"Smart" Parallel battery packs w or w/o BMS?

[Lonemi]

Dear Users,

I was looking for a solution for many days regarding this simple problem.

I have two 1200Wh 8S NCM battery packs and I am trying to wire them in parallel in order to double the capacity; the problem is that I am not sure where and when to use a BMS or an active balancer in the wiring solution.

I have attached 3 wiring options in order to check the pros and cons of each one, and I leave these here for you to judge.

The requirements I have are: Semplicity, low cost and effectiveness.

Option 1: Two active balancer and 1 hardware bms.
Balancing is made from the two balancers and BMS just protects battery pack from under/over current and voltage, temp. protection.

Option 2: One Smart BMS and 1 active balancer
The smart bms takes in charge the balacing and all other features, while the other battery pack is simply balanced by a second active balancer.
Since the second battery pack "should" behave like the first one, an active balancer should be the most cost effective option while keeping the full balance of the first pack via BMS.

Option 3: One Smart BMS
The Smart BMS balance cables are connected to both balance connectors of the two battery packs resulting in a "parallel balance cables" connection.
Even though I am not sure if this can be done, or if this can create damages when balancing, this could be the simpler and most effective option.

Please let me know what you think and what could be the best solution for you.

Thank you very much

 

[MisterSandals]

Why not the standard 1 BMS per 8S battery?
Are you thinking you need or want a balancer beyond what most smart BMSs do?

 

[Lonemi]

Yes, if I use one BMS per battery pack it will surely work, but it will be twice expensive.
In the end the function of the second BMS, since the total voltage of the two battery packs will always be the same, is just to equalize the 8 cells.
No I do not have a need for any special function of the BMS, just the normal ones.

So I was wondering if there was a way to optimize and make the system much simpler and cheaper

 

[MisterSandals]

So I was wondering if there was a way to optimize and make the system much simpler and cheaper

The only way i see to use a single 8S BMS with 16 cells is to configure as 2P8S. I cannot tell if your "packs" are workable to make a different configuration (other than 8S).

In the end the function of the second BMS, since the total voltage of the two battery packs will always be the same, is just to equalize the 8 cells.

Understood but you do so without any protection for the battery without a BMS. You're using a dangerous lithium so that makes it even worse.

 

[Lonemi]

The only way i see to use a single 8S BMS with 16 cells is to configure as 2P8S. I cannot tell if your "packs" are workable to make a different configuration (other than 8S).

The battery pack are bought like so and enclosed in heatshrink, there is no way for me to reconfigure it.

Understood but you do so without any protection for the battery without a BMS. You're using a dangerous lithium so that makes it even worse.

But since the only one BMS is after the parallel connection between the two packs, it should still protect the pack without the BMS from Under/over voltage/current, temperature protection etc. right? In theory it should only not have the balancing function maybe?

 

[MisterSandals]

, it should still protect the pack without the BMS from Under/over voltage/current, temperature protection etc. right?

No, not at the cell level where almost all problems exist. If a single cell in your unmonitored battery goes rogue, there is no protection.

 

[Lonemi]

No, not at the cell level where almost all problems exist. If a single cell in your unmonitored battery goes rogue, there is no protection.

Right, so even if the pack without BMS has an active balancer on it, it cannot disconnect the single pack or even notice a problem.

It seems the only option is to have 2 BMS...