Switched from AGM to lithium mid-build — here's what actually surprised me

Last spring I was deep into fitting out my tiny house on wheels (a converted Iveco Daily, if anyone's curious) and I'd budgeted for two 110Ah AGMs because that's what every YouTube video from 2019 told me to buy. Then I fell down a Fogstar rabbit hole at 1am and ended up ordering a single 200Ah Fogstar Drift LiFePO4 instead. Same rough budget, completely different world.

The bit nobody warned me about properly was the charging profile. My existing Victron SmartSolar MPPT 100/30 handled the switch fine once I updated the charge settings — but the van's factory alternator charging via the split-charge relay was a disaster. LiFePO4 accepts charge so fast that the alternator was working flat out and running hot. Ended up fitting a Victron Orion-Tr Smart DC-DC 30A isolator, which sorted it completely and honestly should be mentioned in every lithium conversion guide.

The usable capacity difference is just embarrassing in the best way. On AGM you're really working with maybe 50% of rated capacity if you want a long life. On the Fogstar 200Ah I'm genuinely using 180Ah day-to-day without a second thought. My 175W Renogy panel keeps it happy through three seasons without shore power.

Curious whether anyone else found the alternator issue caught them off guard, or if there's a smarter/cheaper way to handle it than the Orion? Also wondering if a second 200Ah battery in parallel is worth it for winter or whether I should just add more solar on the roof first.

@DriftGal same boat here, well, same van. Had AGMs in my old motorhome for years and the weight alone should've pushed me to lithium sooner. Two 110Ah AGMs is basically a small child sitting in your vehicle permanently.

Biggest surprise for me switching was the resting voltage thing — kept thinking my Fogstar batteries were dodgy because the BMS cuts so sharp. Took a while to retrain my brain off the old AGM voltage curves.

Also nobody mentions how much faster your alternator charges them. Nearly cooked my B2B first week because I hadn't sized it properly for the charge acceptance rate. Worth double checking your DC-DC charger is actually rated for lithium properly, not just marketed as "compatible."

What capacity did you end up going with?

Really interesting thread, @DriftGal! The thing that caught me off guard when I made the switch was how differently lithium behaves at low state of charge. AGMs start struggling and voltage sags noticeably when you're pulling them below 50% — you almost get a warning. Lithium just delivers full power right down to nearly empty, then drops off a cliff. Took me a while to trust the BMS readings rather than relying on "how things feel." Also worth mentioning for anyone mid-build: your existing AGM-spec charging profile almost certainly needs adjusting. Ran my Victron on AGM settings for a fortnight before I properly sorted it — wasn't doing the cells any favours. @ZFS_OffGrid the weight thing is massive isn't it, especially on a Daily where payload matters.

Fitting out my shepherds hut last year I nearly wept when I realised my Fogstar 100Ah weighed less than one of the AGMs it replaced — suddenly I had room for an actual kettle instead of a battery.

Has anyone else noticed the resting voltage behaviour catching them out after the switch? With my AGMs I could glance at the voltage and get a rough SoC estimate, but with lithium the flat discharge curve means you're basically flying blind without a proper battery monitor.

Ended up fitting a Victron SmartShunt — genuinely one of the better decisions in my motorhome build. Worth factoring that cost in if you're budgeting a switch.

Also curious @DriftGal — did you go with a drop-in LiFePO4 (like Fogstar Drift) or build a pack with a separate BMS? I went drop-in first time round for simplicity, but wondering whether a custom setup gives noticeably better charge/discharge control for solar integration specifically.

Really relate to @WattLiz's point — that state-of-charge guessing game is a nightmare at first. What actually surprised me most was how aggressively the Victron BMV battery monitor earns its money with lithium. With my old AGMs I barely glanced at it, but now it's practically essential. The flat discharge curve means voltage alone tells you almost nothing until you're suddenly at 20% and wondering where the afternoon went.

Also caught out by the charging current acceptance — my lithium just gulps down whatever my solar throws at it, so my Renogy panels that felt "enough" with AGMs now feel borderline undersized during heavy usage days. Anyone else end up upsizing their array after switching?

@WattLiz and @OldSailor75 — yes, the voltage readings are almost meaningless on their own with lithium, but honestly once you've got a proper battery monitor (I use a Victron BMV-712) you'll wonder how you ever managed without one regardless of chemistry. The percentage readout becomes your new normal pretty quickly.

What genuinely caught me off guard was how flat the discharge curve runs right until the end. First time my system suddenly shut off I thought something had failed — turned out I'd just actually hit the bottom. With AGMs you get that gradual fade as a warning. Lithium just... keeps going, keeps going, then stops. Took a bit of getting used to mentally, even knowing it in theory beforehand.

@MoorRoger is right that you get used to it, but the thing that actually caught me off guard switching my shepherd's hut setup over was the charging profile difference. My old Victron MPPT was set to AGM defaults and was just floating the lithium at the wrong voltage for weeks before I noticed. Not dangerous, just inefficient — lost probably 10–15% usable capacity until I dug into VictronConnect and updated the profile properly. Worth double-checking your charge controller settings aren't still running old presets.