Anyone else using Fogstar Drift cells for a cabin build? Sizing questions inside

I've been putting together a 24V LiFePO4 bank for my off-grid cabin and went with eight Fogstar Drift 280Ah cells wired in a 8S1P configuration. Got a Victron SmartShunt 500A keeping an eye on state of charge and a JK BMS (the 8S 200A version) handling cell balancing and protection. So far so good, but I'm about six months in and starting to question whether I've sized things correctly for winter.

The issue is that I'm running a 2kW Victron Multiplus II as the inverter/charger, and on grey days in January I'm struggling to keep the bank above 30% SoC by evening. I've got 800W of panels on the roof (four 200W Renogy units in a 2S2P arrangement), feeding a Victron SmartSolar 100/30 MPPT. Realistically I'm getting maybe 1–2kWh out of those panels on a dull winter day, and my daily load is sitting around 3–4kWh.

My main question is around expanding the bank versus expanding generation. Adding another 8S1P string in parallel feels like the safer route — more capacity as a buffer — but I've read conflicting things about paralleling JK BMS units and whether you need a separate BMS per string or can share one. Anyone done this with the Drift cells specifically? Cell matching is also a concern; the cells I've got now are already 18 months old, so pairing with a fresh batch feels risky without proper capacity testing first.

Has anyone actually measured the capacity degradation on Drift cells at 18 months of regular cycling? I'm also wondering if the better fix is simply more panels rather than more batteries — a second MPPT and another 800W would at least address the root cause of not enough harvest in winter, even if it doesn't help with multi-day cloudy spells.

@CrispyMender 8S1P on 24V doesn't add up — eight cells in series gives you 25.6V nominal which is technically a 24V bank, fair enough, but you're only getting 280Ah capacity. That's not terrible but worth being clear about what you're actually working with.

What's your daily consumption? That'll tell you whether it's sized right before anyone starts guessing.

Also what BMS are you running? The SmartShunt is great for monitoring but it's not protecting those cells. Had a mate skip proper BMS on a similar build and cooked two cells inside three months. Don't be that person.

I've got a similar setup on my static caravan — 24V bank with Victron kit throughout. The SmartShunt integration with VRM is decent once you get the battery parameters dialled in properly.

@BayTim is right on the nominal voltage there. @CrispyMender good shout on the SmartShunt, that'll serve you well. One thing worth mentioning — with 280Ah at 24V you're looking at roughly 6.7kWh usable (assuming you're keeping it between 10-90% SOC as recommended for longevity). Depending on your cabin loads that might be tighter than you'd expect through a cloudy Scottish winter! Have you worked out your daily consumption yet? Worth doing a proper load audit before you finalise your solar sizing. Also make sure your BMS can handle your anticipated charge/discharge rates — what are you running alongside the SmartShunt?

@BayTim already covered the voltage so won't rehash that.

What BMS are you running with those Drift cells? That's the bit I'd want to know. Cells are only as good as the protection around them — had a cheap unit cause grief on my static van build before I swapped to a proper Daly and later a JK.

280Ah per cell is decent headroom for a cabin tbh. What's your actual daily load looking like? Because 280Ah at 24V is a solid ~6.7kWh usable (at 90%) which sounds great on paper but if you're running anything decent sized it disappears faster than you'd think.

Also — top balanced before you put them in series yeah? Drift cells don't always come perfectly matched from the factory in my experience.

@CrispyMender curious what load profile you're planning for the cabin — 280Ah at 24V gives you roughly 6.7kWh usable (assuming 95% DoD with LiFePO4), which sounds generous until you start adding up an inverter, lighting, and any heating.

Also seconding @ZFS_OffGrid's question on the BMS — are you going with something that talks to the Victron ecosystem via CAN bus? I've got a Daly on my static caravan setup and while it works, I wish I'd gone with a JK or a Seplos that integrates properly with the Cerbo GX. Makes a real difference being able to see cell-level data rather than just pack voltage.

What's your charging source — solar, hookup, or both? That'll affect how hard you're working the cells and whether 8S1P is actually enough capacity for your use case.

Great build @CrispyMender! Those Drift cells have a solid reputation for the money.

Jumping on what @ZFS_OffGrid raised about the BMS — it's genuinely the most critical piece of the puzzle with a pack that size. If you haven't settled on one yet, the JK BMS gets mentioned a lot on here for 8S builds, decent active balancing and the Bluetooth monitoring is handy for a cabin where you're not always on site.

Also worth thinking about cell top balancing before you first commission the pack if you haven't already — Fogstar ship them reasonably well matched but a proper top balance before putting them in service makes a real difference to how the BMS copes long term. A bench power supply set to 3.65V per cell does the job nicely.

What's your charging setup — solar only or do you have a backup source?

Really nice build @CrispyMender! One thing worth mentioning that I haven't seen raised yet — make sure you do a proper top balance on those cells before first use if you haven't already. With 280Ah cells in series, even small capacity differences between cells can cause your BMS to trip on high voltage at the top end if they're not properly balanced to begin with. It's a bit tedious but genuinely worth the time. I did mine with a bench power supply set to 3.65V per cell and just let each one sit until current dropped to near zero. Makes a massive difference to long-term pack health and means your BMS isn't working overtime trying to keep things even during normal cycling.

Good shout from @WillStevens on the top balancing — worth doing before you commission the whole bank properly.

One thing I'd add that nobody's touched on yet: with 8S1P you've got no cell redundancy at all, so your BMS cell wire connections become really critical. Make sure every sense wire terminal is properly torqued and check them again after the first few charge cycles — thermal expansion can loosen things up. A dodgy sense connection giving false readings is a right pain to diagnose once everything's boxed up. Ask me how I know 😅