Ladungserhaltung - 24V Bleibatterie aus 12V Quelle

Been mulling something over after a recent rabbit hole on a German forum — got me thinking about my own situation on the boat.

I've got a 12V lithium bank (Fogstar Drift 200Ah) fed by roof-mounted solar, and I'm wondering about the best way to keep my 24V lead-acid bow thruster battery topped up from it. Not bulk charging — just float/maintenance, since the thruster barely gets used outside of locks.

The voltage mismatch is obviously the elephant in the room. A straight connection is out, so I'm looking at a DC-DC converter. Victron Orion-Tr seems the obvious candidate — their isolated 12/24V units look tidy — but I'm wondering whether the 15A model is overkill for pure float purposes, and whether there's a smarter (cheaper) way to do it.

A few questions I'd love to hear thoughts on:

• Does anyone run a similar cross-voltage setup for a secondary lead bank?
• Is there any benefit to using a dedicated maintenance/float profile rather than just letting a standard DC-DC charger cycle as normal?
• Any concerns about leaving it permanently connected, or is the Orion smart enough to not cook a lead battery over time?

The lead bank doesn't need to do heavy work — I just hate the idea of it sulphating away between uses. Canal life means I'm often moored up for weeks at a stretch, so keeping everything topped up passively feels like the right approach.

Curious whether anyone else has tackled this kind of inter-voltage battery nursemaiding — especially on a boat where space for extra kit is always at a premium.

@DorsetCamper — classic rabbit hole situation, German forums are genuinely excellent for deep-dive theory.

The core principle you're hitting is charge conservation through a DC-DC converter. A Victron Orion-Tr Smart 12/24 (or 12/24-15 if you need the amps) will do exactly what you're describing — it's not magic, it's just power in ≈ power out with some efficiency losses (~92-94%).

So from your 12V Fogstar bank, pushing into a 24V secondary bank, you're roughly halving your available amps but doubling voltage. 100A at 12V becomes ~47A at 24V after efficiency losses.

On the narrowboat, make sure you're sizing your input wiring correctly — the 12V side carries significantly higher current, so cable sizing is critical. Victron's Orion range integrates nicely with their ecosystem too if you're running a Cerbo GX.

What's the 24V load you're trying to run?

@DorsetCamper the thread title gave it away before you even explained — "Ladungserhaltung" is basically charge conservation, and it's the crux of why a B2B charger between mismatched voltages isn't just a voltage converter, it's actively managing the power equation.

On your boat scenario specifically — if you're pulling from a 12V Fogstar bank to charge a 24V bank (or vice versa), your current halves (roughly) as voltage doubles. That's not "lost" energy, it's just Ohm's law doing its thing. The Victron Orion-Tr Smart handles this really cleanly and shows you input/output current in the app so you can actually see it in real time rather than just theorising.

What's your intended load on the 24V side? That'll drive whether a 30A or 15A Orion makes more sense for your setup.

Got a similar setup on my cabin — 12V Fogstar Drift feeding a 24V bank via a Victron Orion-Tr Smart. Works well enough but took me a while to get the charge profiles dialled in properly.

One thing worth watching: the Orion-Tr isolated versions give you a lot more flexibility if your grounds aren't common between source and destination. Caught me out initially.

Not sure what the German thread concluded but from my own tinkering — efficiency losses through the DC-DC conversion are real, maybe 8-10% in practice. Worth factoring that into your panel sizing if you're running this long-term on the boat @DorsetCamper.

@Fogstar_Fan the Orion-Tr Smart is fine until you realise you're essentially halving your current capacity and wondering why your 24V bank charges like it's got a grudge against you — been there on the narrowboat, switched to a proper 48V architecture and never looked back.