Basic narrowboat split chargers are voltage sensing switches. The alternator feeds the starter battery with current via voltage.

When the narrow boat battery has reached what the sensor believes to be a constant voltage and is in very basic terms “charged” it allows the excess current carrying voltage to be diverted to the leisure bank.

The Electronic Bit…

Diodes used to be used widely to achieve this but engineers realised that whilst diodes are reliable they are inefficient. As we have seen, fractions of volts matter when battery charging so Voltage Sensing Relays are now commonly used as they introduce little or no volt drop into the circuit.

Top quality high amp voltage sensing relays are expensive but are proven to be reliable as technology advances. Best practice is to specify relays that have a continuous load rating higher than the output of your alternator/charger. Overloading the circuit would cause the relay to shut down, fail or possibly catch fire. Speak to your marine electrician or specialist electrical components supplier if you are in any way unsure what to fit.

Battery To Battery Chargers

Working in a similar way to the split charger, the battery to battery charger is an intelligent way of using excess charge voltage. More expensive than a relay or diode splitter, you get what you pay for. Three stage regulated charging is a feature of the top quality units.