Deciding weather or not to keep one 12v battery in a 24v 5th upgrade

So I'm in the middle of planning my 5th battery upgrade (yes, fifth — the rabbit hole is real) and I've hit a bit of a dilemma I can't quite reason my way out of.

Currently running a 24v system with Fogstar Drift cells, and I've got a lone 12v 100Ah battery sitting in the corner of my garden office that I've been using as a kind of "emergency reserve" for smaller 12v loads — things like the router, a couple of LED strips, and a small fan.

The question is: do I keep it separate and maintain that dedicated 12v setup, or just retire it and run everything through a DC-DC converter off the main 24v bank?

Arguments I keep going back and forth on:

• A dedicated 12v battery feels like a safety net, but it's also another thing to monitor and keep topped up
• DC-DC converters (looking at Victron Orion options) seem cleaner but adds another potential point of failure
• The 12v battery is getting on a bit — probably only holding 70-75% capacity realistically

Has anyone else faced this when scaling up their system? Did you ditch the standalone 12v or keep it? Wondering whether there's a strong case either way that I'm missing. Would especially love to hear from anyone running a garden office setup where 12v loads are fairly modest but uptime really matters.

@MelKing83 Mixing voltages in a series bank is how you end up with a paperweight — ditch the lone 12v, your BMS will thank you before it rage-quits entirely.

@Rob1963 makes a fair point, but I'm curious what the actual use case is for keeping it, @MelKing83?

I've got a small cabin setup where I kept a standalone 12v Fogstar Drift specifically for emergency backup — completely isolated from the main 24v bank. Separate charge controller, separate loads. It's been genuinely useful when I've needed to run a single 12v device without touching the main system.

Is that the kind of thing you're thinking, or are you actually trying to integrate it into the 24v bank? Because those are very different conversations with very different answers.

• If integrated into the 24v bank → @Rob1963 is right, don't do it
• If isolated emergency reserve → completely viable with the right setup

What's driving the decision to keep it?

@MelKing83 the short answer is: don't do it in series, obviously — @Rob1963 has that covered. But there's actually a legitimate case for keeping a lone 12v as a completely isolated auxiliary bank — dedicated to 12v loads (lighting, water pump, USB charging) with its own small solar input and charge controller. I run exactly this arrangement in my static caravan; a little 100Ah Fogstar Drift 12v handles all the "housekeeping" loads while the main 24v bank does the heavy lifting for the inverter.

The key question is whether your 12v loads justify the complexity. If everything's already converted to 24v or runs through the inverter anyway, it's probably more hassle than it's worth. What loads are you actually trying to power with it?

@CliveBaker genuinely curious where you're going with that — what's the "legitimate" use case you're hinting at?

My gut says the only sensible option is keeping it completely isolated — dedicated 12v loads only, charged separately. I've got a small Victron MPPT just feeding a single Fogstar 12v in my van build for exactly this kind of "legacy device" situation. Works fine if you're disciplined about it.

But is that actually worth the faff for @MelKing83's setup? What loads are we actually talking about? Because if it's just a couple of 12v accessories, a simple DC-DC converter off the 24v bank is probably cleaner and you're not managing two separate charge sources.

What's the battery chemistry on the lone 12v? That changes things considerably.

@MelKing83 been there — I ran a rogue 12v alongside my 24v bank for about 6 months powering just my 12v fridge directly. Kept it isolated, charged via a Victron Orion DC-DC. Worked a treat actually.

The key thing nobody's mentioned yet: balancing the charge sources. If you're pulling from the same solar array, you'll need proper isolation or the voltages fight each other and your BMS will throw a tantrum.

Fogstar Drift cells are brilliant if you're rebuilding the 24v side too — just saying 👀

What's the actual 12v load you're trying to keep? That changes everything. Small parasitic draws are honestly better served by a quality DC-DC converter off the main bank rather than babysitting a separate battery.

What's your current BMS situation?