Running dissimilar lithium batteries from a single AC-coupled inverter (~5kWh/48V)?

Bit of an odd situation I've landed myself in and wondering if anyone's tackled this before.

I've got a Victron Multiplus-II 48V sat in my garden office setup, and I'm looking to expand capacity without replacing what I've already got. Currently running a 100Ah LiFePO4 pack (Fogstar Drift cells) but I've been offered a decent price on a second-hand 48V pack from a different manufacturer — different chemistry specs, slightly different charge voltages, and obviously its own BMS.

The question is whether it's viable to run both packs in parallel from a single AC-coupled system, or whether the two BMSs fighting over charge/discharge thresholds is going to cause headaches long-term. I've read conflicting things — some say as long as the resting voltages are close enough it's broadly fine, others say you're just asking for one BMS to constantly cut out while the other does all the work.

A few specific things I'm uncertain about:

• Does the Multiplus-II care about seeing two separate BMSs via DVCC, or does it just follow whoever's shouting loudest?
• Is there a practical way to balance the state of charge before paralleling them up?
• Anyone running something like this in a van conversion or similar space-constrained setup where replacing the whole bank isn't realistic?

Feels like the kind of thing that should work in theory but has a hundred ways to go wrong in practice. Would be curious whether anyone's actually lived with a mixed-pack parallel setup rather than just theorised about it.

@FormerMariner this is a classic "don't do it" situation but there are ways to make it work safely.

The core problem is that dissimilar lithium packs will have different internal resistance and SOC curves, so if you parallel them at the battery level you'll get current hogging — the lower-resistance pack takes the hit every time.

Proper approach: run each pack through its own BMS to a common DC bus, and critically, make sure both BMS units can communicate or at least independently disconnect without creating a floating bus situation.

If one pack is Victron-compatible (DVCC via CAN-bus), you'll need to check whether your secondary pack's BMS plays nicely or if you're running it in "dumb" mode with fixed charge parameters.

What packs are you actually looking at? Fogstar Drift or something more custom? That changes the answer considerably.

Ran two mismatched lithium packs on my narrowboat for a while — the trick is never letting them charge each other directly; isolate them with a Victron Lynx Distributor and let the Multiplus-II treat them as one bus only at the DC level. Separate BMS units per pack, both talking to the Cerbo via CAN, so whichever hits its limits first pulls the emergency brake on the whole system. Fogstar Drift cells are surprisingly tolerant of this arrangement in my experience. The real gotcha is charge voltage — if your packs have different cell chemistry or age, you'll need to set charge voltage to the lower pack's maximum, which means the newer/better pack never quite fills. Not ideal but beats a thermal event in your garden office.

What BMS are you running on each pack? That's what I'd want to know first.

On my boat I've got a similar dilemma brewing — I've been eyeing up a second Fogstar Drift but it wouldn't match my existing pack's cycle count, let alone chemistry if I went a different route.

My understanding is the Multiplus-II can only really "see" one battery bank — so are you planning to parallel the packs physically, or keep them electrically separate somehow?

Also, does your existing pack have a CAN bus connection to the Victron via DVCC? Because that complicates things further if the second pack has a different BMS protocol. The Cerbo GX can only follow one battery's SOC signal at a time, can't it?

Genuinely asking as I might face the same decision soon.

Good points from @SmartSolar_Queen on the BMS question — that's genuinely the crux of it.

What people often overlook is the resting voltage differential between dissimilar packs. Even if both are nominally 48V LiFePO4, a 20–30mV difference at rest will drive equalisation current between them when paralleled directly. With larger capacity packs that current can be surprisingly destructive to the weaker BMS.

My shepherd's hut setup uses a Victron Lynx distributor with individual fusing per pack precisely for this reason — each bank connects to the busbar independently, never directly to each other. The Multiplus sees one combined voltage but the packs can't fight each other through a low-impedance path.

If your BMS units have CANbus or VE.Can, getting them talking to the Multiplus properly transforms the situation — charge limits from each BMS get respected rather than guessed at.

Good shout from @HighlandExplorer but I'd add the voltage calibration piece is often where this falls apart in practice.

Even if both BMS units are "happy," if the resting voltage curves differ between chemistries (say one's LFP and one's a slightly dodgy LiFePO4 clone with different cell grades), the Multiplus has no idea — it's just seeing a combined bus voltage.

On my garden office setup I ran into this even with two packs from the same brand. State of charge reporting became basically meaningless until I sorted proper shunt monitoring with a Victron BMV-712.

@FormerMariner — what are the actual battery models you're pairing? Makes a big difference whether we're talking matched chemistry or genuinely dissimilar cells. Some combos are manageable, others are asking for trouble.