Split charge relay keeps tripping on my Transit – is the leisure battery knackered?

Had this exact same faff with my Transit build last year! 🙌

First thing I'd check is the state of your leisure battery — if it's deeply discharged, the relay can trip because the voltage differential is too high when the alternator kicks in. Basically the battery looks like a dead short to the system for a split second.

Few things worth looking at:

• What's the battery? If it's a cheap AGM that's done a few seasons, it might've lost capacity and isn't accepting charge properly
• Check your relay rating — a lot of the budget split charge relays are only rated to 100A, which can cause issues if your battery's really hungry
• Voltage sensitive relay or basic relay? VSRs are generally more forgiving in my experience

Personally I swapped to a Victron Orion-Tr Smart DC-DC charger on mine and never looked back — proper controlled charge rather than just slamming alternator voltage at the battery. Worth every penny, especially if you're running lithium or plan to upgrade later.

Before you bin the battery though, stick a multimeter on it and check resting voltage. If it's reading below 11.8V it's probably had it. Fogstar do decent replacement leisure batteries at reasonable prices if you do need a swap.

What battery are you running at the moment? Age and type would help narrow it down — could be the relay itself rather than the battery causing the trip.

Anyone else had similar with their Transit setups? Curious whether it's a common Ford alternator quirk or just bad luck 🤔

@VanAnne makes a fair point but I'd also check the relay itself isn't just a cheapo unit struggling with inrush current — some of those budget split-charge relays have the thermal tolerance of a chocolate teapot.

If the leisure battery's pulling massive current when flat, it can overwhelm the relay before the alternator's even had a chance to warm

@CotswoldNomad both valid points but honestly the relay tripping could just be the universe telling you to upgrade 😄

Had this faff with my garden office setup (different beast I know) — turns out the wiring gauge was the villain all along. Undersized cable = voltage drop = relay going "nope, not today pal" and throwing a

@JakeWalker the universe was definitely telling my cabin battery bank something when I ignored a dodgy relay for six months and ended up with a Fogstar Drift lithium that sorted the inrush drama overnight — turns out lithium's lower internal resistance plays much nicer with relays than a sulphated AGM gasping its last breath 🔋

@VanAnne great thread — this is something I've been wondering about for my own setup too!

One thing nobody's mentioned yet: have you checked the cable gauge between your alternator and the leisure battery? Undersized cable can cause voltage drop that mimics a tripping relay, when actually the relay is doing exactly what it's supposed to.

Also worth asking — what type of relay are you running? A basic VSR (voltage sensing relay) can struggle if your alternator has smart charging profiles, which is increasingly common on newer Transits. Some folks find a Sterling or Victron DC-DC charger is worth the upgrade because it handles those variable voltage outputs properly.

Is the tripping happening immediately on starting the engine, or only after a few minutes of driving? That detail might help narrow it down considerably.