# Charger for single cell (balancing)



## polodiy (Oct 25, 2015)

Assembling 8s 100ah pack. Using minibms and cell log 8s combo.

Upon first charge realized that pack is a bit unbalanced.
Can i use 6amp 24v charger to charge single cells a little bit to catch up with higher charger ones?

It will work as 48amp for single cell. I will be next to cells/charger and remove it as soon as cell reaches shunting phase.


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## Karter2 (Nov 17, 2011)

A better idea would be to individually discharge the higher cells one at a time until the (static) voltages match the lowest cell...and then recharge the pack.


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## polodiy (Oct 25, 2015)

Karter2, thank you for your reply.

What can i use to discharge cells? Don't have equipment for that. 
And what are the cons of charging with 24v 6amp charger?


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## dougingraham (Jul 26, 2011)

polodiy said:


> Can i use 6amp 24v charger to charge single cells a little bit to catch up with higher charger ones?
> 
> It will work as 48amp for single cell. I will be next to cells/charger and remove it as soon as cell reaches shunting phase.


Typically a charger configured for 24 volts would not be happy at all if you tried to drop the output voltage to around 3.5 volts. There aren't many topologies of power supplies that would do 48 amps when used at 1/8 the voltage. Other limitations come into play apart from the pure watts in vs watts out calculation. I would suggest you get one of the inexpensive RC charger/discharger and use that to charge and discharge your single cells. These can do capacity testing and are remarkably useful for pretty much all your battery needs.


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## polodiy (Oct 25, 2015)

dougingraham said:


> There aren't many topologies of power supplies that would do 48 amps when used at 1/8 the voltage. Other limitations come into play apart from the pure watts in vs watts out calculation. I would suggest you get one of the inexpensive RC charger/discharger and use that to charge and discharge your single cells. These can do capacity testing and are remarkably useful for pretty much all your battery needs.


dougingraham, thank you for your reply. Could you explain more detailed plz?
I was assuming that 6amp charger designed for 28v pack, will work as 48amp charger when connected to 3.5 cell. Or i am wrong here?


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## Sunking (Aug 10, 2009)

Use your 24 volt supply, get you a Cellpro Powerlab 8, *connect all cells in parallel*, and charge them to 3.6 volts. If you are in a hurry, connect them in Parallel, walk away for a few hours, reconnect them in series, and use the Power Lab 8 to Balance charge them. At 8S if your 24 volt charger can output 50 amps or C/2 charge rate. Your limit will be your charger. 

You cannot use a BMS to do the initial Balance. They are only designed to maintain a Balanced pack. Frankly once you know the capabilities of the Cellpro Power Lab 8, you will toss your BMS in the trash can, and use the Cellpro to do all charging, testing, and monitoring.


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## Sunking (Aug 10, 2009)

polodiy said:


> I was assuming that 6amp charger designed for 28v pack, will work as 48amp charger when connected to 3.5 cell. Or i am wrong here?


Dead wrong. A Charger is not a DC-DC Converter. That is what the Cellpro Power Lab 8 can do. If you connect a 24 volt 6 amp charger to a 3.6 volt cell you will let the Smoke out of the Charger, or it wil go into Current Limit of 6 amps and fold back the voltage to 3 volts. Most likely let the smoke out. 

A DC-DC Converter are Power Converters with limits. Example if you have a 1000 watt supply say 36 volts @ 28 amps, will charge a 3.6 Volt battery up to 1000 watts / 3.6 volts = 277 amps. Same Converter would charge a 24 volt battery at 1000 watts / 24 volt = 42 amps. 
Th eCellpro Power Lab 8 is a RC Hobby Charger aka Power Converter. It has limits. Maximum input is 1344 watts from 26 to 36 volts @ up to 60 amps. Om the Output it can charge any battery chemistry. Smallest battery is a single NiCD at 1.2 volts up to 40 amps. 40 amps is the output limit. So if you connected all 8 cells in parallel for 3.6 volts is 40 amps or if divided by 8 is 5 amps per cell or a total power of 3.6 volts x 40 amps = 144 watts. Connect them in series for 24 volts and you charge them at 40 amps per cell or 960 watts.

With your 24 volt 6 amp charge is going to be painfully slow. If you connect the batteries in parallel and use the cellpro will convert it to 3.6 volts at 40 amps or 5 amps per cell. That is a 20 hour charge rate. Makes no difference if they are connected in series or parallel with your charger. It will be SLOW either way. Your charger and BMS is a One Trick Pony, and a painfully slow pony at that. 

