# Practical Considerations of using batteries in parallel + parallel Vs series

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

1. 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

2. 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.

• As mentioned in the answer, batteries in parallel keep each other balanced. That said, you shouldn't pair together batteries which have very different capacities when tested by themselves. Laptop battery packs contained li-ion cells in parallel, and once they two cells are paired together they are treated as just one cell with around double the capacity of a single cell. That is, they are always charged and discharged together. Jun 4, 2021 at 3:54
• @ErikR tested the capacities of the two cells. They are the exact same model which have a nominal rating of 500mAh. One was 512mAh and the other one was 495mAh
– fred
Jun 4, 2021 at 3:55
• Those are pretty close... you shouldn't have any problem using the in parallel. Jun 4, 2021 at 3:56
• How do you calculate that? As I said, always use them together as a parallel unit. Paralleled batteries work just fine in a lot of consumer devices. Jun 4, 2021 at 4:03

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.

• From this then I assume the designer will have to balance the sate of charge of each battery to ensure they do not reach 0%? I guess one of the reasons as to why this confuses me is because voltage and state of charge are related
– fred
Jun 4, 2021 at 3:48
• @fred Voltage and SOC are related, but the relationship is not linear, and in parallel the batteries only care about having the same voltage. Jun 4, 2021 at 3:50
• 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 nominal capacity. If nominal was 500mAh then some might be 490 some might be 510 etc.
– fred
Jun 4, 2021 at 3:52
• @fred Still not linear. Voltage vs SOC of a battery being nonlinear is in and of itself without need to be different from other batteries chemistries to be such. Jun 4, 2021 at 3:53
• @fred No. Assuming identical cells but different capacities, a 50% full battery has the same voltage as another 50% full battery regardless of capacity. Batteries are like buckets. Volume is capacity, the height of the water is the voltage and the volume of water in it is the charge. If they are the same chemistry, their full charged voltage is the same, ergo same bucket height regardless of bucket volume. Does the 50% full large bucket have a different water level than the 50% full small bucket? Connecting in parallel is like butting up the buckets against each other and removing the divider. Jun 4, 2021 at 3:58