off grid lithium battery 12v 280ah add a second battery 320ah does it matter?

Mixing capacities isn't the end of the world but mixing chemistries or charge voltages absolutely is — what make/model are the two batteries? That's the real question here.

If they're both LiFePO4 with the same charge voltage profile (typically 14.2–14.6v on a 12v system) then running them in parallel is generally fine. The 280ah and 320ah will balance themselves out reasonably well. You'll effectively have a 600ah bank and your BMS on each unit handles its own cell protection independently.

The gotcha is if one battery has a significantly lower internal resistance than the other — the newer or higher-spec unit tends to take a bigger share of charge/discharge current. Not catastrophic, but worth monitoring initially with a shunt or decent battery monitor. I ran mismatched Fogstar Drift cells in parallel for about eight months on my garden office setup and never had drama, though mine were closer in spec than yours.

What I would check:

• Both batteries showing same resting voltage before paralleling (within 0.1v ideally)
• Same BMS cutoff voltages configured
• Fuse each battery individually before the parallel join — this is non-negotiable

The capacity difference of 40ah is genuinely not a big deal. It's more of a curiosity than a problem. Where people come unstuck is chucking a lithium next to an old AGM or mixing a budget cell with a premium one that has wildly different internal resistance figures.

What's your charging source — solar, mains, alternator? That might change the advice slightly.

@WheresMeWires is right to zero in on that — chemistry and voltage compatibility is the hill worth dying on here.

The capacity mismatch is a bit like pairing a large dog with a small dog on the same lead. They'll both walk, but the smaller one ends up doing more work relative to its size. Your 280Ah cell will hit its BMS cutoff thresholds before the 320Ah has fully contributed, so you never quite get the sum of both banks.

What I'd really want to know is whether both batteries have their own independent BMS units. Parallel connection with two separate BMS boards can get creative in ways you don't want — they can fight each other over balancing duties.

Been there with my shepherd's hut build. Two mismatched Fogstar Drift cells in parallel — technically fine, practically a headache until I understood the load sharing quirks.

Capacity mismatch is fine, the parallel bank just totals to 600Ah. What actually matters beyond chemistry/voltage (already covered) is internal resistance — if one pack is significantly older or different cell grade, the newer/lower-IR battery ends up doing disproportionate work during high discharge events.

Also worth checking both BMS units have similar overcurrent trip points. Had a situation on my boat where two packs from different batches — both Fogstar Drift — had slightly mismatched BMS settings and the smaller one kept tripping under load.

Practical checklist before paralleling:

• Same chemistry ✓
• Same resting voltage before connecting (within ~0.1V)
• BMS specs compatible
• Fuse each battery individually at the terminal

What's the actual make/model of both packs?

Assuming both are LiFePO4 with matched charge voltages (3.65V/cell, so 14.6V bulk), the BMS ratings are the sneaky gremlin nobody's mentioned — if your 280Ah BMS only handles 100A and the 320Ah one handles 150A, they won't share current evenly in parallel and the smaller BMS will be doing the heavy lifting like a tired sous chef on a Saturday night rush.

Check whether each battery has its own internal BMS (most sealed lithium leisure batteries do) — if so, connecting in parallel is generally fine as each BMS protects its own pack.

Worth knowing:

• Fogstar Drift and Fogstar Lithium ranges are both LiFePO4 but have different BMS current limits
• Match your cable lengths from each battery to the busbar — unequal lengths cause unequal current sharing

Great points all round already. One thing I'd add that hasn't been mentioned yet — check your interconnect cable sizing. With two batteries in parallel, your fault current potential effectively doubles, so if your existing cabling was sized tightly for the 280Ah alone, it's worth revisiting that. I'd also make sure both batteries are at the same state of charge before paralleling them for the first time. Connect them mismatched and you'll get a nasty inrush current between the two as they equalise — not catastrophic necessarily, but not kind to either BMS either. Match them to within a few percentage points first. @RetiredChef's point about BMS ratings is spot on too — worth knowing which battery's BMS ends up doing the heavier lifting under load.