I suggest you find yourself a good 24 volt DC Power Supply, and Cellpro Power Lab 8. A good match is from the same company, a 24 volt 55 amp Bench DC Power Supply. Between the Power Supply and Cellpro Power Lab 8 is there is no battery you cannot charge, monitor, and diagnose.

Your charger and BMS is a One Trick Pony, and a painfully slow limited Pony


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## PStechPaul (May 1, 2012)

I think there might be problems if you try to connect cells with widely different SOC in parallel. It seems that those with higher voltage and full charge would dump a lot of current into those with lower voltage. I think it would be best to discharge all the cells to their low setpoint, or at least until they all matched within 50 mV or better. If the internal resistance of the cells is 500 uOhms, that would cause 100 amps of current.

Also, when charging multiple cells in parallel, the wiring may be critical. You should connect the charger negative to the (-) terminal of the first cell, and the positive to the (+) terminal of the last cell, to equalize the effect of voltage drop.


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## Sunking (Aug 10, 2009)

PStechPaul said:


> I think there might be problems if you try to connect cells with widely different SOC in parallel.


It would be if they are Wildly Different of greater than .1 volt. Otherwise there is no better or faster way to EQ voltages. I would have serious doubts about my supplier if I received cells more than .1 volt difference. 

Worse thing you can do is connect them in series and use a BMS to Balance the cells. You would either burn up a lot of cells over charging them, or grow old waiting for weeks to EQ.


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## Karter2 (Nov 17, 2011)

polodiy said:


> What can i use to discharge cells? Don't have equipment for that. ..?


 Sorry, but you are building an EV lithium pack, but dont know how to discharge a single cell slightly ??
.....you are going to deal with a lot bigger issues before you are finished ?


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## Sunking (Aug 10, 2009)

Karter2 said:


> Sorry, but you are building an EV lithium pack, but dont know how to discharge a single cell slightly ??
> .....you are going to deal with a lot bigger issues before you are finished ?


I gotta agree. You are building an EV where budget is not much of a concern. Not only do you need the equipment you do not have, you also have to have the knowledge to use it and what it tells you. 

Two things are likely to happen to you, and neither is good. Destroy your batteries, or start a fire you cannot put out. In other words you are about to loose a lot of money. For starters you really do not even have a decent charger at 24 volts @ 6 amps. A 24 volt 100 AH lithium battery is not even large enough for a golf cart. 

So what is the purpose of this? Off-grid solar maybe?


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## dougingraham (Jul 26, 2011)

polodiy said:


> dougingraham, thank you for your reply. Could you explain more detailed plz?
> I was assuming that 6amp charger designed for 28v pack, will work as 48amp charger when connected to 3.5 cell. Or i am wrong here?


It is possible but unlikely. There are always limitations on these things. You never say exactly what the charger is so it is impossible to know for certain. Some of the previous respondents are making an educated guess about what you have and basing their replies on that assumption.

In the case of using a 1000 watt capable RC charger the 1000 watts is generally the peak capability of the device. It probably cant do 1000 volts at one amp or 1000 amps at one volt. It might be able to do 25 amps at 40 volts or 40 amps at 25 volts. That 1000 watt limitation is probably the ability of the electronics to shed the waste heat from the conversion. There will be input voltage limits, possibly 15 volts because it was designed to operate off of a 12 volt car type battery. There will be output voltage and current limitations due to limits of the switching devices. A nominal 24 volt at 6 amp charger would be 144 watts. Extrapolating that 144 watts down to 3.5 volts seems like you could attain 41 amps output. But if the output switching devices have a 10 amp limit then 10 amps is all you can get.

A 12 volt car battery charger is often just a step down transformer with a couple of different taps on the output windings. This means it has a fixed output voltage depending on the tap used. There will be a device that limits the output current so the charger doesn't catch fire and often a sensor that turns off the output when the voltage reaches what would be considered a full state of charge. That kind of charger would simply overheat and shut off for a minute if you tried to connect it to a single lithium type cell. Once it cooled off it would then turn back on until the charger again overheated. It would do this on/off cycling until the lithium cell burst into flames.

