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I am fitting a second "leisure" battery in my car. It will be a 12V 115-150Ah AGM lead acid battery for use when the engine isn't running with loads up to 500W, generously, in practice It will be 250W max and about 50-100W constant. It will be about 5m/6m cable length away from the starter battery which is an "ordinary" wet lead acid 12V battery, guessing around 50Ah. The car/van is a '99 t4 VW 2.5tdi.

My question is are there other benefits of installing a 30a auto switching Relay over a dead mans switch besides preventing discharge of the starter battery while engine isn't running due to not flipping the switch? For example, does it have some sort of resistor that limits the flow to 30amps? How I understand what can happen is with the Car running and the starter battery charged and the leisure, lets say half empty, when the switch is "engaged" large amounts of current can flow (over the already charged starter battery...). I'm not an electrician.

My original thoughts were using 7mm diameter copper wire, a 400amps rated dead mans switch and 2 100amp fuses before each battery. Now I'm thinking to put a 80amp fuse on the starter and a 60amp breaker before the leisure, as I read that supposedly more than, lets say 20-40amps is too much for charging this battery (along with problems with voltage differences, starter battery receiving charge while full(?), the supposed need for a three state(-of-the-art) charge for the leisure, large currents if batteries are at different levels...and so on) So fair to say I'm a bit confused on the subject.

Thank you for your time, Gasper 

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30 amp rating is the capacity of the relay, take it past that rating and it burns up, go for the dead mans switch or a commercial battery Isolator, example here

Generally you want the amp rating slightly higher than the max output of the alternator.

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  • Thanks! Manual switch it is than (I like to be able to manually connect if needed). I understand why the cable should be able to carry more than the alternator can put out. Its hard to achieve in practice though; potentially over 100amps over 6m, that is one fat cable according to most tables online, no? Anyhow, you have put my mind at ease and I will continue with this as planned:) Thanks for the info. – bugy Nov 15 '19 at 17:59

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