Charging two separate batteries with the same MPPT

So this is something I've been mulling over since I reconfigured my van setup last autumn.

I'm running a fairly tidy 12V system — Victron SmartSolar MPPT 100/30, a Fogstar Drift 100Ah LiFePO4, and an Orion-Tr DC-DC charger for topping up from the alternator when I'm driving. All hangs off a pair of busbars, nothing exotic.

But here's where it gets interesting. I've been thinking about adding a separate smaller battery — maybe another 50Ah LiFePO4 — dedicated purely to running my 12V compressor fridge overnight, just to keep the cycles isolated from my main leisure bank.

The question is: can the MPPT sensibly charge both batteries without them fighting each other or confusing the charge algorithm?

My instinct says they'd just equalise across the busbar anyway, making the whole "separate battery" idea a bit pointless unless I throw a VSR or a DC-DC isolator in between. But then you're adding complexity and losses.

Has anyone actually run two discrete LiFePO4 packs off the same MPPT in a van or cabin setup? Did you use any isolation between them, or just let them sit on the same bus and accept they'll behave as one combined bank?

I suspect the cleaner solution is just to go bigger on a single battery and stop overthinking it — but I'd love to hear how others have approached this, especially if you've got real-world experience rather than pure theory.

What's your setup looking like?

@OddJobBob looks like your post got cut off mate — what's the second battery?

That said, charging two separate batteries from one MPPT is doable but gets messy fast. The MPPT sees them as one combined load, so if they're different states of charge you'll get uneven charging. A DC-DC charger (B2B) between the two is usually the cleaner solution — Victron do a decent Orion-Tr Smart that handles it properly.

I've got a similar dilemma with my shepherd's hut and garden office — seriously considered running one MPPT into both but ended up keeping them completely separate. Less faff long-term.

Finish your post and we can give better advice!

@OddJobBob welcome to the forum — great first post topic, and a very common question once people start expanding their systems!

Looks like both you and @WillWebb got cut off mid-sentence, which is amusing. Do finish your thoughts when you get a chance!

That said, the short answer is: it depends entirely on the second battery's chemistry and voltage profile. A Victron MPPT will only run one charge profile at a time, so mixing a LiFePO4 with, say, an AGM on the same controller is genuinely problematic — one will always be charged incorrectly.

If both are LiFePO4 (same capacity, same chemistry, ideally same age), paralleling them before the MPPT is generally the cleaner solution. I did exactly this with two Fogstar Drifts in my van and it's been solid.

Tell us what the second battery is and we can give you proper advice.

Parallel them with proper busbars and let the MPPT see one combined bank — trying to charge two isolated LiFePO4 packs from a single MPPT is a recipe for one battery doing all the heavy lifting while the other sulks in the corner.

If they must stay isolated (different chemistries, different capacities, whatever), you'd need a Victron Orion DC-DC between them — MPPT charges primary, Orion tops up secondary. Done it myself in the motorhome when I added a second Fogstar alongside the original, worked a treat once I got the absorption voltage sorted.

The 100/30 should handle both 100Ah packs in parallel no bother at current, just watch your cable lengths are matched or you'll get uneven charging — physics doesn't care about your feelings.

@FormerCop makes a fair point about paralleling, and that's the cleanest solution if both packs are identical chemistry, capacity, and ideally same age/cycle count.

Worth flagging though — if the second battery ends up being a different chemistry (say, a lead-acid starter battery), you absolutely cannot parallel them. In that case a DC-DC charger (B2B) is the proper approach: the MPPT charges your primary LiFePO4 bank at the correct lithium profile, and a separate Victron Orion-Tr Smart handles the second battery with its own appropriate charge curve.

I ran exactly this arrangement in my Ducato before consolidating to a single 200Ah bank — Fogstar as the house battery, original Fiat lead-acid kept topped up via an Orion-Tr 12/12-18. Rock solid for 14 months.

Curious what the second battery actually is, @OddJobBob — that fundamentally changes the answer.

@DucatoSolar the chemistry point is really important — I've got a mixed situation in my shepherd's hut where I ended up with a Fogstar and an older AGM for a while and you genuinely cannot just parallel those without causing grief.

If you're dead set on keeping them isolated, a DC-DC charger (Victron Orion is the obvious one) between the two batteries solves it neatly — the MPPT charges your primary bank, Orion handles the secondary. Obviously you're losing a bit of efficiency in the conversion but it's clean and safe.

Alternatively two separate MPPTs if you've got enough panel capacity to split. More hardware but each battery gets properly managed charging profiles.

What chemistries are we actually talking about here? That changes the answer quite a bit.