Be extremely careful around batteries. There is no off switch on a battery. If you inadvertently short one it will grunt and usually something else gives like the copper wire or bus bars turns to a super heated copper plasma vapor. With lithium battery types if they catch fire they are exceedingly difficult to put out. You need to cool them enough that they stop producing oxygen. Tossing them into a lake might not be enough and who has a lake handy in their garage to try it?


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## polodiy (Oct 25, 2015)

Sunking said:


> Dead wrong. A Charger is not a DC-DC Converter. That is what the Cellpro Power Lab 8 can do. If you connect a 24 volt 6 amp charger to a 3.6 volt cell you will let the Smoke out of the Charger, or it wil go into Current Limit of 6 amps and fold back the voltage to 3 volts. Most likely let the smoke out.
> A DC-DC Converter are Power Converters with limits. Example if you have a 1000 watt supply say 36 volts @ 28 amps, will charge a 3.6 Volt battery up to 1000 watts / 3.6 volts = 277 amps. Same Converter would charge a 24 volt battery at 1000 watts / 24 volt = 42 amps.


Sunking, thank you for your explanation, i was thinking that charger works similar to/like converter.


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## polodiy (Oct 25, 2015)

dougingraham said:


> It is possible but unlikely. There are always limitations on these things. You never say exactly what the charger is so it is impossible to know for certain. Some of the previous respondents are making an educated guess about what you have and basing their replies on that assumption.
> 
> In the case of using a 1000 watt capable RC charger the 1000 watts is generally the peak capability of the device. It probably cant do 1000 volts at one amp or 1000 amps at one volt. It might be able to do 25 amps at 40 volts or 40 amps at 25 volts. That 1000 watt limitation is probably the ability of the electronics to shed the waste heat from the conversion. There will be input voltage limits, possibly 15 volts because it was designed to operate off of a 12 volt car type battery. There will be output voltage and current limitations due to limits of the switching devices. A nominal 24 volt at 6 amp charger would be 144 watts. Extrapolating that 144 watts down to 3.5 volts seems like you could attain 41 amps output. But if the output switching devices have a 10 amp limit then 10 amps is all you can get.
> 
> ...


dougingraham, thanks for your explanation!


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## MichaelTerry (May 2, 2016)

I'm in a similar situation to Polodiy
I have a pack of GBS lithiums, connected in pairs with one bms board per pair.
I need to find and replace some bad (i think) cell, then balance the pack.
from reading here it sounds like I should be getting a PowerStation 8 to charge the individual cells and do the bottom balancing.

In addition to the tool I need more info. anyone know what voltage I should discharge, then charge the cells too?

I havent been able to find a manual for the GBS 100ah cells

thanks to you all for your help


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## PStechPaul (May 1, 2012)

A battery charger is essentially a current source, usually made as a buck switching regulator, and it should be able to drive a short circuit or a low voltage battery or single cell. The test instructions for the EMW 10-15kW charger I am working on instructs the user to short the output as one step in the calibration process, although this is before PWM starts. 

The charger uses a PWM frequency of about 10 kHz and the duty cycle can be as low as 1%, so the 350 VDC bus voltage will be applied to the 150 uH inductor for about 1 uSec at 1%. The current through the inductor will rise at a rate of I=V*t/L so it will reach 2.3 amps peak. This current initially charges the output capacitors which act as essentially a short circuit. With 9000 uF, the capacitor will charge at a rate of V=I*t/C, so during the off time of the duty cycle of 100 uSec, this current might charge the capacitor to a maximum of 26 mV, or more likely an average of 13 mV. It would take about 300 cycles at 1% duty cycle, or 300 mSec, for the voltage to reach 3.9V to start charging the cell.

Since the charger should measure the output current every PWM cycle, it should be able to stop the PWM when it reaches this point, and then resume when more voltage or current is needed. So, at least for the EMW design, there should be no problem charging a single cell.

A DC-DC converter, OTOH, is designed for a certain range of input voltage, and a fixed or variable output voltage. It may have buck or boost topology, but more likely it will have an isolation transformer and perhaps a SEPIC or flyback topology. There is usually current limiting circuitry as well as maybe an overvoltage crowbar. The output current is fundamentally limited by the components and circuit board traces.

