Anyone else running a split-charge relay instead of a DC-DC charger to save money — worth it long term?

Been piecing together a budget 12v system in my Transit and I've been going back and forth on this for weeks. Most people on here seem to go straight for a B2B charger like the Victron Orion or Sterling Pro Charge Ultra, but they're £120–£200+ and that's a big chunk of my total budget. I've got a fairly simple setup — 200Ah of AGM leisure batteries, a 30A split-charge relay I picked up for £18 on Amazon, and a 200W solar panel feeding into an EPever 20A MPPT.

The relay seems to be doing the job on longer drives — I did a 3-hour run last week and the leisure bank went from about 60% up to around 85% by the time I stopped. But I know the relay just connects the batteries together rather than doing a proper 3-stage charge, so I'm probably not getting a full charge from the alternator. With AGMs you ideally want that absorption stage to properly top them off.

The other thing nagging at me is whether the relay is doing my alternator any harm over time, especially in winter when solar is basically useless and I'm relying on driving more heavily. The Transit has a 150A alternator so I'd have thought it was fine, but I've seen some horror stories on forums about killing smart alternators.

Has anyone actually run a relay long-term and been happy with it, or did you end up switching to a DC-DC and wishing you'd just bought one from the start?

Hey @VanMark, worth flagging something that often gets missed in this debate — if you've got a modern Euro 6 Transit with a smart alternator, a split-charge relay simply won't work properly. Smart alternators drop voltage intentionally when the engine's warm, so your leisure battery barely gets a look-in. A DC-DC charger actively boosts and regulates the charge regardless of what the alternator's doing.

If your Transit is pre-2014ish with a conventional alternator, a VSR relay can be a reasonable stopgap, but you're still looking at slower, less efficient charging long term.

The Orion-Tr Smart isn't cheap upfront, but given how much you're already investing in the rest of your build, skimping on this one component can genuinely cost you more in premature battery replacements. What year's your Transit?

@VanMark ran a split-charge relay in my first van build for about 18 months — a proper old-school VSR setup. Seemed fine until I noticed my starter battery was getting dragged down on longer stops with the engine idling. The alternator on modern Transits isn't designed to bulk-charge a leisure battery and maintain the starter; you're essentially fighting the smart charging system.

Switched to a Victron Orion-Tr Smart 30A and honestly the difference was immediate — proper staged charging, LiFePO4 compatibility, and Bluetooth monitoring so I could actually see what was happening.

The relay saves you maybe £80-100 upfront. But if it damages your alternator or kills your starter battery early, that's a false economy pretty quickly.

For a budget option specifically, the Renogy DCC50S is worth a look — significantly cheaper than Victron and does the job decently.

@VanMark split-charge relay is basically asking your alternator to do a full sprint every time you start the engine — modern smart alternators absolutely hate that, and your wallet will know about it when the alternator bill arrives 🔧

@VanMark I spent years in professional kitchens where we had a saying — cheap kit costs you twice. Same applies here, I'm afraid.

What nobody's mentioned yet is the effect on your leisure battery's actual lifespan. A split-charge relay will bulk-dump unregulated voltage straight in, which absolutely hammers lithium cells if you've gone that route, and even with AGM you're looking at significantly reduced cycle life over time. A decent DC-DC charger like the Orion actively manages the charge profile.

Do the maths properly — if a VSR setup costs you an extra battery replacement in three years, you've already spent more than the B2B would've cost upfront. I learned that lesson the expensive way with kitchen equipment and I'd hate to see you repeat it.

That said, if you're running a small flooded lead-acid setup temporarily while saving up, a relay isn't catastrophic short-term.

Worth adding something that hasn't been mentioned yet — it really depends on what battery chemistry you're running. If you've got a standard lead-acid leisure battery, a VSR split-charge relay is actually a reasonable match, since both batteries want a similar charge profile. Where it genuinely falls apart is with lithium. A DC-DC charger isn't just about protecting your alternator — it's actively regulating the charge profile to suit LiFePO4, which a relay simply cannot do. So @VanMark, what are you running in the back? If you're on AGM or lead-acid and keeping costs down, a decent VSR isn't the disaster some make it out to be. If you're planning to upgrade to lithium later though, factor the B2B cost in now rather than buying twice. @RetiredChef26 isn't wrong about cheap kit costing you twice — just depends which decision that actually applies to in your specific situation.

@WTP_Power raises the most important point in this thread. I'll add some static caravan context — I ran a split-charge relay feeding AGM leisure batteries for about three years before switching to a Victron Orion-Tr Smart 30A. The relay worked, but my alternator was clearly lagging on cold starts and my AGMs were perpetually undercharged because alternator voltage droops badly under load.

Now on Fogstar Drift lithium, a B2B is non-negotiable — the absorption profile simply can't be replicated with a relay.

The maths that convinced me: a decent Orion costs ~£150 and potentially doubles your usable battery cycles. Spread that over five years and the relay "saving" evaporates completely.

If you're genuinely budget-constrained and running AGM, a relay is tolerable short-term. Just don't pair it with lithium.