Worth swapping AGM to lithium in a tiny house setup — numbers don't quite add up for me?

Been running a 200Ah AGM bank (2x 100Ah 12V in parallel, Varta leisure batteries) in my tiny house for about three years. System is fed by 400W of Renogy panels through a Victron SmartSolar 100/30, with a Victron Multiplus 12/3000 as the inverter-charger. Works fine, but I'm losing noticeable capacity to the 50% DoD limit and the batteries are starting to sulphate — resting voltage is sagging more than it used to.

The obvious upgrade people point to is a 200Ah lithium drop-in, something like a Fogstar Drift 200Ah or a Redodo/Enjoybot if budget is tighter. Usable capacity nearly doubles overnight, cycle life is vastly better, and the Multiplus handles lithium charging profiles without complaint once you tweak the absorption/float settings. On paper it's a no-brainer.

Problem is the maths. I'm stationary most of the time so charge cycles are fairly predictable — roughly 80–100Ah out per day in winter. A decent 200Ah LiFePO4 is sitting around £400–£550 right now. Two replacement AGMs would cost me maybe £160–£180 all in from Tayna or similar. Even accounting for the DoD difference and longer cycle life, the payback period at my usage rate feels like it stretches past 6–7 years.

Has anyone actually run the numbers properly on a low-daily-draw static setup? Curious whether the lithium premium genuinely stacks up or whether I'm better off just replacing the AGMs and revisiting in a couple of years when prices drop further.

Hey @LDVAdventure! One thing worth factoring in that people often miss — AGMs in parallel can develop imbalances over time, so your usable capacity is probably already noticeably less than that nameplate 200Ah, especially after three years. Lithium's flat discharge curve also means your appliances run more efficiently right through the cycle, which isn't always reflected in straight Ah comparisons.

Your Victron SmartSolar is already lithium-compatible with a firmware/profile tweak, so no controller cost there. Main outlay is obviously the battery itself.

What's your typical daily consumption looking like, and do you have winter shading issues? That'd help work out whether you'd genuinely benefit or whether your current setup just needs the AGMs replacing like-for-like for a fraction of the cost. Sometimes that's honestly the smarter move. 🔋

@LDVAdventure the usable capacity argument is where lithium really wins on paper, but your numbers need to account for the full charge cycle. AGMs want a proper absorption phase — your SmartSolar 100/30 is spending meaningful time at reduced current finishing that charge. Lithium accepts bulk current almost to the top, so your 400W array is actually working for far longer per day.

Ran the same sums before upgrading my garden office to a Fogstar Drift 200Ah LiFePO4. Real-world difference was roughly 25% more usable solar harvest on cloudy days simply because the acceptance curve is so much flatter.

Counter-question worth asking yourself: what's your worst-case winter day look like? If you're supplementing with a mains hook-up or generator anyway, the AGM inefficiency matters far less and the payback period stretches considerably.