Anyone else running a split-charge relay off a leisure battery with a budget 100W panel — is it actually worth it?

So I've been tinkering with my Berlingo camper conversion for the past few months and I'm trying to keep costs sensible. I've got a single 100W mono panel on the roof going into a 20A PWM controller (a cheap Victron knock-off from Amazon, about £18), charging a 110Ah AGM leisure battery. The split-charge relay pulls from the alternator when the engine's running, and honestly it all seems to work fine on overnight stops — but I'm not sure I'm getting the full picture.

The thing I keep wondering is whether the relay is doing more harm than good. I've read that alternators aren't really designed to bulk-charge a deeply discharged leisure battery, and mine does seem to get a bit warm on longer runs. The battery sits at around 12.4V after a night running a 12V compressor fridge (drawing roughly 4–5A average) and a few USB charges. The panel gets it back up to 12.7V–12.8V by early afternoon on a decent day, which feels okay but I'm never quite sure if the AGM is actually getting a proper full charge.

Has anyone done a proper comparison — maybe with a battery monitor — between relying mainly on the panel versus the alternator top-up? I'm wondering if I should ditch the relay altogether and just trust the solar, or whether on cloudy UK days I'd regret it. Trying to avoid spending big money on a DC-DC charger if the relay is genuinely good enough for weekend use.

@BerlingoDream the PWM/split-charge combo is a classic budget starting point — ran something similar on my narrowboat for a season before upgrading.

Honest truth though: a 100W panel through PWM is leaving real money on the table, especially with a leisure battery that's not fully charged. You'll lose roughly 20-30% compared to MPPT on cloudy UK days, which is exactly when you need every watt.

The split-charge relay side worries me more though — what's your alternator situation? On shorter runs around town you're barely tickling the battery before the engine cuts out again.

If budget's genuinely tight, I'd prioritise swapping the PWM for a cheap Renogy 20A MPPT before touching anything else. Picked mine up for about £35 on offer and the difference in autumn harvests was immediately obvious.

What capacity leisure battery are you running against that panel?

Good setup to start with @BerlingoDream. One thing worth checking — if your split-charge relay is a simple voltage-sensing type, make sure the solar isn't fooling it into thinking the starter battery is full when it isn't. The panel voltage can sometimes trigger the relay even with a poorly charged starter battery. A dedicated VSR (voltage sensing relay) with a proper threshold helps here, usually around 13.3V to engage. Also worth knowing that your 100W panel in UK conditions realistically gives you maybe 200-300Wh on a decent day, so manage expectations accordingly — it's brilliant for topping up but won't power a kettle! What's your leisure battery capacity? That'll help work out if the setup is balanced properly.

Hey @BerlingoDream, one thing I'd add that hasn't been mentioned yet — with a 100W panel and PWM, keep a close eye on your cable runs. Budget setups often fall down not on the components themselves but on voltage drop across undersized wire, especially if your panel's mounted at the back and your controller's up front. Even a modest drop can really hamper a PWM controller's efficiency compared to MPPT. Worth doing a quick calculation based on your actual run length before assuming the system's underperforming. Also, what leisure battery are you running — flooded, AGM, or something else? Makes a fair difference to how well a simple split-charge setup keeps it topped up in practice. Cracking little vans the Berlingos, should serve you well as a first build! 🙂

Worth mentioning the battery chemistry side of things — if you're running a standard lead-acid leisure battery, that PWM + split-charge setup is actually a reasonable match since PWM suits lead-acid better than lithium anyway. I ran a near-identical setup in my van before upgrading to a Fogstar 100Ah LiFePO4 and a Victron MPPT, and honestly the basic rig did the job fine for weekends away.

The real limitation @BerlingoDream will hit is usable capacity rather than the charging side. Lead-acid gives you roughly 50% usable, so a 100Ah battery is realistically 50Ah. For emergency backup that's workable, but if you're planning longer stays you'll feel it quickly — especially in winter when that 100W panel is often producing closer to 30-40W in the UK.

@GrumpyHermit replied:

Worth it? Yes, but only if your expectations are realistic. A 100W panel through PWM will genuinely disappoint you on a grey November morning — and let's be honest, this is Britain, so budget for grey mornings.

The split-charge relay side of things is fine as a starting point but do yourself a favour and check whether yours is a VSR or a manual isolator type — they behave quite differently when both sources are charging simultaneously. @Russ1960 is on the right track there.

What's your actual leisure battery capacity? Because a single 100W panel into anything under 100Ah is reasonably matched, but go much bigger and you'll be waiting an age for a full charge. The maths matter more than people admit when they're romanticising the whole setup.

@BerlingoDream one thing nobody's flagged yet — the split-charge relay side of your setup could actually be your weak link here. A standard VSR will only kick in around 13.7V, which means on dull days your panel might never trigger it properly through a PWM controller.

Ran into this exact issue on my shepherd's hut before I swapped to a Victron DC-DC charger instead. Night and day difference, though obviously not "budget" territory.

For now, manually monitoring with a cheap battery monitor (Votronic do decent ones) will at least tell you if the relay's actually doing anything useful or just sitting idle.

@GlenChild replied:

Good thread, this. One thing I'd throw in that nobody's touched on yet — the split-charge relay side of things can actually let you down more than the solar if the alternator isn't running long enough. Short town drives won't do much for your leisure battery regardless of what's happening on the roof. If you're mostly doing longer runs you'll be fine, but if it's mostly urban driving, the 100W panel might actually be doing more heavy lifting than your relay.

Also worth checking the voltage drop between your panel and controller — cheap cable over even a modest run can quietly rob you of a fair chunk of your actual output. Proper sizing there costs next to nothing but makes a real difference.