Practical Considerations of using batteries in parallel + parallel Vs series

Question

I have some queries regarding charging/discharging LiFePO4 in parallel:

1. Initially, when they are first connected, they need to be bough to the same voltage to avoid high currents. Excluding the problem of high currents, if say there are two batteries in parallel. one has a capacity C1 and the other C2 where C1>C2. If one was charged to 50% and the other to 100%, once connected would they balance to 75% each? I assume no, hence if not:

2. What kind of balancing is needed? Since they have different capacities and they are being discharged at the same rate would the two batteries simply discharge into each other such that they will have the same voltage and more importantly the same SOC?

3. Given the same cells are used in the parallel connection, how much of a capacity difference is too much? I have read that since cells are made in batches there could be up to a 15% difference between battery capacities. Would this be considered unusable in a parallel connection?

The situation I am fearful of is this:

Say two batteries are both at 10% SOC ( state of charge ), but they both have different capacities where one say is 550mAh and the other is 500mAh. Since they are in parallel would they both reach a 0% SOC at the same time or would there need to be extra balancing to make sure that the one with the smaller capacity doesn't continue to discharge as it hits 0% SOC while the other one continues to discharge and is still at say 2% SOC as opposed to 0%. This would essentially kill the battery with the smaller capacity.

Parallel Vs Series

4. Since cells connected in parallel continuously discharge into each other to self balance would this not put a lot of stress on them and reduce both their capacity and max number of cycles possible? essentially just killing them slowly

5. What advantages does a series connection have as opposed to parallel? so far from my experience I have found that series is able to provide a higher voltage but this problem can easily be solved using a DC-DC converter. So why would someone use series over parallel?

When I mention different capacity cells I mean the same type of cells, exact same ones, but of course batteries don't have the same capacity as the nomnial capacity. If nominal was 500mAh then some might be 490 some might be 510 etc.

Answer 1

1. They self-balance based on voltage. Not charge. Because things in parallel want to have the same voltage. Your example of 50% and 100% balancing out to 75% doesn't work like that as a result. Because battery voltage vs remaining capacity is not linear, and even if it were linear the 50%||100% = 75% would only happen for batteries with identical capacities.

2. No, the batteries because batteries aren't discharged at the same rate. Your concept of them discharging at the same rate and then discharging into each other to balance is roundabout. They don't do this. They just discharge at different rates such that their SOC decrease at a rate such that their voltages are the same.

3. Not really a point when too much is too much so much as it's just not a good idea in general, but not nearly as bad as putting different capacity cells in series though.

4. See 2.

5. DC-DC converter is not an "easy" solution. Try powering a car and running all that power through a DC-DC converter. That makes the DC-DC converter huge, complex, expensive, and needlessly wasteful when you could just connect the cells in series.