‘When I made the change to lithium batteries on my yacht I saved space and money’

Simon Hardaker set about making the switch from lead-acid batteries to lithium batteries, here's what he learned throughout the process

We bought our boat, Cassini, a Beneteau Oceanis 50, in 2022 intending to live aboard for long periods from the summer of 2023. After a winter overhaul and a few shakedown cruises to Cornwall and the Channel Islands that spring, it became clear that our existing domestic bank battery would need to be renewed before we set sail for our planned cruise to the Caribbean later in the year.

Our existing lead-acid batteries were AGM type. AGM or Absorbent Glass Mat, is a battery type and construction that is sealed and low maintenance, unlike the old ‘flooded’ cells that need to be topped up with distilled water from time to time.

Our AGM batteries were charging well enough, but the rate and depth of discharge was fast and low. The voltage regularly fell off to less than 50 per cent. This could have been the loads we subjected the battery to, or the batteries themselves reaching the end of their useful life. Either way, they required renewal to meet expected demand for the forthcoming liveaboard period…

Charging cycles

AGM batteries can typically go through between 500 and 1,000 complete cycles of charge and discharge before they need replacing, much fewer if regularly discharged below 80 per cent. Some suppliers claim as many as 1,500 cycles.

For a liveaboard boat, that could equate to just two or three years’ use. Our existing domestic batteries had been replaced two years earlier, but the boat had completed an Atlantic circuit by then. So, by my conservative reckoning, they had used up over half of their available cycles during that period based on usage.

We have all the usual instruments on board our boat: chart plotters, lights and so on, plus two fridges, a microwave, toaster, watermaker, even a washing machine. That’s a lot of power. The domestic battery was made up of five 120 amp-hour (Ah) AGM batteries, totalling 600Ah altogether.

The batteries were charged by 800W of solar panels, a Silentwind 600 (wind generator), Watt & Sea 600 (water generator) and a 100A Balmer alternator on the engine. We have no diesel generator on board.

Our engine start battery is another 120Ah AGM which we weren’t planning to change. Apart from having plenty of lifetime remaining in that particular battery, one of the advantages of AGMs is their ability to deliver high current for a short time – exactly what you need to start an engine.

Lithium does not share this profile so well and is less suitable for engine starting. We hoped, therefore, that our renewable sources could routinely maintain a healthy domestic battery, of whatever kind, with only very occasional need to run the engine to recharge it.

Battery options

Faced with a significant capital outlay, whichever route we decided to take to replace the batteries, we looked at two basic options – either replace the old batteries with more AGMs, or make the change to lithium ones.

There are various kinds, or rather chemistries, of lithium batteries. Marine applications today, that the yachting industry and marine insurers can all get behind, are lithium iron phosphate, LiFePO4, commonly referred to as LFP.

There are other sorts of lithium batteries in use around the world today. Some are commonly fitted in electric cars for example, which have a very different chemistry including cobalt and other elements, that can supply the high current that you need to start a car. These applications have more complex safety and management systems installed and should not be considered for yacht applications. In the early days of lithium adoption, there were a few examples of this kind of battery catching fire on superyachts, so they are best avoided.

There is a wealth of information available online about LFP installations on yachts all over the world now, from YouTube to Facebook and much more in-between.

I used all these resources, together with some of the contacts made on Facebook, to prepare a comparative cost benefit of both systems before choosing a solution to suit us and our liveaboard plans.

Evaluating the economics

LFP batteries, on a one-off cost basis, are more expensive than AGM. Part of our calculation, however, considered that, while we could reasonably expect to get up to a maximum of 1,500 cycles from another set of carefully husbanded AGM batteries, the LFP alternatives could deliver maybe 5,000.

This would represent a lifetime saving since it would have needed three or more changes of AGMs before the LFP ones had used their available cycles. For a liveaboard application like ours, LFP completely made sense. If the application was for more occasional boat use throughout a summer season, the economic case might be slightly different, and a like-for-like AGM solution would be better.

