Fitted a 40A DC-DC charger last week — worth it over a split charge relay?

Been running a fairly basic setup in my Transit-based camper for a couple of years now — 100Ah AGM leisure battery, split charge relay off the alternator, and a 200W solar panel on the roof. It's done the job on weekend trips but I've noticed the AGM barely gets above about 80% SOC on overcast days and the relay setup just dumps whatever voltage the alternator puts out, which isn't ideal.

So last week I finally fitted a Victron Orion-Tr Smart 12/12-40A DC-DC charger between the starter and leisure battery. The difference on a run up the M6 was pretty noticeable — leisure battery hit 14.4V and held it properly through the absorption stage for the first time. Previously the relay was just sitting at around 13.8V and calling it a day.

The cost was the main thing putting me off — picked mine up for about £160 — but honestly I wish I'd done it sooner. Especially useful now that newer vans have smart alternators where a relay is basically useless anyway. My Transit is a 2014 so it's on the edge of that, and I reckon the DC-DC is future-proofing it nicely.

Has anyone else made the switch and noticed a real-world difference in charge times or battery health? And has anyone paired one with lithium — wondering if it's worth upgrading the AGM at the same time or just getting more out of what I've already got?

@MarshPete absolutely worth it, especially if you've upgraded or plan to upgrade to lithium at any point. Split charge relays are fine for AGM but they just lump voltage straight across — no charge profile control whatsoever.

I fitted a Victron Orion-Tr Smart 30A in my Sprinter build last year. The difference was immediately obvious — proper absorption and float stages, and the Bluetooth monitoring via VictorConnect is genuinely useful for diagnosing alternator health.

One thing worth noting: modern Euro 6 Transits have smart alternators that drop voltage intentionally to reduce fuel consumption. A basic relay goes almost dead in that scenario. The DC-DC charger sees that and compensates.

40A is solid headroom too — you'll see meaningful charge even on short runs. What battery chemistry are you running with it?

@MarshPete the other big thing nobody mentions — smart alternators on modern Transits will often see the relay as a fault and pull voltage back. DC-DC charger handles that properly.

Went through the same swap on my setup last year. Victron Orion-Tr Smart 30A here and the difference in charge acceptance was noticeable pretty much immediately, especially on shorter runs.

Your 200W solar will still do the heavy lifting in summer obvs, but having the DC-DC actually pushing decent current into the bank during winter drives when the panel's doing nowt is genuinely useful.

Really glad you made the switch @MarshPete! One thing worth adding to what's already been said — a DC-DC charger gives you proper charge profiling, so your AGM will actually reach a full state of charge rather than just floating around 80% like it often does with a basic relay setup. That alone extends battery life considerably. I noticed a real difference in my own van once I fitted one. Also worth checking your alternator output cable sizing while you're at it — a 40A charger is pulling decent current and an undersized cable will create unnecessary voltage drop and heat. What make did you go for? I've had good experiences with the Sterling units myself, though Victron's Orion-Tr Smart seems to be everyone's favourite these days for good reason.

@MarshPete done the same on my cabin's off-grid van setup — B2B charger was one of those "why did I wait so long" moments.

The bit that surprised me was the charge profile consistency. Split relay just dumps whatever the alternator fancies at you. The DC-DC actually finishes the job properly — my Fogstar lithiums were finally hitting 100% on driving days rather than sitting at 87% looking sorry for themselves.

Worth checking your cable sizing too. Plenty of people fit a 40A unit then run it through undersized wire and wonder why it throttles back. Done it myself. Felt very stupid. Moving on.

Solar + DC-DC together is genuinely a decent combo once you've got the two sources playing nice.

Which charger did you go for? Victron Orion or one of the cheaper alternatives?

@MarshPete worth clarifying the terminology here since it's causing confusion in threads like this regularly — "B2B" (battery-to-battery) and "DC-DC charger" are the same thing, just different names for the same device. Victron call theirs the Orion-Tr Smart, Sterling and Votronic use other naming conventions.

One thing nobody's touched on: if you're running a lithium leisure battery (or planning to upgrade), a DC-DC charger is essentially non-negotiable. A simple relay will happily pump unregulated voltage into lithium cells and cause all sorts of grief.

Also worth checking your cable sizing. Most people underspec the input cable run from the starter battery — a 40A charger needs appropriate wire gauge for the distance, otherwise you're losing efficiency before you've even started.

What's your cable run length from the starter battery?