Switching to 24v from 12V for all but starting battery. Have an aux start on dash to parallel batteries, need input.

Made this exact jump last year — best decision I made for the van build tbh.

Few things worth thinking about:

The parallel switch idea — I've seen a few people do this but it makes me a bit nervous. You're essentially connecting two different voltage systems momentarily which can work, but if your 24v bank is sitting at say 25.6v and the 12v start battery is at 12.4v the current rush isn't ideal. Worth adding a resistor or a proper battery combiner relay rather than a straight parallel switch.

Alternator charging — how are you planning to charge the 24v bank from the alternator? This is where most people get stuck. I went with a Victron Orion-Tr Smart 12|24-15A DC-DC charger and it's been rock solid. Isolates the systems properly too, so no voltage mixing headaches.

Battery choice — I run Fogstar Drift 24v LiFePO4 now and the efficiency gains over my old 12v setup are genuinely noticeable. Less cable losses, smaller wire runs.

What's your current 12v start battery — a standard lead acid? If so the aux start parallel thing might be fine as a backup emergency option but I wouldn't rely on it as primary charging path between the banks.

What inverter are you running? That's usually the thing that forces people to 24v in the first place 😄

Would love to hear what others have done — especially anyone running a split charge relay setup between the two voltages. Surely someone's cracked a cleaner solution than I have.

@VitoSolar — that cliffhanger ending is going to haunt me 😂

The parallel switch concern I suspect you were heading toward is momentarily bridging two different-voltage banks, which is... not ideal, shall we say. If your 12V starter is sat at 12.8V and your 24V aux bank is fully charged at 29V, closing that parallel switch is going to have a very bad time indeed — we're talking potential BMS trips, fuse obliteration, or worse.

What most people do properly is fit a DC-DC charger (Victron Orion-Tr Smart is the obvious choice) to keep the 12V starter battery topped from the 24V bank whilst maintaining complete isolation. Clean, safe, no drama.

The aux start function becomes somewhat redundant then, but honestly that's the right redundant — better than the alternative which involves smoke and regret.

@VitoSolar left us on a cliffhanger worse than a BBC drama finale 😂

But yeah, the parallel switch concern is a real one — momentarily bridging a 12V starting battery with a 24V bank is going to have a very bad time. The voltage difference means massive current spike the instant you close that switch, which is hard

@VitoSolar finish your sentence, mate! 😄

But I think I know where you were headed — paralleling different voltage banks is the concern, right? Even momentarily bridging a charged 24v system with a 12v starter battery could cause a pretty savage current spike depending on the state of charge difference between them. I've seen people use a proper voltage-sensitive relay or a DC-DC charger setup to keep the systems isolated but still allow the starter battery to top up from the leisure bank. Much cleaner solution than a manual parallel switch. What isolator are you currently running between the two banks?

@VitoSolar went full BBC cliffhanger on us there, but let me finish the story for him — because I lived this mistake in my shepherd's hut build before I got it right.

Paralleling a 24V bank with a 12V starter battery, even momentarily through a dash switch, is essentially asking them to fight each other. The 12V side will try to drag the 24V bank down, and depending on your BMS settings you'll either trip protection or — worse — silently stress the cells without knowing it.

What actually works: a proper DC-DC charger (I use a Victron Orion) to keep your starter battery topped from the 24V bank. One-way, controlled, no drama. The aux start switch idea sounds convenient on paper but it's the kind of shortcut that costs you a battery bank.

@MuddyNomad now you've done it too 😂 this thread is cursed

Right, the actual concern with paralleling 12v and 24v banks via a dash switch — even momentarily — is you're not just risking a big inrush current, you're potentially cooking your BMS on both sides. The voltage differential will try to equalise instantly and something will be the sacrificial lamb.

On my narrowboat I run a dedicated 12v starter battery completely isolated from the 24v LiFePO4 house bank (Fogstar Drift cells). Victron Orion-Tr DC-DC charger handles keeping the starter topped up from the house bank — no paralleling, no drama.

A DC-DC isolator is almost certainly a cleaner solution than that aux parallel switch, depending on what the OP is actually trying to achieve with it.