# Lithium Batteries in Parallel



## Siwastaja (Aug 1, 2012)

No problem, just make sure they are at the same SoC before actually connecting them in parallel. For example, you can fully charge them first separately, or measure that they are close enough in voltage (for example, < 0.05 V difference).


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## skooler (Mar 26, 2011)

Siwastaja said:


> No problem, just make sure they are at the same SoC before actually connecting them in parallel.


Correct. Although not 100% necessary - The stronger cell will try to charge the weaker one which can cause a few sparks when you connect them.



Siwastaja said:


> For example, you can fully charge them first separately, or measure that they are close enough in voltage (for example, < 0.05 V difference).


Not correct...

Voltage is not a good indication of state of charge in the middle of the charge curve. At the top it is slightly better but the issue you'll have is that the cells will all have a different capacity meaning that at 3.4v, you could have some cells at 60AH and others closer to 70AH.

When you discharge these cells in parrallel to a lower SOC you'll have issues with the weakest cells and the lowest capacity parallel groups of cells across the series string.

Ideally you'd know the actual capacity of each cell and you can then group them so that the capacity of each parallel group of 3 cells has a very similar capacity.

Otherwise some groups of 3 could be at 180AH and others over 200AH.

Your pack is only ever as strong as the weakest cell (when in series).

I'd advise measuring the capacity of each cell with something like a powerlab8. Group them in threes so that the total capacity of each group is equal with other groups. Then drain the cells down to a low capacity for a bottom balance.

Cheers,

Mike


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## pm_dawn (Sep 14, 2009)

I have done this with Thundersky cells.
A total of 120 40ah cells were all measured in capacity,sorted and combined in triplets so that the total capacity was as equal as possible.
I arranged the pack to be 40s3p 40ah to a total of 40s120ah
I bottombalanced all cells before connecting them and then bottom balanced again when connected in triplets.

Connecting cells of different SOC is not a good idea.
I tried that during the capacity measurements to try to charge more than one cell at a time in order to speed up the process. Not a good idea !!!!
Although the pack of 5cells seemd to be fully charged some of the cells were not. So from there on I always charged/discharged them separately for capacity measures. 
Then put them at very close SOC at the bottom. I think that is where you can best find the point where they have a good indication of SOC.

The car is running fine bottom balanced with per cell monitoring.
Has run over 15000miles in a year now since I put it together.

BUT its a real PITA to put all these cells in parallell and then in series.....
Will never do it again, to much work. You must have gotten the cells really cheap to consider doing that work. Just make sure you understand the number of connections that has to be done to make that pack. And the number of straps that goes into it....


Regards
/Per


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## bdmiko (Jul 17, 2013)

skooler said:


> I'd advise measuring the capacity of each cell with something like a powerlab8. Group them in threes so that the total capacity of each group is equal with other groups. Then drain the cells down to a low capacity for a bottom balance.


Thanks, makes sense. A lot of work and time, but it makes sense.



pm_dawn said:


> BUT its a real PITA to put all these cells in parallell and then in series.....


Not really forward to this but I gotta work with what I've got.

Thanks
David


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## GizmoEV (Nov 28, 2009)

First, I'm assuming you are using LiFePO4 cells. I did this for my Gizmo but only because of the geometry of the box. Also, it doesn't really matter what the relative SOC is of the cells when you connect them. I've tested it and don't know what others are concerned about. I tested it with a full 40Ah cell and an empty one through a 50A 50mV shunt and recorded only ~30A, IIRC, for a very short time. In less than a minute the current between the cells was less than 10A.

As for matching up capacity values you definitely do not want to parallel then capacity check because you will only get the group's capacity and won't know how to match them up. After that, if you bottom balance the parallel set or charge to say 3.6V and hold until the current drops to 0.05C for a top balanced set the cells will be fine. Again, this isn't theory this is what I found in practice. If I had a more programmable charger I would go with a bottom balanced pack.

Oh, be sure to have long enough bolts for the where you have a double strap connection. You don't want to strip out the terminal. If you have to buy replacement bolts go with brass bolts and bronze lock washers or get some nord-lock washers and SS bolts. Either combination will assure you won't have any bolts loosening up. Check my blog for what I did for checking pack balance.


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## pm_dawn (Sep 14, 2009)

The reason I see for not connecting two cells of different SOC is just that fact that there will not be that much of a voltage difference that any significant current can be driven that makes the cells get to the same SOC in a reasonable time. If you connect them at different SOC, make sure you leave them alone for a couple of days so they can balance themselves.

As I wrote earlier, I have tested it and I had problems with that. The cells was not all equal SOC after charging a group of cells in parallell. Probably because they did not have time to balance SOC between them.....

I would strongly suggest trying to sort them to get the least capacity variance in the pack. That is a good help when doing balancing of any kind. Top or Bottom!

Regards
/Per


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## Siwastaja (Aug 1, 2012)

pm_dawn said:


> The reason I see for not connecting two cells of different SOC is just that fact that there will not be that much of a voltage difference that any significant current can be driven that makes the cells get to the same SOC in a reasonable time.


No, it's to protect the cells and your property and health from damage. If the SoCs differ too much, the current going from the cell to another can be enormous, damaging the cells and causing a fire danger.

The cell IR is the only limiter for the current. A lot of waste heat is produced in the cells. If you wanted to connect cells with different SoCs in parallel, you need an active DC/DC converter (bidirectional buck-boost, for example) inbetween.

The cells have indefinitely time to "balance" after they are fixed in parallel configuration, so once you have made the connection safely, you leave them paralleled and think it as a single larger cell in all practical purposes.

This is how it's done.

Always measure the voltages with multimeter just before connecting. You can also carefully make the connection with a tip of the wire so you can see if there is any sparking.


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## GizmoEV (Nov 28, 2009)

pm_dawn said:


> As I wrote earlier, I have tested it and I had problems with that. The cells was not all equal SOC after charging a group of cells in parallell. Probably because they did not have time to balance SOC between them.....


If you charge them to 3.6V or above and let the current drop to 0.05C or discharge them to around 2.7V and check the set after a rest they will be balanced. As you surmised, you either did not let them sit a really long time or get them far enough up or down the voltage curve.

As for what Siwastaja is saying about protecting things note that my test was with 40Ah cells, 1 gauge cable, and a 50A 50mV shunt so the resistance between the cells was more than just the connecting straps.


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## pm_dawn (Sep 14, 2009)

I charged to 3.6v and 1A, but I guess that I did not leave them connected long enough. I think I'm going to do a test about that again......


But yes take care when connecting them so you dont get sparks and arcing.
I have not yet gotten that. I have been using both 40ah and 90ah in parallell.

Regards
/Per


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