very close to each other in voltage. At that point they should be basically balanced with any minor differences taken care of within a few cycles. You shouldn't be doing any range tests for the first 10 or 20 cycles anyway.


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## dtbaker (Jan 5, 2008)

EVfun said:


> I would not leave them for an extended period of time at 3.8 volts....You shouldn't be doing any range tests for the first 10 or 20 cycles anyway.



ok, maybe its not a great idea to let the sit at 3.8 for weeks, but a day shouldn't hurt. I wouldn't recommend 'range testing' top balanced packs EVER if you can help it unless you have a cell level BMS. Much longer life if you stay above 60% or 70% DOD supposedly....


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## dladd (Jun 1, 2011)

dtbaker said:


> it probably would be good to check on them every couple days, but *shouldn't* be a problem if the bench power supply is set to constant-voltage some conservative amount over the target vpc the charger will be shooting for.... say .10volt over the target average so that it is most likely the pack will hit the target 'balanced' (making the BMS redundant really). as the pack balances to a fixed voltage below the 'max', but at least part-way up the knee, the amps will drop to zero, so why are you concerned?


i wasn't so much thinking about damaging the cells due to sitting in parallel on the charger as much as something catastrophic happening. Like a rat running by and bumping the voltage knob up to 5v. 

Not likely, but I'm the guy who unplugs the toaster when we go on vacation...


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## MUTHSEV (Jun 9, 2011)

Sorry, the batteries will not be unattended at all! We will be monitoring them daily. Also, instead of 3.8volts we will be charging the batteries at 3.55v or so. Thank you for you help though!


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## PeterH (Mar 20, 2009)

I'd like to know if you have received your Orion BMS yet. I just tried to order a centralized version of the Mini BMS and my local vendor has suggested I look into the Orion instead.

There doesn't seem to be a lot of experiece with the Orion BMS documented on the diyelectriccar forums.

Have any thoughts on the subject to share?

Thanks,
Peter H.


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## MUTHSEV (Jun 9, 2011)

We have just received our Orion BMS and have not yet installed it. So far from the information we have gathered about the BMS it far exceeds our expectations and should be a very beneficial addition to our MR2. Follow us and our progress on our website and weblog! Thanks


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## bwjunkie (Jul 31, 2013)

I test drove my 144v 45S VW today and had sag down to 120v at 300+ amps when I was driving 50mph up a 3% incline, with brand new batteries on a cold day with the top down, so the batteries were being air cooled to 40deg F . FUN!

I also have an Orion BMS and it's a nice unit with excellent email support, but since it doesn't have an exact profile for my possibly rare-sized 60ah Voltronix batteries, I'm wondering if I should make modifications to the default 40ah TS profile I selected. 

It has the max cell voltage set to 3.65 (is that too low?), so I'm not sure if I'm getting a 95% charge or not.

The excellent news is I drove 20 miles and it still felt strong as I pulled into my garage. Monday I'll be very happy if my appointment/inspection with the CO tech DMV goes well and they clear my car for a E fuel type title. 

Josh


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## dtbaker (Jan 5, 2008)

bwjunkie said:


> I also have an Orion BMS and it's a nice unit with excellent email support, but since it doesn't have an exact profile for my possibly rare-sized 60ah Voltronix batteries, I'm wondering if I should make modifications to the default 40ah TS profile I selected.
> 
> It has the max cell voltage set to 3.65 (is that too low?), so I'm not sure if I'm getting a 95% charge or not.



The voltronix I think are just relabeled Winston cells, so 3.65vpc end of charge is just great.

having different capacity cell (60ah) doesn't change the target voltage. It does change the amount of current the charger may consider 'end of charge'; when held at CV and current drops to C/80 its usually considered 'full' and the charge can end. some curves cut it off even sooner with very little loss of capacity.


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