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Q & A

Battery Charging

My domestic batteries are still going strong after five years but I had to replace my starter battery of the same age at the beginning of this season. This is probably due to the use of a three stage charger on the domestic batteries over the winter. My starter battery doesn't have the same charging facility. My question is would it do any good or harm if I connected a lead between the sets of batteries once the charger was at the trickle stage?

Asked by: Clive Dutson  | 12.32pm, Wednesday 15 August

WW says:

If you have been keeping your cabin batteries charged over the winter with a battery charger, this has certainly contributed to their longevity. The starter battery has certainly suffered through not having the same treatment.
If you fitted a temporary lead connecting the starter battery to the domestic bank during the winter when the boat is not in use; this would be beneficial and will not harm the starter battery as long as it is a standard lead acid, irrespective of the domestic battery type. There are however several problems with this arrangement; there is the danger of forgetting to remove the lead when boating and flattening the starter battery, the battery isolators might be rendered ineffective, your boat insurance might be negated if a claim is made, and temporary battery connections are not allowed by the Boat Safety Scheme.
A better arrangement would be to fit a built in multi-stage battery charger with several outputs. Common units have 3 outputs enabling starter, domestic, and bow thruster batteries to be independently charged (unused outputs can be connected to another). Do ensure that the connections are fused close to the batteries as they will bypass the isolators.

Rupert Smedley  | 3.50PM, Wednesday 15 August

A fairly cheap option would be to use a 10W (or 20W if you moor in a wooded area) solar panel- which could be attached directly to the starter battery (via an inline fuse). The 10W panel would need no regulator (just blocking diodes- usually built-in to the panel), while a 20W panel would need a simple regulator to avoid overcharging.
A small solar panel is ideal for keeping a starter battery topped up, without having to wire-in a second charger. It is also a virtual fit-and-forget option to keep the battery at optimum level, even during the winter, by overcoming the self-discharge inherent in all batteries.

Mark Langley  | 4.40PM, Wednesday 15 August

A split charge relay is energised when the engine is running to connect both batteries to a single alternator. It would be possible to arrange a relay to connect the charger output to the starter battery, but it would only want to be energised when the battery charger is active.
The charger output cannot be used, but possibly an internal supply might be tapped or a 230V relay could be used connected to the supply to the charger. Thus when the charger is switched on, the relay would energise connecting the batteries together.
This is however a course of action that should only be undertaken by a competent person.

Rupert Smedley  | 12.08PM, Friday 17 August

Readers say:

Changing the existing single output charger for multi-output is a rather expensive option - it's probably cheaper to carry on buying starter batteries!
People sometimes charge two sets of batteries from one alternator, is there a similar option with my charger? It's aWaeco Mobitronic with 25 amp max output.

Clive Dutson  | 4.21PM, Wednesday 15 August

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