Worth running a small 12v system alongside a 48v setup for low-draw overnight loads?

Been running a 48v Victron system for a couple of years now — 4x 200Ah Fogstar Drift LiFePO4s, Multiplus-II, the usual. Works brilliantly for the heavy stuff but I've started wondering whether it makes sense to add a small dedicated 12v bank just for overnight low-draw loads.

Thinking things like the router, a couple of LED strips, phone charging — probably 20-40W continuous at most. Right now everything goes through the inverter which feels wasteful given the conversion losses, especially in winter when I'm watching every watt. A small 12v lithium (even a single 100Ah Fogstar or similar) direct-wired to those loads seems cleaner.

The bit I'm unsure about is how to charge it sensibly off the 48v bank without adding a load of complexity. Seen people mention Victron Orion-Tr Smart DC-DC converters for exactly this kind of thing — anyone actually running that setup? Wondering if the quiescent draw of the Orion cancels out any efficiency gains if it's on 24/7.

Has anyone done the maths on whether this is actually worth the hassle, or am I overcomplicating something that a decent low-power inverter setting would solve anyway?

@LazyBodger I'd push back on this slightly. Your Multiplus-II's quiescent draw is what, 11-15W at idle? Running a parallel 12V system means an additional BMS, additional charger, additional points of failure, and you're still losing conversion efficiency anyway — just at a different voltage.

What actually moves the needle is putting your overnight low-draw loads on a dedicated 12V DC circuit fed through a quality 48-12V DC-DC converter (Victron Orion-TR Smart being the obvious choice). Single battery bank, no redundant charging complexity, and the Orion's efficiency sits around 94-95% which is hard to fault.

I run exactly this on my static caravan setup — lighting, router, and a small 12V fridge all fed from an Orion pulling from the main 48V bank. Far cleaner than maintaining two separate systems.

@Compo makes a fair point about the quiescent draw, but there's a hidden cost people overlook — the DC-DC conversion losses if you're feeding a 12v rail from 48v. A decent Victron Orion-Tr Smart will sit at around 93% efficiency, so you're bleeding roughly 7% continuously on whatever your overnight load is.

In my static caravan setup I actually run a dedicated small 12v LiFePO4 (a 100Ah Fogstar) purely for low-draw overnight loads — LED lighting, a router, a couple of USB chargers. Keeps the big 48v Multiplus in sleep mode far more consistently and my overnight idle figures dropped noticeably.

The counterargument is complexity and an additional BMS to monitor. Whether that tradeoff suits you depends heavily on what your actual overnight load is — worth logging it properly with a Victron BMV first before committing either way.

Great thread. One thing neither @Compo nor @LochLinda have touched on yet — the complexity cost. Two separate battery banks means two separate SOC monitoring setups, two sets of fuses and protection, potentially two BMS units to keep an eye on. That's not nothing when something goes bump in the night and you're trying to diagnose it half-asleep.

My honest take: if your overnight loads are genuinely low-draw — think a bit of LED lighting, a router, maybe a Pi — you might get more mileage from simply optimising your 48v system's idle behaviour. Victron's ESS assistant gives you decent control over when the Multiplus actually kicks in. Have you looked at your dynamic cutoff settings at all @LazyBodger? Sometimes the answer's already in the system you've got rather than bolting something else alongside it.

@DustyWanderer raises the complexity point well, and it's the one that killed this idea for me personally when I considered it for my garden office setup.

There's also a balancing problem nobody's mentioned: your 48V bank sits at, say, 90% SoC overnight while the 12V auxiliary bank drains to 40%. Come morning, your solar has to service two chemistries at different states simultaneously. Depending on how you've wired your DC-DC charger priority, you can end up with the 48V bank absorbing charge whilst the 12V is still starved.

Victron's Orion-Tr Smart handles this reasonably well with its input voltage lockout, but it adds another device to monitor, another potential failure point, and another entry in your system schematic.

For genuinely low overnight draws — we're talking sub-5W — just let the Multiplus idle. The maths rarely justify the architecture.