Swapped out my 100Ah AGM for a 100Ah lithium — night and day difference or am I imagining things?

Finally took the plunge last month and replaced the ageing Varta AGM in my Ducato-based van with a Fogstar Drift 100Ah LiFePO4. Same capacity on paper, but in practice it feels like I've doubled my usable storage overnight. Running a 40W compressor fridge, a few USB charges and the odd bit of LED lighting, I was lucky to get through a single overnight on the AGM without it sagging badly by morning. Now I'm comfortably doing two nights without shore power before I even think about worrying.

I kept the same 200W of roof-mounted solar and the existing Victron SmartSolar 75/15 MPPT controller, just updated the battery profile in the app. Resting voltage sits around 13.3V most evenings which I understand is pretty healthy for LiFePO4. The Fogstar came with a basic BMS built in, so I've not added any external protection, though I've been wondering whether I should.

One thing that's thrown me slightly — my Sargent EC160 control unit seems to charge at a fixed absorption voltage of around 14.4V, which I think is fine for LiFePO4 but I'm not 100% sure it's actually getting the cells properly balanced. Anyone else running a Fogstar or similar with a Sargent unit and had issues?

HarbourKate | 847 posts | ⚓ South Coast

@BorderNomad You're absolutely not imagining it! The usable capacity difference is the real kicker — your AGM was probably only giving you 50Ah safely, whereas the Fogstar will happily hand over 80-90Ah without batting an eyelid. That's nearly double the actual usable energy from identical nameplate figures.

The voltage curve is what I noticed most when I switched — AGMs sag noticeably under load, so your lights dim, your compressor fridge works harder, everything feels sluggish as the battery depletes. LiFePO4 just holds steady right until the end.

Worth double-checking your alternator charging setup though if you haven't already — a B2B charger is strongly recommended with LiFePO4 in a Ducato to protect both the battery and your van's smart alternator. Did you sort that side of things? 🙂

GazKelly75 | 312 posts | 🔧 Peak District

@BorderNomad Not imagining it at all mate. The bit that surprised me most when I made the same switch wasn't just the usable capacity — it was how the voltage holds steady right across the discharge curve. With my old AGM, the 12v fridge would start struggling and throwing low voltage warnings when the battery was maybe 60% depleted. The lithium just... keeps going at consistent voltage until it's genuinely nearly empty. Makes a massive difference to how your 12v appliances actually perform day-to-day. The Fogstar Drift is decent kit too — solid BMS on those. Only thing worth double-checking is that your B2B charger or solar controller is actually set to a LiFePO4 charging profile rather than AGM, otherwise you're leaving performance on the table.

HollyWatson | 203 posts | 🌿 Welsh Borders

Great choice with the Fogstar Drift, @BorderNomad — solid bit of kit for the money. One thing worth mentioning that nobody's touched on yet: notice how your alternator charging behaves now? LiFePO4 will happily gulp down whatever your B2B or split charge relay can throw at it during that bulk phase, so you're actually getting meaningful top-ups from shorter drives rather than the AGM's sluggish acceptance rate dragging things out. I found my 40A B2B suddenly felt worth having after the swap! Just keep an eye on your charge profile settings if you haven't already — some older VSR setups weren't designed with lithium's characteristics in mind. Enjoy the extra headroom either way, it genuinely changes how relaxed you feel about using appliances in the evening. 🙂