BMS balance leads safe with secondary device?

[BlitzSSS]

Hi All

Is there a risk in damaging my BMS by running multiple devices that use balance leads over the same battery pack?

Examples might be the BMS and and active balancer or an ISDT charger for one of its functions such as balancing or discharging.

Could any of these types of devices contradict eachother and do damage?

(hopefully not a stupid question)

Thanks

 

[Steve_S]

There IS potential for some trouble here.

BMS wire harness are only intended to handle the voltages / amperage that the BMS if capable of. For example, a BMS with 1.2A Passive Balancing will have 20g or 22g wire. Also having a BMS which has Passive Balancing and adding an active balancer into the mix will more than likely cause a conflict, therefore disabling the passive balancing is almost mandatory. An Active Balancer (depending on model & type) could output up to 10A, so obviously the wire sizing has to support that level. The big QNBBM Balancer recommends a minimum of 17 gauge wire.

A separate Battery Monitor (non charging or interactive with the cells/pack), "may" be okay sharing the same leads but personally, I think that's something you would have to consider if it is worth the potential risk or hassle to debug if something is amiss.

IF you want to connect a BMS & Active Balancer together for a battery pack for example, the most effective method would be to use a "Connection Terminal Strip" as shown below. Run the BMS, Balancer or Monitor leads into one side and then an approrpiate gauge from the block to the cells. You MUST ensure that you keep the wires in proper order, so Cell-1 + from the BMS and Balancer are going to the same lead going to the cell-1, cell-2 and so on. In some instances, it is recomended to use a Fused Strip (with high amp Active Balancers) which is similar to the Molex shown but have ATC type fuses also shown below.



LINKS TO CONNECTORS:
Molex: https://www.digikey.ca/en/products/detail/molex/0391002109/4988621
Hella: https://www.summitracing.com/parts/hla-h84960111 (An Amazingly GOOD DEAL BTW !)
* WARNING * Beware of Cheap Fuses and "discount imports" Use only high quality OEM Quality certified & tested fuses.

CASE IN POINT:
I am using Chargery BMS8T's for my 4 LFP Packs., Each pack is getting a QNBBM-8S Active Balancer. Passive Balancing on the Chargery BMS' will be disabled (they conflict). The Chargery & Balancer leads will go into one side of the Molex Connector and 14Ga (fine strand) out to the cells from there. I am assembling these in their final configuration this week as I am completing my powerhouse overhaul & update.

Hope it helps, Good Luck
Steve

 

[BlitzSSS]

Hey Steve, thanks for your reply but I don't think I conveyed my concern too well. I'm not worried about the current handling of the wire/balance leads themselves more so any conflicts of the devices potentially trying to perform different functions over the battery pack.

For example if I have a BMS (Overkill) connected but also want to periodically plug in my ISDT charger to discharge the pack/cells to a predefined voltage or perhaps use it because it can balance the cells at higher rates than the BMS, the ISDT charger has it's own balance leads and I'm asking about connecting this simultaneously directly to the cells along with the BMS.

Is this safe to do or could one device potentially short out the other for example? or as a matter of basic principal should I always disconnect the BMS?

Thanks

 

[Steve_S]

The overkill BMS has Passive Balancing, per Page 20 of their manual. The ISDT is just another low end Passive Balancer. If you were looking at an Active Balancer they can go up to 10A and require better wiring as a result.

 

[Steve_S]

Li-Ion BMS - White Paper - Dissipative vs. nondissipative balancing (a.k.a.: Passive vs. Active balancing)