Any switching power supply draws about the same input power as output (plus losses). So a buck converter with a shorted output may draw only a few watts input, and with a 100 watt load the input might be 110 watts with an efficiency of 90%. With a 110 volt input it will draw about 1 amp, while at 220 volts it will be about 0.5 amps. The output theoretically could be 100 volts at 1 amp, or 1 volt at 100 amps.

[edit] As for the voltages for charging and discharging LiFePO4 cells, here is a discharge curve for a little 18650 cell rated at 1800 mA-H but actually only about 1100:










You can see that fully charged is about 3.3 volts and fully discharged is about 2.5 volts into a 1.5 amp load (about 1C). The open circuit voltage is pretty close to 3.4 volts fully charged or mostly discharged. There might be 10% more capacity left but it is not good to discharge much lower than 2.5 volts.


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## polodiy (Oct 25, 2015)

guys thank you for all replies.
Cellpro PowerLab 8 is a bit too expensive for me, i just need charger to charge single cells a little to match the higher charged cells (to do initial top balancing). 
I will be not using the after that.



dougingraham said:


> I would suggest you get one of the inexpensive RC charger/discharger and use that to charge and discharge your single cells.


dougingraham, i am thinking about buying Turnigy Accucell 8150 for that.
I will charge pack with my regular charger till 1st cell reaches shunting phase. And than charge other 7 cells individually with Turnigy Accucell 8150 to shunting phase. After that miniBMS should keep them balanced.
Does it sound ok?



Sunking said:


> I suggest you find yourself a good 24 volt DC Power Supply, and Cellpro Power Lab 8. A good match is from the same company, a 24 volt 55 amp Bench DC Power Supply. Between the Power Supply and Cellpro Power Lab 8 is there is no battery you cannot charge, monitor, and diagnose.
> Your charger and BMS is a One Trick Pony, and a painfully slow limited Pony


Sunking, i know Cellpro PowerLab 8 is very useful piece of equipment, but i don't feel good to invest $250+ just to use it once a year.
About miniBMS been limited - agree, but for the price it should do it's job.
Lastly you are saying 24v 6amp charger is too weak for 100ah cells. I ordered such a small charger on purpose, cause read that lifepo4 loves low C rate charging. MiniBMS should keep the pack to 80DOD max, so daily i will have to charge max 80ah. It's 14 hours charging a day. (overnight charge + couple of extra hours) Sounds ok to me? Or it's still too weak?


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## Sunking (Aug 10, 2009)

polodiy said:


> Sunking, i know Cellpro PowerLab 8 is very useful piece of equipment, but i don't feel good to invest $250+ just to use it once a year.?


Once a year? You would use it everyday to charge and monitor the batteries. You would not have needed anything else but a Power Supply as the source of charger power. You would not have needed anything else. 



polodiy said:


> Lastly you are saying 24v 6amp charger is too weak for 100ah cells. I ordered such a small charger on purpose, cause read that lifepo4 loves low C rate charging. ?


 Who told you that? Slow charging on LFP is C/2, not C/20. C/2 on your pack is 50-amps. Granted 50 amps is pricey, but 20 to 30 amps is about right on a 100 AH pack.

To fully charge a LFP you apply a constant current of some rate, let's say C/5 or 20 amps in your case. When the voltage reaches 3.6 volts you hold 3.6 volts until current tapers to 3-5% C or 3-5 amps for you. It is going to take longer than you think of 14 hours. When you hit 3.6 vpc the Constant Voltage phase, you are roughly 80% SOC, and things start slowing down. At C/2 would take an additional hour or two to saturate 3% of C. At C/20 or 6 amps, much longer. 

Not sure where you got this slow charge thing from or what kind of cells you have, but most manufactures recommend C/2. You can charge slower if you have the time, but the time calculation is not straight forward H = AH/H. That only gets to 80%. 80 to 100% is much slower as current tappers off.


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## MichaelTerry (May 2, 2016)

Thanks for the info, I will get myself a powerlab to work on this pack
Today I went out and tested the resting voltage of the cells. There are 88 gbs cells in pairs, with one mini bms board per pair. Most of the cells were at 3.27 to 3.29 volts, but 3 pairs were between 3.21 an 3.24, one pair at 1.48, a pair at 1.2, and the biggest looser was a pair at 0.16 volts. the vent cap had popped off one of the cells of the lowest pack.

I guess the next step will be to separate the low functioning pairs and see which ones can be saved.

I will surely need some new cells.

how do I test for self discharge?

