Adding a mismatched battery to EG4 off-grid system

Been wrestling with this exact dilemma for my garden office setup. Currently running a 48V system with a pair of Fogstar Drift 100Ah LiFePO4 batteries and a Victron MultiPlus, and I've been tempted to add a spare 50Ah battery I've got sitting around from a previous build.

From what I've read, mixing capacities can work if the chemistry and voltage are matched, but the BMS on each unit will still protect itself independently — which means the smaller battery could hit its cutoff threshold well before the larger ones, effectively taking itself out of the loop early. You'd essentially be paying for capacity you're not fully using.

A few questions I keep coming back to:

• Does anyone have real-world experience running mismatched capacities on the same bus bar, rather than just theory?
• Is there a meaningful difference between connecting them in parallel at the battery level vs. letting the inverter/charger handle it?
• Would it be worth just selling the 50Ah and putting that money toward a matched unit?

I'm also wondering whether the EG4 BMS handles this differently to, say, a Victron Smart Lithium setup — the EG4 ecosystem seems to have its own quirks when it comes to battery communication.

Anyone here actually running a mixed bank long-term? Curious whether it causes headaches down the line or whether it's genuinely fine in practice once everything's dialled in.

@SaltyRigger mate, don't do it — mismatched LiFePO4 batteries on a shared bus is basically asking two mates with different appetites to share one plate of chips, the greedy one (higher capacity/lower internal resistance) will always hog the charge current and you'll cook your BMS before you can say "warranty void" 🔥

If you're dead set on expanding, grab another matched Fogstar Drift and parallel it properly — Fogstar's own guidance actually recommends identical batch numbers where possible for balanced internal resistance.

Alternatively, keep the new battery on a completely separate Victron SmartShunt-monitored string and let a DC-DC charger bridge them — more faff but your existing setup stays happy and your Victron doesn't throw a tantrum.

@BoatPaddy is right, and I learned this the hard way on my boat when I shoved a spare Fogstar alongside a Renogy — one of them was basically freeloading while the other did all the heavy lifting like a tired intern.

Different internal resistance = different charge/discharge curves = your BMS units having a proper argument at 2am when you're trying to sleep. If you must expand, grab another matched Drift 100Ah and wire them in parallel properly, or look at a separate battery bank with its own MPPT and let the Victron sort out the DC coupling — at least then everyone's playing by the same rules.

@StormyDrifter that's exactly the kind of horror story that keeps me up at night honestly

Worth adding — if you're absolutely determined to add capacity, separate battery banks with isolated DC-DC charging between them is the cleaner workaround. Victron's Orion-Tr Smart handles this reasonably well. Each bank talks to its own BMS properly that way.

I did something similar expanding my shepherd's hut system. Two separate Fogstar banks, Orion between them — belt and braces approach but it works flawlessly as an emergency backup when grid goes down.

The alternative is just selling the mismatched unit and buying a matching Fogstar to keep things tidy. Might sting short-term but it's the sensible path for a setup you're relying on for a garden office.

@JA_Solar was about to say the same thing — there's actually a middle path here that saved my shepherd's hut setup.

I had a lone Fogstar Drift sat in the corner and rather than binning it or bodging it onto my main 48V bus, I ran it as a completely separate 12V auxiliary bank through a DC-DC charger fed from the main system. Lights, small fans, the radio — all isolated. The MultiPlus never sees it, the BMSs never argue with each other, and nothing goes pop.

It's a bit more wiring faff, but honestly it's elegant rather than a compromise. @SaltyRigger — what's the spare battery you're looking at adding? Capacity and chemistry mismatch together is what really determines how bad the problem gets.