Battery Cable

[Moodymike]

I want to make up an emergency battery bypass switch and cable. Confused over what the spec is of existing cables in order to purchase. Advise please.

[diverphil]

Hi. Not sure what you mean to do or go from where to were.? A possible answer would be measure the diameter of the wire you have already. Then work out the cross section area in mm2. There A the gauge of wire you will require.

[SYJetzt]

Why not use this?

[Baddox]

The cable thickness required depends on the length of cable and the current that will run through it.

Start by finding the current your starter motor pulls; there should be details of this online or in a handbook.

Next, measure how long the cable run is from where your batteries are, to the starter motor.  Double this length as there will be a positive and negative wire to complete the circuit.

Now look up the optimal cable size online to suit the length and current.  There’s no harm in using a heavier cable but avoid using a thinner one.

Tinned cable is the most corrosion resistant and welding suppliers are often a cheap source.  You need to crimp lugs onto the end of the cable and seal with heat-shrink.  Suitable hydraulic crimpers will set you back about £25 from ebay, or the cable supplier may crimp them for you.

[Salty]

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Next, measure how long the cable run is from where your batteries are, to the starter motor.  Double this length as there will be a positive and negative wire to complete the circuit.

.................You need to crimp lugs onto the end of the cable and seal with heat-shrink.  Suitable hydraulic crimpers will set you back about £25 from ebay, or the cable supplier may crimp them for you.

Attached below is a diagram of the bypass system I installed several years ago to enable emergency starting of the engine in the event of the engine start battery going flat, though I’ve never actually needed it. The system was installed following a recommendation from a coding surveyor at a time when my boat was coded for charter use, and which remains switched off at all times other than in an emergency.

The cables I used for connecting the domestic battery’s to the engine start battery via the bypass switch were the same cross sectional area as the existing battery connecting cables, and by connecting to the existing engine start battery it allowed me to continue to use the existing starter cables, thereby keeping the cost down. The separate battery switch was sized suitable for the starter motor current, and to ensure that the bypass switch was only used in an emergency, the switch key was screwed to the woodwork within the boats battery locker near where it would be needed, and thereby making the use of the system a deliberate action in an emergency rather than something that would be switched on automatically without any real consideration for why it was being switched on.

Some wires and cables have been omitted from the drawing for clarity