Many thanks for your advice

Michael Terry


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## dougingraham (Jul 26, 2011)

polodiy said:


> dougingraham, i am thinking about buying Turnigy Accucell 8150 for that.
> I will charge pack with my regular charger till 1st cell reaches shunting phase. And than charge other 7 cells individually with Turnigy Accucell 8150 to shunting phase. After that miniBMS should keep them balanced.
> Does it sound ok?


This is the kind of charger I was thinking of.

I believe you will find many more uses for a charger than you know.


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## polodiy (Oct 25, 2015)

Accucell-8150 arrived yesterday, very useful tool indeed. Thank you guys for all your replies and recommendations.

Strange thing i mentioned:
Last time i've charged the pack, all cells reached 3.55v+ voltages and one hit HVC causing miniBMS cut off the charger.
Since i didn't have the way to balance cells, i've let it sit like that until Accucell-8150 arrived. 10-14 days passed, only miniBMS modules/head, relay, 24v-12v converter, charger were connected to pack and cells show voltages 3.196-3.199v. 

I though normal resting voltage is 3.3v or so. Is it normal behavior? Or drain from miniBMS?


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## alvin (Jul 26, 2008)

polodiy said:


> Accucell-8150 arrived yesterday, very useful tool indeed. Thank you guys for all your replies and recommendations.
> 
> Strange thing i mentioned:
> Last time i've charged the pack, all cells reached 3.55v+ voltages and one hit HVC causing miniBMS cut off the charger.
> ...


 
That is not normal. Mine can sit a long tome and still be above 3.3.
Something is draining them.


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## dougingraham (Jul 26, 2011)

polodiy said:


> Last time i've charged the pack, all cells reached 3.55v+ voltages and one hit HVC causing miniBMS cut off the charger.
> Since i didn't have the way to balance cells, i've let it sit like that until Accucell-8150 arrived. 10-14 days passed, only miniBMS modules/head, relay, 24v-12v converter, charger were connected to pack and cells show voltages 3.196-3.199v.
> 
> I though normal resting voltage is 3.3v or so. Is it normal behavior? Or drain from miniBMS?


A fully charged LiFePo4 cell will after several days of resting end up at about 3.4 volts. A pretty much completely drained LiFeP04 cell will after several days of resting end up around 2.7 volts. The storage voltage is generally considered to be about 3.3 volts and this will be about half charged.. Those numbers are a little squishy and depend on brand, temperature, and age and to a lesser extent the algorithm used to charge and discharge.

I can't comment specifically on the Mini BMS but if the BMS is discharging the pack it is probably doing so with a different discharge current on every cell and is the cause of the imbalance in a pack. Properly operating LiFePo4 cells that are connected to nothing will hold their resting voltages for years because there is essentially no self discharge.


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## polodiy (Oct 25, 2015)

Just did the measurements

Relay + DC converter + MiniBMS head draining 275mA
Is it acceptable for daily use for 100ah pack?

Will be disconnecting when not using for more than week.


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## Moltenmetal (Mar 20, 2014)

minibms head draws from the aux battery. DC/DC draws from the main pack, and in my case keeps my aux fla battery charged to 13.5 V all the time, so I don't have to pull it and put it on a maintainer. That loss from the fla battery is the biggest waste of pack energy during storage in my car. My draw is nearer to 100mA from the main pack- that's not accurately measured, just watching my Ah meter bounce around between 0 and its minimum displayed current of 0.1 A. Haven't back calculated it from readings of Ah vs time during storage - will do that sometime. My pack at 18.5 kWh will supply this ~10W plus another - 1 W for my Lee hart bridges plus the parasitic draw of the miniBMS celltop boards from each cell for a long time, but it does mean special considerations if I'll be away for months in winter.


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## Sunking (Aug 10, 2009)

polodiy said:


> Accucell-8150 arrived yesterday, very useful tool indeed. Thank you guys for all your replies and recommendations.
> 
> Strange thing i mentioned:
> Last time i've charged the pack, all cells reached 3.55v+ voltages and one hit HVC causing miniBMS cut off the charger.
> ...


Keep in mind a BMS cannot balance an out of balance pack. They can only keep a Balanced pack balanced. 

Not sure what your Mini BMS Bypass Current is but they generally range from 50 to 150 ma. So if you have say even a minor 2 AH in-balance would take up to 40 hours to balance with only 50 ma.


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