Multiple jk bms battery in parallel

Running multiple JK BMS packs in parallel in my shepherd's hut and honestly it's less drama than I expected — the JKs do a decent job of keeping things civil between packs if you let them balance out before lumping them together.

Key thing I've learned the hard way: don't just slam two packs together at different states of charge — pre-charge resistor or at minimum let them sit at the same voltage first, otherwise you're basically watching your cells have an argument at full volume.

A few things worth discussing:

• Are you wiring battery to busbar rather than daisy-chaining? Daisy-chaining parallel packs is a classic mistake that causes uneven loading
• What gauge cable are you running between packs and the busbar? Keep them equal length or the physics will punish you
• Anyone using the JK's active balancer across parallel packs or just letting the BMS protection handle it?

My setup is two 280Ah Fogstar Drift packs with JK 200A BMS units — running into a Victron Multiplus — and the main gotcha was getting the JK low-voltage disconnect thresholds matched between both units, otherwise one BMS trips early and the other pack ends up doing all the heavy lifting like a tired carthorse.

What's your pack sizes and how many are you planning to parallel up? Makes a difference to whether you need a proper pre-charge circuit or can get away with being a bit cavalier about it.

@TorFinn interesting to hear that — I've been wondering about this for my motorhome backup setup. Quick question: are you connecting them straight to the same busbars, or are you using any balancing resistors / fuses between packs?

I've seen some folk recommend putting a small resistance in each parallel leg to stop one pack dumping into another on connection — apparently the inrush can stress the MOSFETs on the JK if the packs are at different states of charge when you hook them up.

Also curious whether you're running the JK BMS units as standalone or daisy-chaining them somehow to a common Victron setup? I've got a MultiPlus-II and wondering how to get meaningful comms from multiple BMS units without it becoming a nightmare. Does the JK even support that configuration natively?

@ExTrucker73 was going to ask if you're running them direct parallel or through a busbar, I'd guess?

For what it's worth, in my static caravan I've got two JK-protected packs tied together at a busbar and the trick that made the biggest difference was letting them sit connected but unloaded for a good few hours before first use. They'll drift toward each other naturally rather than fighting. Had one pack trying to push 40A into the other when I rushed it initially — not ideal.

The JK's active balancer actually helps here more than people give it credit for. Just make sure your cable lengths from each pack to the busbar are identical — same gauge, same length. Sounds pedantic but it genuinely matters for passive current sharing.

Been solid for about eight months now on my Fogstar cells.

@TorFinn done the same with my marine setup — two JK packs sitting side by side like a couple of old sailors who've decided to stop fighting over the last biscuit.

The key thing nobody mentions: let them rest at the same state of charge before you connect them. Skip that step and you've basically introduced two packs with very different opinions

@AZY_Marine that's a great way to put it 😄

Running two packs myself in the cabin — let them sit at the same SOC before connecting and they've been pretty well-behaved since. JK's active balancer definitely helps smooth things over once they're in use.

One thing worth mentioning — I'd strongly recommend a proper busbar over direct parallel wiring if you're adding more than two packs. Keeps cable lengths equal and avoids one pack doing more work than the other. Grabbed some decent copper busbars off Amazon, nothing fancy.

@ExTrucker73 for a motorhome backup I'd probably keep it to two packs max unless you've got the space and wiring sorted properly. Victron's monitoring makes it easy enough to keep an eye on both packs independently too, if you're already in that ecosystem.

Worth noting the JK BMS has configurable cell overvoltage and overcurrent thresholds — if your packs aren't exactly matched in capacity, the weaker one will hit its limits first and drop off. Had this exact issue on my narrowboat before I sorted the settings.

Recommend setting both units identically and leaving a bit of headroom on the overcurrent threshold so neither BMS trips under load. The "start balance voltage" parameter matters too — keep it consistent across both units or they'll be fighting each other during absorption.

@TorFinn what cell chemistry are you running? LFP I'd assume, but worth confirming since the voltage windows differ quite a bit between manufacturers even for the same chemistry. Fogstar cells for instance sit slightly differently to EVE at top of charge in my experience.