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electrics - earthing

My domestic battery's are earthed to the engine viv the neg terminal. Which then go to shunt and busbar with no current running to earth. The starter battery earth is on the busbar but the alternator for the starter is earthed through the engine and I am getting current going to earth. What I would like to know is do I have to run a new lead from the alternator to the busbar to stop this, or does it not matter.

Asked by: anthony spanswick  | 12.25pm, Monday 8 December

WW says:

There should be only one earth from the batteries, this is usually part of the starter circuit.
I am not entirely clear as to how it is all wired up from your description; could you please draw the circuit and email it to rupert.smedley@waterwaysworld.com

Rupert Smedley  | 2.18PM, Monday 8 December

Thinking a bit more about your problem; it sounds as though your starter circuit alternator might be faulty. It is possible for one or more of the diodes within the alternator to fail and for it to still work, albeit not so well. This will lead to a drain on the battery when the engine is not running.

Rupert Smedley  | 5.11PM, Tuesday 9 December

The current you are seeing is the return current from the alternator charging the starter battery since it is only detected when the engine is running.
The domestic alternator has a separate earth connection to the negative bus bar so the return current from the domestic battery charging does not go through the earth lead; whereas the starter alternator is earthed to the engine and so the return current must flow through the earth cable. There is no danger to the boat hull from this current, corrosion will only occur due to return current flow if the hull is used as a conductor which is not the case.

Rupert Smedley  | 4.11PM, Sunday 14 December

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