# best way to discharge for bottom balance



## dougingraham (Jul 26, 2011)

I can't claim that this is the best way but it is probably the fastest with least oversight needed. Parallel all the cells and then drain to 2.5 volts (or whatever you feel comfortable with). Disconnect the load and let the voltage bounce back up. You may have to put the load back on if it bounces back up more than you want. Typically this will be 2.7 to 2.8 volts. Let it sit a day and this should be good enough for a bottom balance. When in this state you have an opportunity to discover cells that exhibit self discharge behavior. After you un-parallel the cells wait a couple of hours and take a voltage reading of every cell. Wait a couple of days and read the voltage again. Normally the cell voltage will continue to rise for several days after you disconnect the load. Cells where the voltage has gone down are suspect and should be marked in some way. Dropping voltage is an indication of an internal leak and is a manufacturing defect. I have heard that CALB will replace cells that behave this way. I would not use that cell because it will continue to self discharge and unbalance the pack.

Note: Your load should be connected to opposite ends of the parallel string. If you don't you will see a small difference in cell voltage, increasing as you measure farther from the load. When you connect the load to opposite ends each cell should see the same load.


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## steelneck (Apr 19, 2013)

I started the discharge by simply driving the car to a low SoC, then in the garage i used the Soliton idle function to slowly discharge even more while running around with a voltmeter measuring cells. I aimed for 2.75V and as soon as any cell hit that i disconnected it and kept on going with the rest. At the end i used an old resistor to drain down single cells, but also a capacitive Bad boy charger to give single or groups of low cells a bit of charge to have more of them reaching 2.75 at the same time. My cells did go down in voltage *very* quickly once down to 2.8V, there is very little energy difference between 2.8 and 2.75. 

You could also use the cabin heater of the car do drain the pack, together with a single cell, or bad boy capacitive charger to give low cells a little juice while the rest of them are going down in voltage. 

Edit. And as Doug says above, cells will bounce up a bit again, so it will take several days to bottom balance properly. There is no quick and automatic method as far as i know.


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## onegreenev (May 18, 2012)

I'll add to that. You should for safety reasons connect up a JLD404 and have it connected via a contactor so it will trip off the contactor when your target voltage is reached. If you parallel 97 cells and they are 100ah cells you will have a 9700ah cell to drain. It would take days and if you miss the target end point manually you could loose 97 cells in the process. Put in a way to shut it down if you happen to miss the target. Even the volt meter will do the trick. You would don't need a shunt with this and the cost is cheap insurance. I use mine all the time. I use mine with the JLD404 but the 404 is only used to check amperage and AH in my setup. 

http://www.lightobject.com/Programm...th-dual-control-Good-for-HHO-System-P408.aspx

Get yours from the source. Much cheaper than the other place we all know about.


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## sabahtom (Mar 1, 2011)

> http://www.lightobject.com/Programm...th-dual-control-Good-for-HHO-System-P408.aspx
> 
> Get yours from the source. Much cheaper than the other place we all know about.


I just saw your video on this topic which I'll put here in case someone else asks this same question soon.

http://onegreenev.blogspot.com/2013_06_01_archive.html


Thanks for the tip, I actually can't find something similar anywhere else at that price so I guess this lightobject is the place to buy it. 

My cells are between 3.20v and 3.26v right now.

I'm thinking of making 4p 24s strings to get down 12v, then the rest of the way with the JLD404 to cut off, unless I can find a motor that will do 3v 30A.

I saw this video - he seems to be using two lengths of rebar as resistors at 125A 20v. He doesn't say how hot they get, I would have thought enough to melt his multimeter clamp, but apparently not.

http://www.youtube.com/watch?v=to0Vlq8J79E


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## onegreenev (May 18, 2012)

sabahtom said:


> I just saw your video on this topic which I'll put here in case someone else asks this same question soon.
> 
> http://onegreenev.blogspot.com/2013_06_01_archive.html
> 
> ...



Use the volt meter if you don't care to know the AH in or out. The volt meter is more accurate than the JLD404 for registering voltage. I'd get them both. Links are on my blog for both meters. The rebar setup would actually require a healthy voltage to do some serious amp draws. With a little battery it won't do much. You might get about 30 amps out. Maybe. I have resistor wire and with a single cell I got only about 24amps out. Much too slow for my needs. If I pumped in a high voltage Im sure I could get a good draw of amperage. If possible disengage your motor in your electric vehicle and see if a cell can turn the motor. If its a warp motor it may not do a good job. My one video you saw showed that warp moving slow. Thats because the warp is advanced for higher RPM in a conversion. My Big GE was neutral timed and that little cell ran that motor just fine. I did not want the motor to sit without moving because I did not want to stress the commutator but with low voltage it really did not do that. 

So not sure what you can use. Experiment but be sure you can connect the whole thing to a contactor to shut it off when needed. 

Pete


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## dougingraham (Jul 26, 2011)

sabahtom said:


> My cells are between 3.20v and 3.26v right now.


To give you an idea of how wide a range this is.

3.2009 volts is 8.2% SOC
3.2557 volts is 15% SOC
3.2704 volts is 20% SOC

Those results are from a several day rest followed by a 25 amp discharge to 2.5 volts with GBS 100 AH batteries compensated by cell capacity.

So you do need to balance those cells.


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## sabahtom (Mar 1, 2011)

dougingraham said:


> To give you an idea of how wide a range this is.
> 
> 3.2009 volts is 8.2% SOC
> 3.2557 volts is 15% SOC
> ...


Much appreciated Doug. These are 3.2v nominal cells. As far as I can see the average cell in my pack will go from 3.23 to 3.25 and settle there after about 400mah of charge. I suspect I've already lost capacity because I haven't maintained them properly, will know this week I hope.


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## sabahtom (Mar 1, 2011)

*discharged cells*

So I discharged the cells slowly (20A) over a period of two weeks as I had time. They were paralelled and at 2.897 give or take 2mv.

I disconnected them all on Sunday but only had time to test a few, which were at 2.897 +/- 2mv. Most are still at 2.897, some at 2.895. A few at 2.894 I have charged singly at 1A, giving them 20ma each. They were at 2.897 after resting from charge for 30 minutes. I'll see how they look tomorrow.

It looks like they're all close to 0 SOC, the V is not rising after the load was disconnected, but right at the end the load was only 10A which may explain why the V is not rising much.

I have marked the ones that are below 2,895 because I think the difference of 2mv is likely due to differences in resistance due to Al oxide on the terminals, so they may not be self-discharging. I didn't wirebrush them before I paralelled, bad idea.


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