DC-DC charger sizing for EV charging at a static caravan — where do I start?

Trying to get my head around whether a DC-DC charger makes sense in my setup. I've got a static caravan on a plot with no grid connection, running a Victron 12V system with 400Ah of Fogstar lithium and a 600W solar array. The caravan is the priority load, but I've also got a small EV (Nissan Leaf, 24kWh battery) I'd like to trickle charge when there's surplus.

The issue is I'm not sure how to size a DC-DC charger correctly for this use case. Most of what I read assumes you're in a van topping up a leisure battery from your car's alternator — I want to do something more like the opposite, pushing energy from my house bank into the EV's 12V system or using some kind of intermediate step. Is that even a viable path, or am I thinking about this completely wrong?

I've seen the Victron Orion range mentioned a lot. Would something like the Orion-Tr Smart 12/12-30 be relevant here, or is that too small to be worth the bother given the Leaf's battery capacity? Wondering what sort of charge times I'd realistically be looking at.

Has anyone actually done EV charging from a standalone solar/battery system at a static, rather than a grid-tied setup? Curious what approach you landed on and what the bottlenecks were.

Hey @BlownFuse, welcome to the rabbit hole! 😄

First question I'd ask is — what EV are you charging and what's your target charge rate? Even a modest DC-DC charger (say a Victron Orion-TR Smart 30A) only pushes about 360W at 12V, so charging an EV directly from your house bank isn't really viable unless you're just topping up a small leisure vehicle battery or an e-bike.

For actual EV charging you'd typically need a much beefier dedicated DC system or an inverter feeding an AC EVSE, which means your 400Ah Fogstar bank sizing and solar/alternator input become critical.

Could you share a bit more about your generation sources — solar panels, generator, anything else? That'll help massively in figuring out whether DC-DC even fits your use case or whether we need to think differently. 👍

Really good question @BlownFuse. Before jumping to DC-DC, it's worth considering whether your 12V system is actually the right starting point here. EV charging draws serious current even at low rates, and a 12V bank will struggle to deliver that without massive cable losses and a very large battery. Most people doing EV charging off-grid are running 48V systems at minimum — it just makes the numbers far more manageable.

What solar generation have you got behind those 400Ah? That'll be the real bottleneck rather than the charger itself. A DC-DC charger is only as useful as the energy you can actually put back into the bank between charges.

Would definitely second @PW_Sparks on knowing which EV too — some are much more forgiving of slow trickle charging than others.

Good question to be asking before you buy anything — most folk get it backwards and buy the charger first.

The bit nobody mentions upfront: your source battery capacity is the real bottleneck here, not the DC-DC charger rating. I learned this the hard way at my cabin when I sized a 30A Victron Orion-Tr Smart thinking it'd crack on regardless — it does, but it hammers your state of charge faster than you'd expect if solar input is modest.

What's your daily solar harvest looking like, @BlownFuse? A static caravan plot in the UK means you're working with maybe 1.5–2 peak sun hours on a grim winter day. That 400Ah Fogstar bank sounds healthy until you're also running lights, fridge, and trying to nudge an EV.

DC-DC can work, but it's a slow trickle game rather than proper charging.

Great points all round already. One thing I'd add — don't overlook your solar and alternator input capacity when sizing this up. A DC-DC charger pulling hard from your 12V bank is only sustainable if you can realistically replenish what you're taking out. 400Ah of Fogstar is a decent starting point, but EV charging is a hungry beast even at modest rates.

What inverter are you running alongside this, @BlownFuse? And are you grid-tied anywhere else on the plot, or fully island? That'll massively affect whether DC-DC is actually your best route, or whether something like a dedicated solar-to-EV solution might serve you better in the long run. Sometimes the "obvious" answer isn't the most efficient one once you map out the full energy flow. 🙂

Good shout from @Dizzy there. One thing worth flagging specifically for your setup @BlownFuse — with a static caravan you're not going to have an alternator in the picture, so your DC-DC charger would presumably be fed from a vehicle you drive onto the plot? If so, the cable run between the vehicle and your caravan system becomes really important. Voltage drop over a long run will absolutely hammer your charging efficiency, and potentially confuse the charger's input sensing. Worth measuring that distance before you decide on a charger rating, because you might find the practical usable current is lower than the charger's rated input. What's the rough distance from where you'd park to your battery bank?

Something I learned the hard way with my shepherd's hut setup — the DC-DC charger is only ever as useful as the battery headroom available when the EV plugs in.

With 400Ah of Fogstar lithium, you've got decent storage, but an EV will drain that faster than you expect. I'd be modelling a worst-case scenario: overcast day, EV arriving with 10% charge, what does that actually look like in Victron VRM at midnight?

The DC-DC route only really shines if you're managing charge windows carefully — not just plugging in and hoping.

Really good thread this. One angle I don't think's been touched on yet — what's your anticipated charging frequency? If you're only plugging the EV in occasionally rather than daily, you can potentially get away with a smaller DC-DC unit because your house bank has longer recovery time between sessions.

Also worth thinking about your 12V system voltage sag under load. A 400Ah Fogstar bank is decent but if you're pulling hard from a DC-DC charger simultaneously with other loads, you want to make sure you're not dragging the source battery down to a point where the charger cuts out mid-session.

What EV have you got @BlownFuse? The onboard charger capacity will dictate your realistic AC-side ceiling anyway, which kind of works backwards into what DC-DC rating actually makes practical sense for your situation.