Advice needed on battery matters

[IrasciBill]

Looking for advice on a couple of related electrical issues for which I'm sure some of you will have knowledge or experience.
Our new (to us) motorhome has been fitted with a battery isolator switch in the start battery circuit. Being on a Ford Transit base this is undoubtedly a worthwhile additional security device. However, when operated, it kills everything powered by that battery; including the trip computer and clock. Although this will minimise battery drain during longer periods out of use, it is a bit of a nuisance.
I am proposing to fit a fused link across the isolator switch terminals to maintain these functions but not allow starting of the engine. I was considering a fuse rating of about 2 or 5 amps as being adequate for this purpose but I notice that commercially available ones normally use a 15 amp fuse. I realise that things like the electric windows (and possibly even the ignition circuit) would possibly blow the 2 or 5 amp fuse if I forget to switch on the isolator before operating them. Is this why a 15 amp fuse is preferable or is there some other reason?
Secondly, with regard to battery drain, I am proposing to install a trickle charger for the start battery. (The factory fitted 12V supply / battery charger only charges the leisure battery.) Is it advisable / desirable that the trickle charger (whether powered or not) be disconnected from the battery before starting the engine?

 

[autorouter]

The starter motor current will easily blow a 50A fuse, so a 2A or 15A fuse will certainly do the job. I suppose the 15A fuse is fitted because they don't want it to blow unless the starter motor runs. So you don't want the fuse to blow if the radio is on loud, or the window winds down, or the wipers and fan operate, etc.

The trickle charger will have a diode (an electrical one-way valve) that ensures that charging current goes the right way. It should be quite safe to leave it connected all the time.

A popular solution is a BatteryMaster, which automatically trickle-charges the starter battery whenever the leisure battery is being charged by the mains charger or solar panel. It's a fit-and-forget solution.

 

[Misterg]

Is this why a 15 amp fuse is preferable or is there some other reason?

Central locking?

 

[IrasciBill]

Misterg No it doesn’t have central locking.

 

[Camping Gaza]

Anything you put across the battery isolator switch is probably unwise as whatever you operate if you forget to switch on the isolator will try to draw max current through that wire and get hot and blow or possibly catch fire, especially your starter motor, cigarette lighter, cab heater motor etc etc.

You need another solution, this will involve additions circuits and a relay of some kind.

If its just to stop your starter motor operating would it not be better to remove the isolator switch from the main battery lead and insert it between the starter motor main cable to disable the starter motor until switched in? Not easy to do I suspect.

I would get a smart charger for your battery when on hookup or whatever as this will never cook your starter battery and maintain it at the correct float voltage summer and winter. It is advisable to remove the charger before starting as it is possible to damage these chargers through surge etc.

Your best bet is a solar panel and regulator feeding your starter battery if you want something permanent as it does the same job as a smart charger maintaining your battery

^{_{Subscribers  do not see these advertisements}}