Fitting a second 100Ah lithium on a 26ft bilge keeler — worth the faff?

So I've got a 2006 Westerly Griffon that I live aboard about four months a year, mostly down at Lymington. Currently running a single 100Ah LiFePO4 (a Fogstar Drift) charged by a 200W solar panel on the pushpit and a Victron SmartSolar 100/20. That setup just about copes when I'm careful, but I'm running a 12V compressor fridge, chart plotter, AIS, and VHF pretty much constantly, and by day three of overcast weather I'm watching the SOC drop below 30% and getting nervous.

I'm thinking of adding a second 100Ah battery to give me 200Ah total. The locker where the current battery lives is tight — I can just about squeeze another one in if I use the same Fogstar Drift form factor, which is roughly the same dimensions as a standard AGM. The main faff I can see is upgrading the cabling, sorting the busbars, and making sure the BMS units play nicely together when wired in parallel. Has anyone run two separate LiFePO4s with independent BMS units in parallel and had issues with them falling out of balance or one BMS cutting out and dumping load onto the other?

I've also wondered whether it's worth ditching the 200W panel and going to a single 175W flexible on the coachroof instead, purely for aesthetics and to stop the rigid panel acting like a sail when I'm motoring into a stiff southwesterly. But I'd obviously be losing a bit of capacity there, which seems daft if the whole point is to have more reserve. Probably a separate thread that one.

Mate, I fitted a second battery in my shepherd's hut and the only thing harder than justifying it to my wallet was justifying it to my wife — go for it, parallel Fogstar Drifts are practically made to be paired, and four months liveaboard on one 100Ah is basically camping with pretensions.

DebbieEvans | 847 posts

@WillStevens Short answer: yes, absolutely worth it for liveaboard use. Four months on a Griffon means you'll be running nav lights, instruments, the odd bit of cooking kit — that single 100Ah will have you watching the SOC like a hawk every evening.

Practically speaking, the Fogstar Drift pairs nicely with itself, so a second unit should balance well if you wire them properly in parallel with matched cable runs. The bilge area on those Westerlys can be tight though — measure carefully around the companionway step area before committing.

One thing worth checking: is your solar controller rated for the increased battery bank? Some cheaper PWM units get grumpy when you double the capacity. If you've got an MPPT you're probably fine.

The "faff" is a weekend's work. The peace of mind at anchor in the Solent is indefinite. 🙂

@WillStevens The wiring side is the main faff on a bilge keeler — awkward battery compartments and getting proper cable runs sorted. But 100Ah is genuinely tight for liveaboard use, especially if you're running a fridge overnight.

One thing worth considering: check your existing charger/MPPT can handle the combined bank first. A 200W panel will struggle to top up 200Ah efficiently in typical UK coastal weather — you might want to uprate to 300W+ at the same time.

I run a Victron SmartSolar which makes it easy to monitor state of charge across both batteries, and honestly that alone justified the upgrade for me. Fogstar Drifts are solid units so pairing a second one should be straightforward — just make sure both batteries are the same age/cycle count before connecting in parallel, otherwise the older one will drag the new one down.

@WillStevens Done exactly this on my 26ft — two 100Ah in parallel is a proper game changer when you're doing multi-day stays without shore power.

One thing nobody's mentioned: check your alternator output first. If you're running the engine to charge, a single-stage alternator dumping into two LiFePO4s can cook itself once the batteries start accepting bulk charge properly. A Victron Orion DC-DC charger between alternator and battery bank is worth every penny to protect it.

Also — Fogstar Drift cells parallel together well in my experience, just make sure your cables to each battery are identical lengths to balance the load properly. Sounds fussy but it genuinely matters long-term.

The faff is real but it's a weekend's work, not a project.

PeteDixon89 | 312 posts

@WillStevens One thing worth considering that nobody's mentioned yet — check your alternator's output and whether you've got a proper B2B charger in the mix. Parallel LiFePO4s can hammer an unprotected alternator when they're sitting low and demand current hard. A Victron Orion-Tr Smart between the engine and the bank will save your alternator from an early grave, especially if you're motoring into Lymington after a few cloudy days with both batteries depleted. Adds a bit of cost but genuinely worthwhile on a liveaboard setup. The Fogstar Drifts are solid batteries BTW, good choice for the second one too — keeps your chemistry consistent across the bank.

Neil1978 | 847 posts

@WillStevens Agree with @VoltChloe that it's worth doing, but I'd flag one thing specific to the Griffon — the battery compartment under the companionway steps is notoriously tight. I've seen people struggle to fit a second LiFePO4 in there without some creative rearranging of the bilge pump wiring. Worth measuring twice before you order.

Also, if you're primarily on solar with that 200W panel, your charge times with double the capacity will obviously increase — might be worth considering whether a small DC-DC charger from the engine circuit would help top things up on passage days. @PeteDixon89's alternator point is well worth heeding too.

The Fogstar Drift is solid kit, good shout matching it with another of the same.