PBO verdict

In simple terms, all the batteries on test delivered within 3-4Ah of their claimed maximum outputs at 25A and 100A, with some even achieving this at 150A.

Also as the lithium battery discharges, its voltage remains nearly constant, only falling away during the last 10% of the time. In contrast a lead-acid battery voltage falls in nearly a straight line, so the actual power available in a lithium battery is greater.

However, even though prices are falling, the headline figure for lithium batteries still looks daunting at more than five times the price of seemingly equivalent lead-acids.

But if you get ten times the number of cycles, then the whole-life cost becomes half. And of course there is the reduction in weight. Straight from the box, a lithium battery is around half the weight of lead, but factor in the increased capacity and the weight saving becomes even greater.

Best buy

We normally try to give a suggested best buy in our tests, but in this case there is no one clear winner, and no single test measurement that we could fairly apply.

For instance the Sterlings give you more Ah for your money, but the Victron, Super B Epsilon, Mastervolt and Relion 100HP give you higher continuous currents.

All the units deliver almost exactly their rated Ah, within our experimental tolerances, but the claimed cycle lives vary. None of these cycle life figures could be verified in our test, and may have been estimated using different depth of discharges. But in any case all reckon to give you at least 2,000 cycles, which for most leisure users is probably at least 20 years of normal use.

Similarly the prices could vary as soon as retailers start discounting them, and again prices are all moving rapidly – usually downwards – as the market expands.

It is generally agreed that those with multi-cell construction will have longer ultimate lives, but at a higher price, and again this is relative and unlikely to be significant for private use, but could become more important for something like a daily use ferry for instance.

Worth the money?

So is lithium for you? Well, being realistic, if you only use your boat 10-20 times a year, and just run lights and navigation equipment, then lead-acid still probably wins by being more cost effective.

But buy the best you can find, with a gel-type top of the list. Keep it well charged up and with sensible use it will last ten years.

But once you start going out more often, or running an inverter, then lithium becomes worth looking at.

And bear in mind that new lithium models are arriving, and prices are falling almost by the day.