Swapping out a tired 100Ah lead acid for lithium in a Transit-based van — worth the hassle mid-trip season?

Background: I've been running a 100Ah AGM under the bed in my Transit conversion for about three years. It's now struggling to hold more than about 60–65Ah usable before voltage sags badly — classic end-of-life behaviour. I've got a Victron SmartSolar 100/20 MPPT feeding it from a single 175W panel on the roof, and a Sargent EC450 control unit managing the split-charge from the alternator.

The obvious upgrade is a 100Ah LiFePO4 — I've been looking at Fogstar Drift 100Ah as the price-to-quality ratio seems decent, and it's a proper UK-stocked option rather than waiting weeks for something from overseas. The problem is the Sargent unit. As far as I can tell, it uses a standard VSR for the split-charge side, which won't play nicely with lithium's flat charge curve — you risk either undercharging or never getting a proper bulk cycle done.

Has anyone running a Sargent-based system made this switch? I'm wondering whether a DC-DC charger (B2B) like the Victron Orion-Tr Smart 12/12-18 is the cleanest fix, or whether there's a way to reconfigure the Sargent to behave sensibly with LiFePO4. The MPPT side should be fine — I can just update the charge profile in VictronConnect.

Also curious whether 100Ah LiFePO4 is actually enough to bother with, or whether I should just go straight to 200Ah whilst I've got the system partially stripped out anyway. Running a compressor fridge (around 35–45Ah/day), laptop, lighting, phone charging — no inverter loads to speak of.

Ella1966 | 847 posts | ⭐ Regular

@SuffolkCruiser Three years is pretty typical for a van AGM getting worked hard, so no surprises there. My honest take — if you're mid-season and heavily reliant on it, the swap is absolutely worth doing sooner rather than later. A degraded battery will only get worse, often quite suddenly.

One practical tip: make sure your B2B charger (if you have one) or split charge relay is actually lithium-compatible before you swap — plenty of Transit conversions are wired for lead acid charging profiles and you'll undercharge a lithium badly otherwise.

Drop-in 100Ah LiFePO4 is fairly straightforward to fit on a weekend. You'll notice the difference immediately — proper usable capacity right down to 20% without the voltage sag you're describing now. Worth every penny in my experience. 👍

LazyWarden | 203 posts | 🌱 Member

Not van-based myself but I've swapped out tired lead acid for lithium in a garden office setup and the difference was immediate — proper usable capacity rather than that frustrating voltage sag you're describing.

One thing worth checking: is your Transit's alternator charging sorted? I nearly wrecked a new Fogstar battery because my existing setup wasn't regulated properly for lithium. You'll likely need a B2B charger — Victron Orion-TR Smart is the obvious choice — otherwise you're potentially cooking the new cells or undercharging them.

Also worth asking: what's your solar situation? If you're relying heavily on alternator charging mid-trip season, the B2B question becomes even more critical before you pull the trigger on the swap.

BatteryDoug | 1,247 posts | ⭐⭐ Enthusiast

Mid-trip season is actually ideal timing if you ask me — you'll feel the benefit immediately rather than waiting until autumn. The swap itself is straightforward on a Transit layout, but do check your alternator situation first. Many B2B chargers like the Victron Orion-Tr Smart are worth fitting alongside, as lithium's low internal resistance can pull harder than your alternator's happy about on a long motorway run. Also double-check your existing solar controller is lithium-compatible — many older PWM units aren't. The usable capacity jump from your current ~60Ah effective to 80–90Ah from a 100Ah lithium will feel transformative day-to-day. What's your current charging setup — solar only, or are you running shore power hookups too? That'll affect which battery chemistry and BMS spec makes most sense for your use case.