What relay rating do I need for a twin leisure battery setup

[Robimoo]

Hi, this is my first post as I have just registered. I’m a newbie doing a van conversion.

currently I have a basic electrical control system that has a relay rated at 80v that connects the leisure battery to the van charge circuit once the engine is running. I want to add a second leisure battery in parallel to the first but was not sure if the 80A relay will be up to the job. Can I use this existing circuit or will I need to fit a Voltage Sensitive relay with a much higher rating , 140A for example ?

Thanks

Rob

 

[DBK]

It should be fine but there's only one way to find out! I have two 90Ah batteries and the fuse was 100A.

I suggest fitting the second battery and see how you get on. Worst case the fuse will blow but that is likely to happen only if the batteries have been drained far below what is wise.

You could fit a bigger fuse but only after checking the wiring and relay are up to it.

And welcome!

 

[Lenny HB]

The charge current will be limited by the size of the cable and the distance between the starter and leisure battery. If the leisure battery is only 2 to 3 metres from the starter batteries and the cables are 25 mm sq you might see 40 amps on startup soon reducing to 20 amps. So an 80 amp relay will be fine, use smaller cables and the current will be a lot lower due to the voltage drop.

 

[Robimoo]

Thanks for your answers folks. I feel much more confident now. Unfortunately because of the layout of the van I have one battery under the passengers seat and one right at the back of the van, not ideal I know. So I guess because of the long cable lengths to the second battery, it will be pulling less than the one under the seat during charging. I can imagine less than 40A. I shall give it a go. Thanks again

 

[Lenny HB]

With the batteries that far apart make sure you take the positive feed for the load off one battery and the negative off the other. Normally with just 2 batteries you don't bother but that far apart I would.

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[pappajohn]

A regular split charge (which is what you're doing) is a 30a automotive relay.

 

[gus-lopez]

 

[Robimoo]

Thank Lenny, will do. I will order my wire lengths accordingly. Presumably it doesn’t matter which way round. I mean I can bring either the positive or negative feed from the battery at the rear of the van as long as I use the opposite pole from the one under the seat. So is it always better to use a negative wire than to use the van body ?

 

[Robimoo]

Hi Pappajohn, the relay is marked 14v/80A, so not 30A.

 

[Lenny HB]

A regular split charge (which is what you're doing) is a 30a automotive relay.

Only in Brit vans with their undersized wiring, German vans use 70 amp relays.

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[Lenny HB]

Thank Lenny, will do. I will order my wire lengths accordingly. Presumably it doesn’t matter which way round. I mean I can bring either the positive or negative feed from the battery at the rear of the van as long as I use the opposite pole from the one under the seat. So is it always better to use a negative wire than to use the van body ?

No it doesn't matter, no don't use the van body.

This site may help you, wire size calculator on the page. Normally for 12v you aim for a votage drop of no more than 3% but for decent split charging you want to aim for quite a bit less

 

[Hymerbell]

And don’t forget positive to positive and negative to negative....or was it the other way around..

 

[Robimoo]

No it doesn't matter, no don't use the van body.

This site may help you, wire size calculator on the page. Normally for 12v you aim for a votage drop of no more than 3% but for decent split charging you want to aim for quite a bit less

Hi Lenny, which site, which page ?

 

[Lenny HB]

Hi Lenny, which site, which page ?

Sorry forgot to paste the link, on the first page.

Cable Size Calculator - Cable sizing and selection | 12 Volt Planet

Cable Size Calculator - Read our guide to selecting cable of the correct type and size for your automotive, marine or leisure wiring application. Auto and Marine Electrical Components from 12 Volt Planet

www.12voltplanet.co.uk

 

[pappajohn]

Only in Brit vans with their undersized wiring, German vans use 70 amp relays.

And the OP never said what it was in the 1st post.

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[Robimoo]

With the batteries that far apart make sure you take the positive feed for the load off one battery and the negative off the other. Normally with just 2 batteries you don't bother but that far apart I would.

View attachment 400536

Sorry, may be a silly question, were do I connect the negative from the van starter battery with this setup ?

 

[Lenny HB]

Sorry, may be a silly question, were do I connect the negative from the van starter battery with this setup ?

Negative direct to the leisure battery, on the positive side fit at least a 50amp fuse close to the starter battery.