The question came up on the pontoon at La Trinite last June, the way it always does. A Dutch couple two berths down had just spent a small fortune on lithium and wanted to know if I thought they had wasted it. The boat next to them had four fat AGM batteries that had cost a fraction of the price. Both crews were heading to the same anchorages off the Morbihan for the same six weeks. So who got it right?
I have run both. My answer is that it depends entirely on how you cruise, and that most people who agonise over the choice are asking the wrong first question.
The number that actually matters
Forget the headline capacity printed on the case. What matters is usable capacity, and the two chemistries are not even close.
An AGM battery does not like being run flat. Drain it past 50 per cent regularly and you shorten its life sharply, so a 100Ah AGM gives you roughly 50Ah you can actually touch. Lithium iron phosphate, the LiFePO4 chemistry that now dominates the leisure market, will hand you 80 to 100 per cent of its rated capacity without complaint. A 100Ah lithium therefore delivers nearly double the working energy of a 100Ah AGM of the same nominal size.
So when you read that lithium "costs twice as much", remember you are also getting close to twice the usable amp-hours per unit. The cost per usable amp-hour is much closer than the sticker price suggests, and over a battery's life it is often less than half.
Cycle life over a French season
Here is where the gap turns into a chasm. A decent AGM bank gives somewhere between 300 and 1,500 charge cycles depending on how kindly you treat it. LiFePO4 routinely delivers 3,000 to 7,000 cycles, and the better marine cells claim more.
Translate that into a French summer. If you anchor off Belle-Ile and the Glenans for six weeks, charging and discharging once a day, that is around 40 deep cycles a season. An AGM bank pushed hard might give you four or five seasons before it sags. The lithium, on those numbers, will outlast the boat. I have watched friends replace AGM banks twice in the time my lithium has sat there unbothered.
Weight, and why it matters more than you think
A 100Ah LiFePO4 battery weighs around 12 to 14 kg. A comparable AGM lands closer to 28 to 32 kg. That is not a vanity statistic. On a 35-foot boat, swapping a 400Ah lead bank for lithium can take 60 to 70 kg out of the bilge, often low and central where it most affects trim. Lighter ground tackle of the electrical kind, if you like.
For a visiting cruiser this matters in two ways. You probably loaded the boat for a long trip, so you are already heavy. And French summer cruising involves a lot of anchoring, where waterline and trim affect how the boat sits and swings. Taking weight out of the wrong end is a quiet upgrade.
The cost reality in 2026
Let me be honest about money, because that pontoon conversation always comes down to it. Marine-grade LiFePO4 batteries with proper monitoring still sit between roughly 800 and 2,000 euros each for a quality 100Ah unit, though basic units have fallen well below that. AGM is cheaper up front, sometimes dramatically so, and that is the genuine pull.
But you must add the hidden cost of going lithium properly. You cannot bolt LiFePO4 onto a tired old charging system and hope. You need a charger and an alternator regulator that understand the chemistry, and a battery management system to protect the cells. Budget for that supporting kit. The batteries are only part of the bill, and the install is where people get caught out.
When AGM is the right call
I will not pretend lithium is always the answer. AGM still makes sense in several real cases.
- You marina-hop more than you anchor. If you plug into shore power most nights and rarely drain the bank, AGM's weaker cycle life never gets tested. Tour the water, electricity and showers in French ports and you may decide you simply do not need the lithium headroom.
- The boat is a stopgap. If you are buying a used boat in France for a single season and selling on, the lithium investment may not pay back. Read buying a used sailboat: hull inspection 10 tips before you spend big on either.
- Your charging system is original and you have no budget to upgrade it. AGM is forgiving. Lithium is not.
There is also a fire-risk conversation worth having honestly. Quality LiFePO4 with a proper BMS is very stable, far safer than the lithium-ion in your laptop, but cheap cells and bad installs do cause incidents. Buy known brands and have the install checked.
A worked example off the Morbihan
Numbers help, so here is the actual comparison from those two boats at La Trinite, scaled to a typical 35-foot cruiser burning 120 amp-hours a day.
The AGM crew carried 400Ah of lead, which on the 50 per cent rule gave them 200Ah usable, comfortably one and a half days at anchor before the engine had to run. To get there they hauled around four batteries weighing close to 120 kg, and on the cube of a hot summer the fridge alone clawed through nearly 100Ah of that. They ran the diesel most mornings to take in a fast slug, then watched the charge rate collapse as the bank filled past 80 per cent.
The lithium crew carried 300Ah of LiFePO4, lighter at well under 50 kg, but on the 90 per cent rule they had 270Ah usable from a smaller, cheaper-to-ship bank. Their alternator and panels poured charge in at full rate almost to the top, so a couple of grey days never forced an engine start. Over six weeks the lithium boat ran its engine for charging perhaps three times. The AGM boat ran it most days.
Neither was wrong. But the lithium boat's summer was quieter and its bilge lighter, and on a cost-per-usable-amp-hour basis, spread over the years the cells will last, it was not the extravagance it looked on the invoice.
How they pair with solar and an engine
The chemistries behave differently when you feed them. AGM accepts charge slowly as it fills, so the last 20 per cent crawls in. That is the curse of engine charging: you run the diesel for an hour, take in a quick slug, then watch the charge rate collapse. Lithium accepts a high charge rate almost all the way up, so your alternator and your panels fill it fast and you spend less time listening to an engine over a sunset that deserved silence.
This is exactly why lithium and solar are such natural partners. If you are building a system from scratch for a season of anchoring, the case for pairing them is strong, and I worked through the sizing in detail in my guide to solar and lithium for a French summer cruise. The same logic applies to the continuous loads of a passage, which I covered in power management on a French coastal passage.
If you are going further and adding thirsty kit, the calculation tilts harder toward lithium. A watermaker for French cruising or serious refrigeration for a French summer cruise both want a bank that can deliver and refill quickly, which is lithium's home ground.
My verdict for a French summer
If you anchor more than you berth, fit lithium and do the supporting work properly. The usable capacity, the cycle life and the weight saving all line up for the way a French summer actually unfolds: weeks in the Glenans, the lee of Belle-Ile, the calanques near Cassis, charging off the sun rather than the engine.
If you berth more than you anchor, or the boat is short-term, AGM is the sensible, cheaper choice and you should not feel you are missing out.
The Dutch couple at La Trinite, for the record, anchored almost every night. They got it right. So did their neighbours, who plugged in and slept easy. The boats were the same. The cruising was not. Work out which one you are before you spend a euro.
