Anyone else struggled to balance two different battery banks on the same system?

I've ended up in a bit of a pickle with my setup. I've got a 200Ah lithium (a Fogstar Drift 12V) that I fitted last spring, but I've also still got an old 110Ah AGM knocking about that I don't really want to waste. Thought I'd try running them together off the same 40A MPPT controller (Victron 100/30) and a 400W panel array on the van roof, but the two batteries are obviously charging at different voltages and it's causing me grief.

The lithium wants to sit at 14.2V absorption and the AGM is happy up around 14.7V. When I bridge them together the Victron is basically stuck trying to please both at once and neither is getting a proper charge. State of charge readings on the BMV-712 are all over the place because it doesn't know what total capacity it's dealing with either.

Has anyone actually made a mixed chemistry setup work properly, or is it just a fool's errand? I've seen a few people mention using a DC-DC charger between the two banks but I'm not sure if that's overkill for a relatively modest van build. Happy to hear if anyone's bodged something that actually works or if I should just sell the AGM and be done with it.

@Boycie57 oh mate, I lived this exact nightmare on my boxer camper build. Short version: don't mix them.

The Fogstar Drift will charge beautifully at its lithium profile, then your AGM will either get cooked or starved depending on which chemistry your charger prioritises. Neither outcome is fun at 11pm on a campsite in the rain.

What actually works:

• Keep the AGM as a dedicated starter/backup bank, isolated with a Victron Cyrix or similar battery-to-battery charger
• Let a Victron Orion-Tr Smart feed the AGM from the lithium at the correct AGM voltage
• Your main loads run purely from the Fogstar

The 110Ah AGM doesn't have to die — it just needs its own lane. Running them in parallel directly is asking for chemistry chaos. The lithium will bully the AGM constantly.

Just to add to what @BoxerCamper is saying - the fundamental problem is that lithium and AGM have completely different charge profiles and resting voltages. Your Fogstar Drift wants to charge up to around 14.6V, but pushing that into the AGM regularly will cook it. Go lower to protect the AGM and you're leaving your lithium perpetually undercharged.

If you're dead set on using both, a DC-DC charger between the two banks is really the only sensible way to isolate them properly - treat the AGM as a separate secondary bank fed from the lithium rather than running them in parallel. Victron make decent ones but there are cheaper options.

Honestly though, given the AGM is already old, I'd question whether the hassle is worth it. What were you planning to use the 110Ah for specifically?

Great points from @BoxerCamper and @Gibbo already. One thing worth adding - even if you were tempted to try isolating them with a battery-to-battery charger (B2B), you'd essentially be treating them as two completely separate systems anyway, so at that point you might as well just retire the AGM altogether.

That 110Ah AGM isn't going to owe you anything - it's done its time. The Fogstar Drift is a cracking bit of kit and deserves to work properly on its own without being dragged down or confused by mismatched chemistry.

Could the old AGM serve a different purpose entirely? A dedicated starter battery, a separate leisure circuit for something low-draw, or even just sold on? Plenty of folk on here would snap up a decent used AGM for the right price. Keep it simple - your future self will thank you!