How do parallel battery packs not have issues?

[Legolessed]

This is me being curious and not being able to find much information about the subject online. With batteries in series I know you can use a balance plug to make sure one cell doesn’t charge more than another but what do you do with parallel battery setups. Do you just make sure they are all the same voltage when you start? Sorry if this is a really dumb question.

[WereCatf]

1 minute ago, Legolessed said:

This is me being curious and not being able to find much information about the subject online. With batteries in series I know you can use a balance plug to make sure one cell doesn’t charge more than another but what do you do with parallel battery setups. Do you just make sure they are all the same voltage when you start? Sorry if this is a really dumb project.

Since they're all in parallel, they automatically balance themselves.

[Legolessed]

1 minute ago, WereCatf said:

Since they're all in parallel, they automatically balance themselves.

Oh wow I’m dumb. What happens if you have batteries with different capacities though?

[WereCatf]

Just now, Legolessed said:

Oh wow I’m dumb. What happens if you have differently sized batteries though? 

They still balance automatically. If you're discharging the battery-pack, there are some efficiency-losses with different capacity cells because the smaller-capacity one will empty faster than the higher-capacity one, so there will be some flow of power from the higher-capacity one into the lower-capacity one to keep it at the same voltage while also discharging into your load. Still, it's usually not much of a concern.

[Legolessed]

1 minute ago, WereCatf said:

They still balance automatically. If you're discharging the battery-pack, there are some efficiency-losses with different capacity cells because the smaller-capacity one will empty faster than the higher-capacity one, so there will be some flow of power from the higher-capacity one into the lower-capacity one to keep it at the same voltage while also discharging into your load. Still, it's usually not much of a concern.

Thank you so much! 

[Unimportant]

17 minutes ago, Legolessed said:

Do you just make sure they are all the same voltage when you start?

Rather importantly, yes! When you connect the batteries together to make a pack they should be balanced. If you put 2 batteries with different charge levels (different voltages) in parallel the 2 batteries will balance themselves, but with no current limit, it would be violent. Once connected in parallel there is no issue - look at them as communicating water vessels.

[Hackentosher]

16 minutes ago, Legolessed said:

Oh wow I’m dumb. What happens if you have batteries with different capacities though?

Yeah it's fine, but you can't parallel batteries with vastly different voltages though because when you connect them, they will dump a shit load of current the instant you connect them, potentially causing a thermal runaway. Don't cause a thermal runaway. 

 

Rule of thumb for parallel charging batteries, only connect batteries that are +/- 0.2V from each other.