# Unbalanced currents in paralell group



## sabahtom (Mar 1, 2011)

I've connected my pack in paralell (100x100AH China Hipower) and was checking to see if I could guess how long it would take for the cells to balance.

I did as in the diagram below, across a group of 17 cells and got 19mv from cell 1 + to cell 17 + and 9.6mv from cell 1 - to - cell 17 -. Sorry the diagram is misleading the 17 cells are all connected in paralell.

I tried this for a few groups and got differences each time, but no fixed ratio.

I haven't balanced a pack before so I have no idea what to expect, or where to properly measure the voltage. I guess that since all the cells in between have various V, between 3.23 and 3.27, the V between connections will be different wherever I measure it, but I expected to only find a voltage from cell 1+ to cell 17- for eg.

Any ideas? My measurements are wrong maybe?


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## Jenny (May 9, 2013)

sabahtom said:


> I've connected my pack in paralell and was checking to see if I could guess how long it would take for the cells to balance.
> 
> I did as in the diagram below, across a group of 17 cells and got 19mv from cell 1 + to cell 17 + and 9.6mv from cell 1 - to - cell 17 -. Sorry the diagram is misleading the 17 cells are all connected in paralell.
> 
> ...


Hey,had you make the battery pack 1S17P? for series,it relates to voltage,for paralell,it adds Ampere Hour.for example. if we make battery pack LiFePO4 16S2P which made of A123 20Ah cells, so it will be 48V(16S*3=48V) 40Ah(2P*20=40Ah). So I am confused for 17 paralell you did.


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## sabahtom (Mar 1, 2011)

Jenny said:


> Hey,had you make the battery pack 1S17P? for series,it relates to voltage,for paralell,it adds Ampere Hour.for example. if we make battery pack LiFePO4 16S2P which made of A123 20Ah cells, so it will be 48V(16S*3=48V) 40Ah(2P*20=40Ah). So I am confused for 17 paralell you did.



The pack is 98 cells in paralell, to slowly balance them before initial charging. After two days or so I'll reconnect them in series to charge at 320-365v.


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## TEV (Nov 25, 2011)

Jenny said:


> I am confused for 17 paralell you did.


It's normal to be confuse, if you don't read or don't understand the question


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## TEV (Nov 25, 2011)

Tom, read this http://www.diyelectriccar.com/forums/showthread.php?t=82260&highlight=balancing

I didn't have time to read all of it, but this part is certainly very relevant



EVfun said:


> Parallel cells still get slightly varying charges based on the connection resistance at the terminals and the differences wiring resistance between each cell and the charger. I could see slight current movement across the interconnects on a block of 6 cells in parallel. You can detect it with a meter that can read down to the ten-thousandth of a volt, across the jumpers between the cells.
> 
> One thing that may work better is to use a single 3.6 volt cell charger that shuts off when done and charge each cell with that.


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## Duncan (Dec 8, 2008)

Hi Guys

This is why you have to either 
Charge the cells until they are past the "knee" of their charge voltage (top balance)
Or discharge the cells until they are past the lower "knee" of their voltage(bottom balance)

In between the "knees" the voltage/charge slope is so flat that you cannot balance the cells


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## sabahtom (Mar 1, 2011)

Duncan said:


> Hi Guys
> 
> This is why you have to either
> Charge the cells until they are past the "knee" of their charge voltage (top balance)
> ...


Hi Duncan

Thanks for the tip. At this point I'm paralelling them for two reasons:

1. I've taken voltage readings from the cells and I plan to charge them up by 30AH from here. If there are no wild differences (hopefully meaning the cells are all ok) I'll bench test my motor.

2. I don't have a 3.6v charger/discharger and I live far away from any place I can get one (except online of course, and the shipping will be more than the purchase price).


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## Ziggythewiz (May 16, 2010)

As Duncan said, you can't balance them as they arrive. They'll be ~50 or 60% soc and on the flat part of the charge curve, where the differences are so minute any balancing current will be negligible. You have to take them to a knee to achieve a known and consistent SOC.

I top balanced mine with a 12/6V charger. It's not difficult.


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## sabahtom (Mar 1, 2011)

Hi Ziggy

Thanks for the advice. I wish these had just been delivered, they've been on the shelf for 2.5 years now.

