Club 80-90 Forums

If I was to install leisure batteries in the engine bay could I use the starter post for the feed into a voltage sensing split charger relay?

Saves rooting new cable up to starter.

Yes or alternator output.

marlinowner wrote:Yes or alternator output.

Perfect. I was a bit unsure how much current would be drawn over the relay and wiring to the starter. Why are some relays 140 amp! What is the most that would go to the leisure battery from the alternator / main battery?

If you connect a full starter battery to a flat leisure battery, a lot of current will flow between them. The current doesn't stay high for very long, because the voltage difference will reduce quite quickly as the two batteries start to equalise, but it would be long enough to damage the wiring or the relay if they're not rated appropriately. So if you've camped for a week and depleted your leisure battery, when you start the engine to drive home you need a split charge system that can cope with that initial surge of current. That's why you see 140A relays. And of course it would need to be cabled to suit.

If you use a smaller relay, just protect it with a smaller fuse, and accept that on the odd occasion when your leisure battery is really flat, you may blow that fuse.

Hi Chris...the 'switching voltage' for the smart relay is typically over 13 volts (13.00 - 13.5) something like that. When the engine is cranking the voltage at the Smart relay will be well below the 'switching voltage and the batteries will remain disconnected, so no worries at all that the relay or split charge wiring will be forced to supply any cranking amps.
Sure, once the engine has started, the alternator voltage of 14 volts plus will switch the relay and connect the two batteries, and whilst the initial draw can be higher (especially with a heavily discharge leisure) you're never likely to see much over 40 amps and then only for a few seconds. Your experience may vary if you are using several large Tojans but the above is what I've measured on my setup with an amp clamp.
The big heavy wiring is there for the starter, the charge and leisure circuits do not need anything like the same gauge. I would always recommend overating by 50% so 60amp wiring in most cases will be more than enough.

Martin

Auto Sleeper fit a 70A relay and 30A fuses to their leisure battery setup. You do want the fuse to blow if there's a problem, not to blow if there isn''t