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On a commercial build with an ignorant owner, absolutely. It's reasonable to expect the car "just work". Then again, EV West's customer service seems to be "We'll take your money but ignore you after that" anyway.But then again, EV West often doesn't even use BMSes with lithium batteries, which I find to be honestly idiotic and dangerous.
On a DIY project, ehn, not a big deal. Power is flowing in and out of the whole string equally, the BMS is really only there to touch up and adjust microscopic changes in how the cells age over time, and is only relevant at extremes. If you have the skills to manually check the cells once a season, or before a long roadtrip where you'll be using up the entire pack, you're probably fine.
What I know for certain at this point:
- You need a BMS for each parallel string[/quote]
What you shouldn't do is connect the endpoints of each pack together and presume it will be okay.
But if you also connect a balancing lead for each cell to its corresponding mirror cell, (making the pack in parallel cell-by-cell as well) I don't think you'll ever see a problem running only a single BMS.
There's a debate about it here in the past, it would have to be the most extreme of extreme cases where the wires you connect them with won't be able to supply enough balancing for the amount of power being used and would melt. This would require a massive, sustained amp drain (racing), on already failed or failing cells, that you somehow haven't noticed yet. If cell #13a is weak on one side, and #13b is trying to rapidly add energy to it because of the voltage imbalance, the balancing wire might try to supply more current than the conductor can carry. I see it as all but impossible to be concerned about. The thicker the balancing wire used, the less a concern it would be.
Why would it be dangerous to leave two parallel strings connected?
Batteries are already in parallel, so it would have to be some circumstance that is unique to having a second pack there.