2P2S 4 x 12v 300ah Lithium Configuration for 24v with balancers. Heeeeeeeelp

Been down this exact rabbit hole with my motorhome build last spring. Four 280Ah cells in 2P2S gave me grief until I stopped fighting the configuration and just accepted that parallel first, series second is the only sane way to wire it.

The balancer conversation is where it gets spicy though. I've got a Victron Smart BMS on mine and honestly the active balancers are doing most of the heavy lifting between the parallel pairs before the voltage even sees the series connection. Fogstar cells were surprisingly well-matched out of the box, which helped enormously.

A few things worth checking before you pull your hair out:

• Are your parallel pairs truly balanced before you connect them in series for the first time? This step trips up almost everyone
• What BMS are you actually running? Some handle 24v 2P2S natively, others throw a wobbler
• Busbar sizing between the pairs matters more than people admit

The real question I'd throw back to the group is — are the balancers you're using passive or active? Because passive on a 2P2S 24v setup is a bit like putting a sticking plaster on a leaky pipe. Works until it really doesn't.

Someone on here mentioned using Daly active balancers across the series connection rather than each individual cell — curious whether anyone's actually done that and measured the results over a full season?

@Nessa68 parallel-first is the only sane way to go with prismatic cells if you value your sanity.

Did mine in the van eighteen months back. Four Fogstar 12V 200Ah batteries — same topology you're describing. The thing most people don't mention is that your parallel pairs need to be balanced to within 0.05V before you ever connect the series link, otherwise you're just shoving current through the interconnects like a fool and wondering why your BMS is throwing fits.

I used a cheap Victron SmartShunt to monitor each bank independently before committing to the final configuration. Took an afternoon. Dull, tedious work.

The balancers help maintain equilibrium — they won't fix a fundamentally lopsided starting point. Get the cells matched first, then let the balancers do their job.

What BMS are you running?

@Nessa68 280Ah cells aren't 300Ah cells though, worth being precise about that.

On the actual question — parallel first, then series is correct for 2P2S, yes. Your parallel pairs balance passively at cell level before you ever bring the series connection in. That's the point.

What balancers are you actually running? Because slapping an active balancer across a 24v pack without understanding where it's connected is how people cook cells. Seen it happen on here more than once.

Also — are these cells properly top-balanced before assembly? Because if not, no balancer on earth is going to save you from a headache. Sort that first, everything else second.

What brand are the cells? Fogstar Drift or similar, or something off AliExpress with optimistic capacity ratings?

@BatteryTim pedantry appreciated but my Fogstar cells shipped as "280Ah" and arrived as "302Ah" so perhaps the cells didn't get the memo either.

@ThistleVicky that capacity variance is completely normal with prismatic LiFePO4 — most reputable cells ship slightly over-rated. My Lishen 280Ah cells on the narrowboat all tested between 295–308Ah on first cycle.

Back to the actual topology question though — parallel-first is settled science for this configuration, but the bit nobody mentions is busbar quality. Sloppy connections in a 2P2S arrangement mean your parallel pairs never truly equalise under load, which defeats the purpose entirely. I used 25mm² flexible tinned copper between my parallel pairs before seriesing them up to 24V.

Also worth noting: a single BMS across the whole 24V pack is cleaner than dual BMS on each parallel pair — less opportunity for protection conflicts under high discharge.

What cells are you actually using? That'll determine whether the internal resistance matching between your pairs is even worth worrying about.

@PN_Camper yeah this is pretty standard stuff — my narrowboat runs a 2P2S 24v bank and the key thing nobody mentions early enough is cell matching before you even wire anything up. Internal resistance matters more than capacity when you're paralleling.

On the balancers — passive or active makes a difference here. I've got an active balancer across each parallel pair and it's been rock solid. Victron kit talking to it nicely via the BMS.

Parallel first is definitely the right call for this config, OP. Gets your capacity sorted before the series step and the cells naturally self-balance within each parallel pair.

What BMS are you actually running? That'll change the advice quite a bit.