Making up series parallel battery arrays.

[opampsmoker]

Hi,
We have a load of used lithium cells that we wish to build up into series parallel arrays. We will just charge them all up, and put them in series parallel. Will there be any problems? We wont bother with any resistive "balancing" between the cells prior to paralleling them. Will we be OK?

 

[Pommie]

If you parallel a good one with a mediocre one then a huge current will flow. Can you afford to let that happen?

Mike.

 

[Diver300]

You should certainly find out how good the cells are before connecting up and using them. You need to measure the capacity of each one at a very least.

There is certainly a problem if you put a cells (or parallel arrays) in series with others where the capacities differ a lot. They will go badly out of balance as they are used.

It's not such a problem with the cells in parallel. If they start at the same voltage, little current will flow when they are connected. A lower capacity cell will contribute less, and a higher one more, as they discharge, but as they stay at the same voltage all the time, they will also stay at a similar percentage of their capacity.

 

[rjenkinsgb]

As Diver says, if they are charged to the same voltage, the actual battery assembly will not be a problem.

However in use, the overall capacity will be limited by the capacity of the lowest parallel set, plus the balance current and charge time will be much greater if there are any faulty cells in the overall assembly.

Any cells that have been left flat, below 3V while stored, are likely to be wrecked and should not be re-used.

It will save later time and trouble if you measure each cell capacity and internal resistance first, discard any that are significantly lower capacity or higher resistance than they should be, and build each parallel set with closely matched total capacity.

Something like an Opus 3100 charger can do cell capacity and internal resistance tests.

And note that you must always use a balance charge system and regulated charger, not just a "battery protection" module that switches off with under or over voltage.

 

[Nigel Goodwin]

If you're doing this, and you should, it's EXTREMELY common practice - it's VERY, VERY much recommended that you use brand new identical cells.

I would also suggest you use a balance protection module, as even with new cells you're likely to run into issues.

Using second hand cells, in series, is fraught with problems - particularly if they are different makes and types. It's not so bad if you're just paralleling them.