I've got a 12v charger but I think it's too intelligent, it cuts off around 13.6v so no use to me. Maybe I can find a cheaper one somewhere that I have to watch closely. I'll definitely charge up past the knee and balance at a later point. Right now the car has been sitting in a friend's workshop (maybe not my friend anymore.....) for over a year because of the many dead end routes I've taken to get this far. Being in SE Asia means that very few people have freehold properties - I'd give a minor body part for a garage of my own.

The reason for charging up halfway at 365v without balancing at 12v is that I will have a few rare hours tomorrow to watch the charge and if I can at least show them the wheels spinning the friends may regain enthusiasm. I can't often get there to supervise/reconnect a 12v charger, it would probably take me two weeks to get the whole pack done.




Ziggythewiz said:


> As Duncan said, you can't balance them as they arrive. They'll be ~50 or 60% soc and on the flat part of the charge curve, where the differences are so minute any balancing current will be negligible. You have to take them to a knee to achieve a known and consistent SOC.
> 
> I top balanced mine with a 12/6V charger. It's not difficult.


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## sabahtom (Mar 1, 2011)

*The forum was right, of course...*

I went back this morning and checked the V of some of the cells. Nothing had changed.

http://www.rcsmart.com.my/webshaper/store/viewProd.asp?pkProductItem=6516

My pack is at 15-20% SOC so I plan to charge it up at 365v for 60AH and see how the cells look. 

The 365v (max) charger was in storage, I just had a look at it. It doesn't have a CAN bus so I'm thinking a top balance might be the way to go, and use this ICharger to discharge and balance, then add the BMS as a last step. I have 98 okay-looking cells (no bulging) so at 3.65v * 98 cells I have a safety margin of 7.3v. 365v-(3.65*98)


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## Duncan (Dec 8, 2008)

*Re: The forum was right, of course...*

Hi Sabahtom

I would suggest you take your cells out of the car - 
take them home - this is going to take a while!
You have 98 off 100Ah cells,
Use your 12v charger to charge *3* cells at a time - if it is charging at 4 amps and you are at 50% charge it will take 12 hours to charge each group of 3 cells - Keep an eagle eye on the first batch (probably worth taking readings every hour and plotting them on a graph)- the later ones you will know what to look for and you will be able to leave alone for a while,

The other way would be to wire up 3 series - 32 parallel - that will take about two weeks continuous charging - but it will be very unlikely to damage your cells!

This is what I did as I am cash poor - so I have got to spend the time (I have a MUCH smaller pack)


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## sabahtom (Mar 1, 2011)

*Re: The forum was right, of course...*



Duncan said:


> Hi Sabahtom
> 
> I would suggest you take your cells out of the car -
> take them home - this is going to take a while!
> ...


Hi Duncan

Thanks yes, that's a good idea. I did try it, and it did take days to figure out it wouldn't work with my charger. my charger refuses to go above 13.56v for longer than an hour, maybe to do with the lead acid charging algorithm. It's too intelligent. I've gone around looking for other 12v chargers (specially cheaper ones) but when I test them, none gives 14v so I didn't buy them. Maybe they'd give a higher V once connected to my cells but I only took my multimeter along.


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## Duncan (Dec 8, 2008)

*Re: The forum was right, of course...*

_13.56v for longer than an hour, maybe to do with the lead acid charging algorithm. It's too intelligent. I've gone around looking for other 12v chargers (specially cheaper ones) but when I test them, none gives 14v so I didn't buy them.

_That is why you use a cheap charger to charge *3*(three) cells in series and why you have to keep an eye on it3 cells = about 11v - well below the level the charger cuts off at


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## EVfun (Mar 14, 2010)

*Re: The forum was right, of course...*

Perhaps the cheap chargers we get in the USA are different, but I would recommend *4* cells in series and never 3 cells in series. Most of the cheap chargers we get are basic transformer, rectifier, timer, and thermal breaker chargers. They will hit 16 volts at nearly zero current and hit about 15.6 charging a typical car starting battery that won't go much below 2 amps at that voltage. 

I've done a charge of some cells this way. I use a dumb auto charger set to *6* volts and a small variac to dial the voltage down a little. I use that to charge 2 cells in series (usually 2p2s so I'm charging 4 cells at a time.) I have a few load resistors that I use as manual shunt regs to hold the voltage down first cell or cell pair to shoot up over 3.5 volts. I can adjust the current with the variac to help that work. Once I get them both to 3.5 I can run them up together to 3.6 volts until the current drops to about 3% of the stated capacity and then terminate the charge.

