Before planning a total upgrade to lithium batteries, carry out a consumption audit –  an in-depth evaluation of your electrical needs.

Modern appliances consume less power than older models, so you might want to consider using some of your repowering budget in the updating of your kit.

Compressor-driven fridges, for example, can consume less than a third of the budget of thermo-electric cool boxes.

Do a consumption audit before a total electrical upgrade

As battery capacity is nearly always quoted in Amp-hours (Ah) it’s often easier to do your calculations in Ah, rather than Watts and Watt-hours.

Firstly, decide on a period (commonly 24h) between charges.

Then, so long as you know what current a device draws, you simply multiply the time it will be in use over that period to get the number of Ah consumed over that period.

Battery capacity

Having totalled up your proposed consumption, double it if you plan to use lead-acid type batteries, or for good quality Lithium-ion batteries that will allow an 80% discharge, multiply your consumption figure by 1.25.

That will provide your required battery capacity for your chosen period but I would always add a further 25% for contingencies.

Make sure your shore-power charge controller is suited for lithium batteries. Credit: Alamy

Please note, capacities on deep-cycle batteries are usually quoted in C20 discharge rates, which is a pretty reasonable guide to follow.

Some ‘leisure’ batteries (compromise between a deep-cycle and a start battery) might also quote a CCA (Cold Cranking Amps) figure, which is a good indication that they aren’t proper deep-cycle ‘traction’ type batteries.

They can be used in smaller craft, they’re just not the best for liveaboard cruising yachts.

Charging lithium batteries

AC battery chargers

Lithium-based batteries are usually charged at constant current of between 0.5C-1.0C (C = capacity in Ah) until the current drops to 0.03C, at which point charging must cease so as not to overcharge the cells.

These figures may differ depending on the type of cell and the instructions.

Some recommend you stop charging at 0.1C to reduce stress on the cells and help extend their lifespan.

The State of Charge (SoC) cannot be determined by battery voltage, as this reaches its peak when the battery is only half-charged.

A good quality charger will be programmable to suit your batteries. Credit: Emrhys Barrell

Furthermore, a fully charged 12.8V Lithium-ion battery has a rested voltage of between 13.3V-13.4V, way above the 12.6-12.7V of a lead-acid battery.

At 20% SoC it will still read 13.0V, whereas the LA will be at 11.8V. If you already have a multi-stage, shore-power charger for lead-acid batteries installed you may still be able to use it providing it gives you sufficient control over the charging regime so you can limit the bulk charge to 14.6V and disable the float function.

One vital function to check if using an existing shore power charger is whether it is designed to perform an automatic desulphation cycle (wherein the charging voltage is increased to 15.5V or more for several hours), as this will completely destroy the Lithium-ion cells.

Alternator charging

Although a regular marine engine alternator/regulator arrangement outputs more or less the required voltage for Li-ion batteries, it is usually only intended to recharge a starter-type LA battery by supplying high current initially, then dropping to a level below 50% of peak output.

A Li-ion battery, however, will suck everything it can from the alternator until it is nearly full, which is a quick way to destroy it.

For this reason, a suitable Battery Management System (BMS) or Protection Circuit Module (PCM) is essential.

The Integrel alternator can produce much more power than a conventional one

Not upgrading the charging source and control system will not only risk the rapid destruction of your Li-ion batteries, but also negate their principal benefit – their capacity for rapid charge acceptance.

A good quality BMS will offer reverse polarity protection, individual cell balancing, charge voltage and current limitation, management and emergency disconnection, battery and alternator temperature sensing, discharge current limitation and management, and visual/audible alarms.

Often mistaken for a BMS, a PCM in fact offers far less comprehensive protection – usually only under/over voltage disconnect, overload and temperature monitoring.

Some battery manufacturers integrate a BMS into their battery casings with the idea that DIY installers needn’t worry about sourcing and integrating one into their system.

Worryingly, these are often sold as ‘drop-in’ replacements for older tech LA batteries, supposedly requiring no changes to your existing charging system.

Unfortunately, this is rarely the case.

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Instead, it’s recommended you use an external alternator regulator designed specifically for use with Li-ion cells, such as the Wakespeed WS500 or Balmar MC-614-H.

Either of these will ensure you get the optimum alternator charge without risk of overheating.

Some providers recommend you incorporate a battery voltage/SOC monitor with programmable parameters and a relay that enables you to disconnect the charging source once the battery reaches 95% SoC.

However, they aren’t accurate enough as Li-ion batteries should be managed at individual cell level.

The main concern is what happens when the batteries reach 100% SoC?

Some cheap BMS just abruptly disconnect the charge source which, while fine for solar, will instantly blow your engine alternator diodes.

Solar charging

Because of Lithium’s ability to accept a high current bulk charge, some owners choose to charge their Li-ion bank exclusively via solar energy.

This works well (in reasonably sunny climes) in that you can crank as much power into the bank as the panels will produce during their most productive part of the day.

It means that the BMS can simply switch out the PV panels when the batteries have reached the chosen SoC.

Mixed chemistry

Although a good quality BMS will turn down or switch off the field supply to the alternator from an external regulator, you may wish to divert the charge to a LA start battery.

With the same battery chemistry all round this usually just involves installing a VSR but with a LA/Li-ion mix this can’t be done as the resting voltage of the Li-ions rarely drops below 13.2V and, with the default VSR disconnection threshold commonly 12.8V, would result in the relay never disengaging.

Anchor windlasses require high battery output, so make sure you specify batteries that can deliver. Credit: Graham Snook

You could, though, install a BMS-controlled ACR or a Mosfet zero-resistance diode isolator.

Duncan Kent is a technical writer for marine publications and websites

That said, the currently recommended method for charging the vessel’s other LA batteries is to connect all charging sources to the Li-ion house bank and then link the LA batteries via a battery to battery (DC-DC) charger, which will provide user programmable, multi-stage charge parameters for your chosen battery.

To save installing an external alternator regulator some owners take the alternator charge to the start battery and then on to the Li-ion bank via a DC-DC charger.

However, this limits the amount of charge available for the Li-ion bank to whatever the rating of the DC-DC charger.

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