Most narrow boats will have two battery banks. One bank is responsible for providing power to the engine for starting and running, the other bank provides power for domestic systems such as water pumps, televisions & lighting. Some higher specification narrowboats will have a third battery bank dedicated to powering a bow thruster motor.

Battery banks may comprise of more than one casing. For example, a 12VDC starter battery bank may comprise of one 12VDC case. A 12VDC leisure battery bank may comprise of four 6VDC cases so for simplicity when we talk here about batteries we are not talking about how many black or red boxes may be on-board, we are talking about the bank as a whole.

Traditionally, batteries we will use on our narrowboat will be the lead acid type, in one form or another. These batteries produce DC or Direct Current. Direct Current flows in one direction only and the main reason it is used for leisure and automotive (engine) applications is because it can be produced and stored in a battery and is relatively safe. 12 volts has become the traditional unit of energy due to the compromise between the way lead acid batteries work, their cost and overall systems safety.

However, it is worth mentioning here that some narrow boat systems are based on 24VDC.

Lead acid engine starter batteries are usually situated in the narrow boat engine bay, very close to the engine starter motor. Heavy diesel engines require lots of “oomph” to start and the “oomph” is stored in the battery in the form of cranking amps. When the key is turned, the amps are allowed to flow into the starter motor which turns over the engine & causes it to fire into life.

After starting the narrow boat engine, the battery will be depleted of the power it used to crank over the engine. This power needs to be replaced and this is usually done by the running engine.

The engine will have a small electricity generator called an alternator fitted to it. When the engine is running, it will turn a belt which is connected to a pulley on the front of the alternator. As the alternator spins, it generates electricity which is converted internally in the alternator to DC and is fed back to the battery. A short period of running the engine is therefore required to replace the power used in starting it.

Lead acid domestic batteries, conventionally, are also situated in the narrow boat engine bay. They can be located in the cabin area but this type of installation requires adhering to very strict regulations managed by the Boat Safety Scheme. Batteries are a mix of volatile substances and therefore must be installed and managed with the utmost regard to safety.

Anyway, back to the engine bay. The domestic battery is depleted every time we place a load on it. Whether that be running the water pump, powering the microwave or simply switching on a light. How long the battery will last before it needs recharging will depend upon the load on it and how much power it had in the first place.

This “oomph” to run televisions & lights etc. is also measured in amps. The more amps you have, the more you can use. The more amps you use, the longer it will take to put those amps back into the battery by recharging!

See our Power Audit section for information on striking the right balance between what you want, need, use and therefore have to replace.

Recharging of domestic batteries can also be done by an narrow boat engine alternator. This alternator, or engine driven generator, could be the same one that recharges the starter battery or it may be a separate, stand-alone unit.

Alternatively, amps can be put back into batteries by solar, wind turbines, 230VAC battery chargers and diesel or petrol generators. We cover methods of charging in our dedicated Charging section.