Ensuring that you have a sufficient reserve of power in the batteries to start the engine is an essential safety factor.
The boat should have at least two batteries. One for the domestic supply and one as a dedicated engine battery. Normally these batteries are of different types.
The domestic one is a deep cycle battery. It is designed to give out charge when not being charged, it will then recharge when the engine is running, if it is used to start the engine, it may take a long time to recharge.
The engine start battery is designed to give lots of power in short bursts, if it is discharged very low by being used to run the lighting, it may not recharge properly.
As a consequence of this, there must be a system of starting the engine on the dedicated battery then switching to the domestic one when the engine is off.
A common practice is to have the engine charging both batteries, there are some risks in this practice.
- The most common problem is forgetting to switch the battery to the domestic battery when the engine is stopped. This will lead to both batteries being discharged.
- If a fault develops in the charging system, you will be discharging both batteries. This may also lead to two flat batteries. This has happened to me on two occasions whilst on deliveries.
I believe that a safer approach is to charge one battery for a period, then switch over to charge the other. I have also been told by an engineer that this approach results in charging both batteries more completely.
To maintain a good charge in the battery it is a good idea to connect to shore power whenever possible and recharge the battery slowly.
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