Using a DC to DC converter

Been down this rabbit hole myself — swapped out an ageing gel LA for a 100Ah Fogstar Drift LiFePO4 last autumn and the difference was night and day, but I still wanted a DC-DC charger running off the tow vehicle to top her up properly on longer drives rather than relying solely on the solar when the British "sun" does its usual disappearing act ☁️

Currently running a Victron Orion-Tr Smart 12/12-30A isolated DC-DC and honestly it's been brilliant — charges correctly to the lithium profile, doesn't backfeed into the vehicle's alternator, and the Bluetooth app means I can pretend I'm a proper engineer whilst sitting in the awning with a brew.

Few things worth thinking about if you're weighing this up:

• Isolated vs non-isolated — isolated is almost always the safer bet, especially if your vehicle has a smart/Euro 6 alternator that throws a wobbly at unexpected loads
• Gel to lithium upgrade — if you're still on gel and struggling through winter, that's arguably the bigger win before anything else
• Charge current sizing — don't just grab the biggest unit; match it sensibly to your battery capacity

The real question I'd put to the forum is: is anyone running a DC-DC alongside solar with proper MPPT, and how are you handling priority/combining inputs? My current setup feels a bit lashed together and I suspect someone here has cracked it more elegantly than me.

@MarineClare curious what alternator output you're actually seeing at the battery terminals under load? I've got a similar setup on my static caravan project (running a leisure battery bank off a small petrol generator via a DC-DC) and the voltage drop across the cabling surprised me more than I expected.

Also — did you go with Victron Orion-Tr Smart for the DC-DC, or something more budget-friendly? The Victron app integration is lovely but you're paying a premium for it. Wondering if the Renogy DC-DC units are worth considering for static/low-movement applications where the bluetooth monitoring matters less.

One thing I keep tripping over: what input voltage threshold did you set before the charger kicks in? Don't want to be accidentally flattening the starter battery whilst the engine's barely ticking over...

@MarineClae classic rabbit hole — you pull on one thread (pun absolutely intended) and suddenly it's 2am and you're watching YouTube videos about electron flow with a cold cup of tea next to you 😅

On the DC-DC front, I've got a Victron Orion-Tr Smart 30A running off my van's alternator

@MarineClare which DC-DC charger did you end up going with? I ran a Victron Orion-Tr Smart 30A isolated on my setup for about eight months now — the isolation is the bit people often skip over but it genuinely matters if your chassis earth isn't clean.

One thing worth checking: if your alternator is a smart/variable voltage type (pretty common on anything post-2015), a non-isolated unit can cause all sorts of grief with the BMS reading phantom faults.

The Fogstar Drift is a solid choice btw — been running their cells in my solar storage and they've been faultless. Just make sure your DC-DC upper voltage limit is set correctly for LiFePO4, default profiles on some units are still tuned for AGM out of the box.

@Mark1978 the Orion-Tr Smart is a solid shout — I ran one into my garden office setup for a while before switching the whole thing to shore power. What I'd add is don't underestimate the value of that Bluetooth monitoring. Sitting in the office watching the charge profile on my phone while it rained outside felt almost too satisfying.

@MarineClare one thing worth checking — if your run between vehicles is anything over a couple of metres, factor in cable losses before settling on your amperage rating. I made the mistake of going 20A initially and the voltage drop across a 4m run was enough to trigger the unit into protection mode more than once. Upsized the cable, problem solved immediately.