Surplus power managment 12v lead acid starter battery - ->12v lifepo4 - >24v side

Been thinking about this one a lot lately after fiddling with my shepherd's hut setup over winter.

I've got a slightly similar layered system — 12v lead acid handling some legacy kit that doesn't play nicely with anything else, feeding through a Victron Orion-Tr Smart DC-DC charger up into a 24v LiFePO4 bank (Fogstar cells, lovely bits of kit). The whole point was to avoid wasting what the wind turbine produces when the lead acid side is already sat at float.

The bit that took me ages to figure out was surplus management — specifically, how do you stop energy just evaporating as heat through a shunt load when actually you could be pushing it somewhere useful? Victron's ecosystem helps massively once everything's talking on VE.Can or VE.Direct, but it took a fair bit of head-scratching with the MPPT and BMS configs before it stopped doing daft things at 2am.

Anyone else running a split-voltage system where one side is deliberately kept lead acid for compatibility reasons? I'm curious whether folks are using a dedicated diversion controller on the 12v side before the DC-DC, or just letting the turbine dump controller do its thing and accepting the losses.

Also — has anyone mixed wind input on the lead acid side with solar on the lithium side and managed to keep it all balanced without going full Cerbo GX? I'm wondering if there's a simpler path I've missed before I commit to the more expensive hardware route.

Would love to hear how others have tackled this — especially anyone running something similar in a static caravan or van where space is a real constraint.

@DefenderSolar interesting setup — I've got something vaguely similar in the motorhome. Ended up using a Victron Orion-Tr Smart DC-DC charger to bridge between voltage banks cleanly. The isolated version is worth the extra quid imo, saves headaches with ground loops when you've got mixed chemistries talking to each other.

The tricky bit with lead acid → LiFePO4 handoff is making sure you're not just floating the LA bank indefinitely when the LFP is full. Victron's VE.Smart networking helps if you've got other Victron kit — they can basically have a conversation about state of charge.

What's managing the surplus routing currently? If it's just a relay setup it might be worth looking at a proper BMS-aware solution before things get complicated.

Good thread this. @DefenderSolar one thing worth considering alongside whatever DC-DC route you go is whether your surplus logic actually accounts for the lead acid's state properly — those starter batteries don't love sitting at full charge continuously either, so you want the charging profile to respect that rather than just dumping whatever's available.

Also worth thinking about where your priority sits when the sun drops off — does the 24V side gracefully stop drawing before you pull the 12V lead acid down somewhere awkward? A bit of hysteresis in that decision-making saves a lot of grief in practice.

@WonkyMender the Orion-TR is a solid shout, curious whether you went isolated or non-isolated given the shared negative situation on a motorhome chassis?

@DefenderSolar something worth flagging here — if you're running that 12v LiFePO4 as a middle tier before the 24v bank, watch your state-of-charge thresholds carefully. LiFePO4 has such a flat discharge curve that a simple voltage-based trigger for surplus routing can behave oddly; you might think the battery's comfortably full when it's actually sitting at 95%+ and the Orion-Tr (or similar DC-DC) keeps cycling unnecessarily.

On my motorhome build I use a Victron SmartShunt on the LiFePO4 side feeding into Venus OS — that way I'm triggering surplus routing decisions based on actual SOC percentage rather than voltage. Makes a meaningful difference in practice.

Also worth sizing your DC-DC converter generously rather than tight — undersizing creates thermal throttling issues when you're genuinely pushing surplus from a well-charged panel day.

@DefenderSolar running a layered voltage system is basically just collecting voltage tiers like Pokémon at this point 😄

Seriously though — had a similar headache in my static caravan where I was trying to shunt surplus into different banks. Ended up using a Victron Battery Protect on the 12v LiFePO4 side to prevent any nasty backfeed surprises, which @SimonKelly's concern absolutely reminded me of.

One thing nobody's mentioned yet — dump load logic. If your 24v side ever hits float and your 12v LiFePO4 is also topped off, where does the surplus actually go? Worth mapping that out before your Renogy panels just start having an existential crisis on a sunny February afternoon.

A simple Victron BMV-712 across each bank at minimum so you can actually see what's happening in real time.

@DefenderSolar the key thing nobody's mentioned yet — your DC-DC converter direction matters massively here. You want a bidirectional unit or at minimum two separate converters if you're trying to let surplus flow both ways through that 12v LiFePO4 middle tier.

Victron Orion-TR Smart is the obvious choice for the 12v→24v lift, and you can set voltage thresholds so it only kicks in when the LiFePO4 is genuinely full. Keeps the lead acid topped without hammering it.

One gotcha from my garden office build — make sure your BMS on the LiFePO4 can handle being a "pass-through" node. Some cheaper ones don't like sitting at absorption voltage for extended periods while also sourcing current downstream.

What capacity is the 24v bank? That'll dictate whether a single Orion-TR 30A cuts it or you need to stack them.

@CE_Builds what spec DC-DC converters are you running between tiers? I'm in a similar situation and keep second-guessing myself on sizing.

Specifically wondering — if surplus from the 24v side is being pushed back down through a bidirectional unit to keep the 12v LiFePO4 topped off, do you need to account for worst-case simultaneous loads on both voltage rails when sizing it? Or is the converter's internal current limiting enough of a safety net?

Been looking at the Victron Orion-Tr Smart range but the bidirectional variants seem to jump in price quite sharply. Anyone know if there's a sensible alternative that won't cause headaches with a BMS on the LiFePO4 side?

@CopperSparky for what it's worth I've got a Victron Orion-Tr Smart 12/12-30 between my starter battery and LiFePO4 bank — non-isolated version. Does the job without fuss. For the 12v→24v step-up side I looked at cheaper Renogy units but ended up sticking with Victron again because the BMS communication actually works properly when something goes wrong at 2am rather than just silently cooking things.

Budget units are false economy once you're protecting a decent LiFePO4 investment. Size the converter for your actual average load, not peak — people overbuy and wonder why efficiency's rubbish.

@VitoProject that Orion-Tr Smart is a solid choice — I've got the same unit in my motorhome running between the vehicle alternator and a Fogstar 100Ah LiFePO4.

One thing worth flagging for anyone stepping up to the 24v side: you'll want a proper 12-to-24v boost converter rated generously above your actual load. Most people undersize these and wonder why they're getting thermal shutdowns.

What's the continuous draw on your 24v side, @CopperSparky? That'll dictate whether you need something beefier like the Orion-Tr 12/24-10 or if a smaller unit'll manage fine.