I will buy one of those off the shelf single cell automatic shut-off chargers and do the whole pack one cell next time I set up a pack. I would charge my way through the pack one cell at a time, which would take some time, then go through the whole pack again, which should take less than a day. Then I would consider them charged and top balanced and treat them only as a pack.


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## sabahtom (Mar 1, 2011)

TEV said:


> Tom, read this http://www.diyelectriccar.com/forums/showthread.php?t=82260&highlight=balancing
> 
> I didn't have time to read all of it, but this part is certainly very relevant


Thanks I saw a RC charger model that is good for the job in that thread too.


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## Duncan (Dec 8, 2008)

*Re: The forum was right, of course...*



EVfun said:


> Perhaps the cheap chargers we get in the USA are different, but I would recommend *4* cells in series and never 3 cells in series. Most of the cheap chargers we get are basic transformer, rectifier, timer, and thermal breaker chargers. They will hit 16 volts at nearly zero current and hit about 15.6 charging a typical car starting battery that won't go much below 2 amps at that voltage.


Cheap charger here are the same - 
but they are designed to limit the current to the maximum on the label (normally 4 amps) - so the two I have work fine on three cells - you could damage the cells if you left them on long enough - but that is what you need to watch

I charge my auxiliary battery using a fancy RC charger - powered by an old computer power supply,

But that charger cost $40 - a cheap car charger is $15


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## sabahtom (Mar 1, 2011)

*Re: The forum was right, of course...*



EVfun said:


> I will buy one of those off the shelf single cell automatic shut-off chargers and do the whole pack one cell next time I set up a pack. I would charge my way through the pack one cell at a time, which would take some time, then go through the whole pack again, which should take less than a day. Then I would consider them charged and top balanced and treat them only as a pack.


My main problem at this point is logistical, my car is 15km away (30 min with the traffic here) drive from my house, and there's no internet there so I can't do anything but sit in the heat and stare at the charger 

I need to charge the pack up enough to get it to my house now that most of the work is done, then I can do the one-at-time way with an RC charger.

I guess if I charge the pack up using the 365v charger up to around 60% SOC (carefully checking individual cell V, maybe using the BMS) I can get away with not balancing it yet. Then I can bring it home and balance it properly with an RC charger as you say. I'm not confident enough to try putting resistors in a mains circuit yet


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## kennybobby (Aug 10, 2012)

That sounds like a good plan--just put some charge into the pack in order to get it home, then you can do all the tweaking and balancing from there. 

Since you only have about 10 miles to get it home you can estimate how many Ahrs you need if you know the weight of the vehicle and plan to drive slowly. Then double that estimate to have a margin. 

For example if 3000 lbs, then you might need about 330 Whrs/mile. To go 10 miles would require 3300 Whrs. If the 98-cell pack is at 313 volts (3.2 vpc) then you need 10.6 Ahrs to get home. So put 22 Ahrs into the pack and get it home. If you want more margin put 30-40 Ahrs in. 

What is the pack voltage right now? Put 10 Ahrs in and measure the pack voltage again. If it is below 3.30 volts per cell then put in 10 more and remeasure. If you want you can measure each cell. If you have one running way out of line just remove it from the pack and go with the rest. 

Repeat until you get to your margin/comfort level, but there should be plenty of energy to get you home with 3.2 to 3.3 volts per cell. No need to go above 3.3 and the short drive won't pull the pack down to damage any cells on the low end.


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## EVfun (Mar 14, 2010)

*Re: The forum was right, of course...*



Duncan said:


> Cheap charger here are the same -
> but they are designed to limit the current to the maximum on the label (normally 4 amps) - so the two I have work fine on three cells - you could damage the cells if you left them on long enough - but that is what you need to watch


I see. Most of our cheap car chargers are 10 to 15 amps and they don't limit current (except by the circuit breaker cycling on and off every few minutes.) 3 cells pretty much insures the breaker keeps popping and resetting, and near the end of charge the voltage rises like a rocket at 10 amps.


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## Duncan (Dec 8, 2008)

*Re: The forum was right, of course...*



EVfun said:


> I see. Most of our cheap car chargers are 10 to 15 amps .


Those are more expensive - the cheap ones here are 4 amp


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