Anyone else finding lithium prices have finally dropped enough to make the switch worth it?

Been running a pair of 110Ah AGM batteries on the boat for about four years now and they're starting to show their age — taking longer to charge and the resting voltage is sagging more than it used to. So I've been properly looking at replacements this time rather than just buying another set of AGMs out of habit.

Spotted a 200Ah LiFePO4 from Fogstar for around £380, which honestly surprised me. Even 18 months ago I was seeing similar capacity going for £550–£600 and I just couldn't justify it on top of the inverter, the solar panels, and everything else I'd already spent. The usable capacity difference is massive too — I'm only really getting maybe 55Ah out of each AGM before I'm nervous about damaging them, so two of them gives me roughly 110Ah usable. One 200Ah lithium would more than double that in the same footprint.

My main concern is the BMS behaviour in cold weather — we're moored on the Blackwater in Essex and it does get properly chilly from November through to March. Has anyone had issues with charging cutting out on cold mornings? I've got a Victron SmartSolar 100/30 so I could theoretically set a low-temp cutoff, but I'd rather hear from people who've actually dealt with it in practice before I commit.

@EssexBoater mate, four years of AGM on a boat is basically a full life sentence served — swap to a Fogstar Drift 100Ah and your charging woes will feel like a distant nightmare from a previous vessel.

GlenLover | 312 posts

@EssexBoater those voltage sag symptoms are pretty classic end-of-life AGM behaviour — you've definitely got your money's worth out of them!

One thing worth considering beyond just the upfront cost is how much usable capacity you're actually gaining. Your AGMs were probably only giving you 50-55Ah of genuine usable power without damaging them, whereas a lithium of the same rated capacity lets you pull 80-90Ah comfortably. So you might actually need fewer batteries than you think to replace your current bank.

Also worth factoring in your charging setup — do you have a DC-DC charger or a lithium-compatible shore power charger? That's often the hidden cost people forget about when switching. What's your current charging situation on the boat?

AshChild | 847 posts

@EssexBoater worth doing a proper capacity test before committing — a cheap battery analyser will tell you actual usable Ah versus nameplate. I ran my AGMs about 18 months past what most would consider sensible, but knowing the real capacity meant I could plan properly.

On the lithium pricing question — yes, meaningfully so. I picked up a pair of Fogstar Drift 100Ah cells earlier this year at a price point that would've seemed optimistic 18 months ago. The BMS integration with my Victron Cerbo was straightforward enough.

One narrowboat-specific consideration nobody mentions: weight distribution. Swapping 220Ah AGM for equivalent lithium lithium frees up noticeable weight in the stern locker — more relevant on a 57-footer than you'd think for trim.

The maths on cycle life versus upfront cost has genuinely shifted in lithium's favour now.

LochSpirit | 1,204 posts

@EssexBoater one angle nobody's mentioned yet — marine environments are particularly brutal on AGM internals due to vibration and condensation cycling accelerating plate corrosion. Lithium's sealed cell chemistry handles that significantly better long-term.

On pricing: I picked up a pair of Fogstar Drift 200Ah units for my garden office build last spring and the per-kWh cost had genuinely dropped to a point where the maths finally stacked against AGM even at a three-year payback horizon, factoring in DoD differences (you're realistically comparing 50% AGM usable vs 80-90% lithium).

One thing worth verifying on a boat installation specifically — check your alternator isn't going to see a damaging current spike when hitting a cold lithium bank. Victron's Battery Protect or a proper DC-DC charger like the Orion-Tr Smart between alternator and bank is worth budgeting for.

LindaClark90 | 203 posts

@EssexBoater we made the jump to lithium on our narrowboat last spring and honestly wish we'd done it sooner! We went with two Fogstar Drift 100Ah units and the difference in usable capacity versus our old AGMs is just night and day — you're not limited to that 50% discharge anymore.

One thing nobody warned me about though — did you check your alternator setup? We had to add a battery-to-battery charger because lithium charges so fast it was pulling way too hard on the engine's alternator. Cost us an extra £150 or so but absolutely essential apparently.

Also worth checking what your solar charge controller is — if it's still programmed for AGM charge profiles you'll need to change those settings before connecting anything new. What are you running for charging currently?

MeganStevens | 512 posts

@EssexBoater the charging profile difference is worth factoring into your decision too — lithium wants a proper CC/CV charger and if you're running an older alternator setup on the boat, you'll likely need a B2B charger in the mix to avoid cooking it. Adds to the upfront cost but it's a one-time job. We fitted a Victron Orion on ours and it's been faultless. Also worth checking your solar charge controller is lithium-compatible before you commit — some older PWM units just aren't set up for it. Once everything talks to each other properly though, the usable capacity difference versus AGM is genuinely remarkable. You get nearly the full rated capacity rather than that frustrating 50% rule.