Connecting batteries in parallel — do's and don'ts

Spot on points from everyone. Real talk from my van setup — I learned this the hard way with mismatched LiFePOs.

The bit that caught me out was internal resistance creep. Even identical batteries drift over time, and parallel strings mask this until one battery's tanking while the other's still chirpy. That's when things get toasty.

What actually saved my rig: monitored each string separately with a cheap shunt before they hit the main bus. Victron's SmartShunt does the job. Means you can clock which battery's drifting before it becomes a problem rather than after you've got a fire hazard.

Also worth noting — if you're running dissimilar chemistries (which I wouldn't recommend but people do), the BMS on each needs its own isolated contactor. Learned that from watching someone else's van nearly go up in flames at a gathering.

Keep the runs short and identical gauge too. Every mm of cable length difference adds resistance variation.

Been there with my static caravan setup — mixing battery ages is asking for trouble. @BMS_Geek's spot on about BMS firmware versions too. I'd add: parallel strings need identical cable lengths and gauges, or you'll get circulating currents that'll cook one bank while the other sits idle. Learned that one the expensive way.

Learned this one painfully with my shepherd's hut setup. Parallel batteries need matched internal resistance or you'll get current imbalance — older cells discharge faster and cook themselves. My Victron shuts down now if they drift more than 0.1V apart. Worth fitting a proper BMS from the start rather than bodging it later.

Aye, the "it'll be fine" method works great until your battery bank decides to become a glorified space heater. I've got three matched Fogstar units in parallel here—took me ages to source identical stock dates. Worth every penny when you're not constantly babysitting voltage creep at 3am.