There are other advantages of LFP over lead acid batteries though. All lead acid cells, including AGMs, can realistically only be discharged to 50 per cent of their capacity – so of the 120Ah, you can really only safely use 60Ah. LFP batteries can go all the way down to 10 per cent of their capacity. Indeed, LFP batteries can actually be recovered from almost zero per cent too. In effect, this meant that for the same usable capacity in our domestic batteries, we could actually reduce the number of batteries, and Ah, that we needed.

So, 400Ah of LFP would provide more usable power – down to that 10 per cent – than 600Ah of AGM, which we would have to try to maintain at over 80 per cent to prolong battery life.

Installation considerations

The simple comparison of the lifetime cost might seem to make lithium batteries an obvious choice, but not so fast!

A change to LFP is not generally a drop-in solution, unless perhaps you adopt what’s called a hybrid system, which has other complications not covered in this article.

If you are thinking of choosing a hybrid option, get further professional advice. One of the features of an LFP battery is that it can accept very high charging current rates. Your engine alternator can quickly burn out trying to deliver this high current unless adequate protection is provided.

If you have other older methods of charging your house battery, either from generators or shore power chargers for example, there may not be LFP profile settings to provide this. Our originally fitted shore power battery charger did not have lithium settings. The technology was probably not so readily available back in 2006 when our boat was built, so it would need to be replaced.

Thankfully the other charge controllers (sometimes referred to as MPPTs) which regulate the battery charge coming from all the other power sources on board, all had lithium settings which could be changed through their respective apps.

Making the change

Our upgrade costs, together with the new batteries provided by LiFe Power, had to include a new battery charger, a Victron 50A Phoenix Smart IP43, to replace the old charger, and a Victron Orion TR Smart Isolated DC-DC 30 charger to protect the alternator from the high currents drawn by LFPs. (LiFe Power no longer exists as a business, but LiFe Power’s previous owner is now part of Fogstar, one of the fastest growing lithium battery suppliers in the country.) It’s important to bear in mind when installing the DC-DC charger, that it gets quite warm in operation so it should be sited in a reasonably well-ventilated space.

Each of our new LFP batteries have their own smart Bluetooth battery management system (BMS) which we can easily monitor and control via an app.

Each of the new Victron units also have Bluetooth control, enabling remote set-up for LFP applications. This and performance data can then be monitored via their apps.

There were other considerations. Given the high levels of current being delivered to the whole battery bank during charge, and potentially high discharge, it’s essential that the cables used to transfer the power are appropriately sized. There are plenty of online references to check the gauge and length of wire is sufficient.

If you research ‘How to install LFP batteries’ online, you’ll find a plethora of sources and information. There are various ways to adapt the boat’s electrical system to incorporate LFP batteries and we chose to employ a partner, Marine Electronic Installations Ltd, to support our upgrade and ensure we would have a safe system that would also be compliant with marine insurance needs.

We chose one of the simplest installations, which is to leave the alternator charging cables in place to charge the AGM engine start battery. This is connected to the DC-DC charger via a fuse. Once the AGM is fully charged, the current trickles over to charge the LFP batteries.

This rather limits the speed that the LFP batteries can be charged, since the AGM charge profile reduces current once the charge reaches about 90 per cent, but it is a simple solution. When on shore power, the Victron Phoenix Smart charges the AGM first and then trickles over to the LFP batteries.

The Victron website provides a range of online resources to help guide safe installations of LFP.

Long-term test

The last 18 months of cruising over 10,000 miles, including crossing the Atlantic twice, and long periods at anchor, have been a really good test.

We very rarely have to use the engine to recharge our LFP batteries, and only really after a long period at anchor without the benefit of the Watt & Sea input.

While I occasionally think more capacity could be used, it’s usually only because we want to be doing something like cooking with the Ninja air-fryer at the same time as trying to use the watermaker – both high current applications. A little advance planning avoids such conflicts. We also now have plenty of space to fit more capacity and since the additional batteries would just need connecting in parallel to the existing ones, it would be a straightforward upgrade.

Total cost

The cost of the battery upgrade was under £3,500, which included the two additional Victron units and the invaluable support from Marine Electronic Installations Ltd to help install the four LFP batteries.

Encouragingly, the cost of LFP batteries continues to fall as wider adoption makes for economies of scale in production.

LFP batteries of an equivalent size and capacity to the ones we installed can now be found for less than £400 per battery from some UK suppliers.

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