Anyone ever use cells in parallel in a battery pack?

[sunsurfer]

I was thinking the other day, why not use some cells in parallel to increase ah capacity without having to build multiple batteries(multiple bms)?

Then it occurred to me, this could be useful if there are weaker cells in a pack since there would be a parallel neighbor to help carry the work.

Example; a 12v of 16 cells with 4P4S. Only issue would be a self discharge issue but does lithium even have that problem like lead acid? If a cell went bad, would it bring down the others in parallel?

 

[MisterSandals]

12v of 16 cells with 4P4S.

There are quite a few of us that run 2PnS battery configurations.

4PnS are less common but a search will find a bunch.

The trick to a successful parallel first configuration is to match the cells into groups so that the combined capacities are equal to the other groups.

For a 2P battery, its pretty straight forward: pair strongest with weakest, pair second strongest with second weakest...

There can quite a few ways to make the group capacities equal. I don't recall any tools to help suggest groupings.

This of course requires capacity testing all the cells and this is a lengthy process. I've tested dozens of cells and paired a few 2P batteries so if you have questions please ask. There are many others that have good info and experience too.

 

[Supervstech]

I was thinking the other day, why not use some cells in parallel to increase ah capacity without having to build multiple batteries(multiple bms)?

Then it occurred to me, this could be useful if there are weaker cells in a pack since there would be a parallel neighbor to help carry the work.

Example; a 12v of 16 cells with 4P4S. Only issue would be a self discharge issue but does lithium even have that problem like lead acid? If a cell went bad, would it bring down the others in parallel?

Tesla uses a 14p6S packs...

 

[timselectric]

I wouldn't parallel cells, without placing a fuse between them.
If a cell developed a short, the others in parallel will dump all of their power into it. Without protection between them, this will be a very bad day.

 

[MisterSandals]

If a cell developed a short, the others in parallel will dump all of their power into it. Without protection between them, this will be a very bad day.

Agreed, but have you heard of this happening and do you have something you can reference that shows this has happened?

 

[timselectric]

Agreed, but have your heard of this happening and do you have something you can reference that shows this has happened?

No
But I haven't looked.
Knowing that the risk exists, is enough for me.

 

[time2roll]

I have 2x 2p4s batteries doing fine three years running. No fear.

 

[sunsurfer]

Well this is encouraging

 

[pollenface]

In all batteries I've dismantled never seen a fuse between parallel cells.

 

[timselectric]

In all batteries I've dismantled never seen a fuse between parallel cells.

Sometimes the connection is designed to be the fuse.
Like in the 18650 cell batteries. The connection will melt, if the current exceeds the limit.

 

[Ampster]

I currently run a 3P16S pack. I am not concerned about the risk of a cell shorting in that configuration.

 

[sunshine_eggo]

Tesla uses a 14p6S packs...

Ummm... Model S 6S Modules are 444 cells... 74P6S :D

 

[pollenface]

Sometimes the connection is designed to be the fuse.
Like in the 18650 cell batteries. The connection will melt, if the current exceeds the limit.

Didn't know that, thanks

 

[lotsofsparks]

Tim electric brings up a good point, A 4s would have 1/4 of a single cells internal resistance and have 4 times the short circuit current. In AC breakers there is a KA rating (The maximum current arch the breaker can extinguish) that must selected to be higher then highest possible short circuit current. DC breakers would have to have some similar rating. 12 volt systems have more power losses but I have 12 volt gear so I wouldn't mind investing in 16 EVE cells 4p4s and when my inverter dies just reconfiguring it to 16s and upgrading to 48 volts instead.