Bms requirement

[Charleswgibbs]

When connecting multiple batteries in parallel, each with their own bms, is there still a requirement to have an additional bms for the overall bank to make sure each battery is equal to the other?

I know that with parallel batteries the main pos and neg cables need to be hooked up to opposite sides of the bank to ensure the current gets shared equally but wasn't sure if this alone was enough to ensure the batteries stay balanced. Is checking the batteries on a regular basis with a multimeter to ensure equal charge enough or is additional equipment required?

Thanks!

 

[BentleyJ]

First a clarification: Most BMS products do have some kind of cell balancing built in but with larger batteries they tend to be inadequate. For this discussion lets assume that the multiple batteries you mentioned have a BMS that will take care of things inside each individual battery without further intervention.
Now when those "self contained" batteries are connected in parallel they do not need any further cell balancing or BMS linking them together. This is because when in parallel the batteries will self correct based on their individual voltage and state of charge.
For example, lets say you connect 2 x 100Ah batteries in parallel. One battery is at 50% SoC and the other is at 80% SoC. As the absorption cycle proceeds at constant voltage, the higher SoC battery will build voltage faster forcing a larger portion of the current into the lower SoC battery. Eventually, both batteries will reach full charge and the controller will switch to float or turn off. Its possible to parallel batteries even if their Ah capacities are not the same.

 

[sunshine_eggo]

When connecting multiple batteries in parallel, each with their own bms, is there still a requirement to have an additional bms for the overall bank to make sure each battery is equal to the other?

No. When in parallel, the two battery voltages MUST be the same (they're in parallel). Each BMS monitors its own cells, and there really isn't such a thing.

I know that with parallel batteries the main pos and neg cables need to be hooked up to opposite sides of the bank to ensure the current gets shared equally but wasn't sure if this alone was enough to ensure the batteries stay balanced. Is checking the batteries on a regular basis with a multimeter to ensure equal charge enough or is additional equipment required?

"balance" refers to multiple things above.

Again, both batteries must be at the same voltage at all times as they are in parallel. If one goes lower, the higher one discharges into it bringing it up.

Opposite post connections is to balance the current flowing through the bank with the idea that the batteries will share equal portions of the current. This is mostly a function of circuit resistance. Ensuring your crimps are high quality and your connections are properly torqued is a good way to ensure balanced loads.

"Balance" as it pertains to a BMS/LFP battery, is that the cells are all at the same state of charge, or at at least are when all cells hit true 100% SoC. That's a function of the BMS.

 

[Charleswgibbs]

Thanks for the fast replies! That makes perfect sense about them self leveling and the clarification about the battery terminal orientation is appreciated.

Thanks again!

 

[Charleswgibbs]

Had another quick question regarding a bms, I was watching wills tutorial here:

I was a bit confused by a few things in the video. He uses a balancer to balance the cells initially but then doesn't use a bms on the battery. I thought a bms was a requirement for lifepo4 batteries?

The other was towards the end of the video at 24:30 he says it's bad to connect in parallel and recommends instead buying larger cells. Does he mean connecting cells in parallel isn't recommended, but connecting batteries in parallel is still ok? I see tons of setups where people run a lot of batteries in parallel so I wasn't sure what he meant.

Thanks again!

 

[sunshine_eggo]

Description:

 

[Charleswgibbs]

That's what I get for not reading! Sorry!

What about the warning against running in parallel? Was it just referring to cells in parallel and not batteries?

 

[sunshine_eggo]

"This video is outdated."

 

[Charleswgibbs]

Gotcha, thank you.

 

[Goboatingnow]

do not connect Li batteries of differing SOCs together , huge currents can flow as one charges the other . Li batteries must be separately charged to near identical voltages and then paralleled, once they are permanently connected they will remain at the same terminal voltage , if they ever get of balance, recharge them separately to a common terminal voltage

you don't "need " a BMS, but then you have no safety features, at a minimum for LFP you need cell monitoring , for high voltage , low voltage , over temp and over current, Active cell balancing isn't generally needed in low C environments , it can be done manually now and again if needed