Most boats these days have at least two batteries on board – and for good reason. That way, the theory goes, you can run as many lights, instruments, stereos and fridges for as long as you like and still be able to start the engine next morning. But to do that requires some sort of battery switching system.

Things have come a long way since the humble battery switch. Here are four options to upgrade how you switch between batteries.

1. Battery switch

The traditional way to switch between batteries is with a switch that allows you to select one or both batteries. This has worked well for decades, but relies upon you remembering to switch it to one battery to start the engine and back to the house battery when the engine goes off. The facility to combine the batteries on ‘both’ is useful, but runs the risk of sailors forgetting to switch it to the house battery later – and waking up to discover two flat batteries.

Pros: Cheap and simple.

Cons: Relies on the user remembering to switch it over and select the right battery.

2. Split-charge diode
Split-charge diodes have been around since the 1960s. Put simply, a diode allows current to flow in only one direction. Most of these types of splitters contain two separate diodes which, when connected to an alternator, split the charge in two and deliver it to two separate batteries which are totally isolated from each other.

This sounds like a great solution – it’s a ‘fit and forget’ way of charging two or more battery banks independently – but it’s not entirely perfect. Diodes by their nature create a volt drop of up to 1V as the current passes through, which means that the batteries won’t receive the alternator’s full charge – and thus, will never become fully charged by the engine if you use a machine-sensed alternator. You can get around the problem by using a battery-sensed alternator, which will do a much more efficient job and enable the batteries to charge fully.

It’s also advisable to fit a switch so you can combine the batteries in an emergency – they will otherwise be completely separated and you won’t be able to start the engine using the house battery.

Pros: A ‘fit and forget’ way to isolate and charge two battery banks.
Cons: Risk of batteries not being fully charged if you use a machine-sensed alternator.

3. Twin alternators
Some people fit an additional alternator to their engine. This has its advantages – it provides a totally independent means of charging the start battery, as well as providing a level of built-in redundancy in case a battery or alternator is to fail.

PBO’s editor, David Pugh, decided to fit an extra 10A alternator to his 13hp Lombardini engine. This totally separates the start and house batteries and their circuits – but they can be combined if one battery was to fail using a switch with the facility to combine batteries. This also has the advantage that if one alternator was to fail, the batteries can be combined and one alternator used to charge both batteries. Better still, you then have a dedicated alternator which will deliver the right amount of charge to each battery.

David’s justification runs as follows: ‘Our engine is slightly overpowered for the boat, so we had plenty of excess power available to run another small alternator. It gives us two separate circuits and should give us a constantly-charged, dedicated engine start battery. As such we can use a smaller, motorcyle-sized battery, reducing weight on board.’

Pros: Totally isolated battery banks, which means you can’t discharge the start battery by accident.

Cons: The engine needs to be able to take another alternator, and powerful enough to take
the extra load.

4. VSR
Voltage-sensitive relays (also known as ACRs or automatic charging relays) keep two battery banks isolated when not charging, but use a high-current relay to connect them together when charge is available. When a charging voltage is detected on the start battery, some units will immediately combine the house and start batteries to charge both, while others will wait until a preset voltage is reached at the start battery before combining them. Once combined, if one battery is less charged than the other, it will take a greater share of the charge until they are equal.
When the engine is switched off and the voltage falls, the batteries are isolated once again. Some VSRs also include a ‘low-voltage cut-out’ which will prevent the VSR from combining the batteries if either of them is at an unacceptably low charge level.

VSRs come into their own when a small inboard or outboard engine is used which has combined starter motor and alternator wires to the battery. In this case, a diode system will only let the current flow one way, whereas a VSR will allow both charging and starting with only one set of wiring.

Pros: Simple solution that takes human error out of the equation.
Cons: Should one battery be past its best or have a faulty cell, the other may not charge fully.

Click here to see how to install a VSR on board