Ok thats all good but how can I stop the second battery charge current going too high.I would be happy to keep the charge at maybe an amp or two as the battery wont be used on a regular basis so can basically be like a trickle charge .
I really couldnt see myself having to jump start numerous cars.
I hopefully would never require the second battery to be directly connected the main battery. If the main battery went flat for whatever reason I would actually jumper myself so to speak. That way I wouldnt have to run heavy leads between the batteries, only the charge lead .
Re your second battery charge rate, depending on what type of battery you end up with, this,
Redarc BCDC1206 3 Stage Battery Charger should do the trick.
Father in law recently tried to do what you are proposing running a Caravan fridge (15 amp load constant) from a voltage sensing relay on primary battery to fridge in van, he has 6mm twin cables all the way, but I think the voltage drop was too high and fridge would not run, so check that the wiring to the items is capable of load required.
As your voltage drops, the amps required to run the item increases, a fridge that draws 15 amp at 13.7 volt will draw 17.125 @ 12v and 18.65 @11 volt
it does not sound like much, but an extra few amps here can really knock you about on LONG runs.
If your second battery is going to be mounted in the rear tray area, run large cables, high quality with SOLDERED joints, not crimped, ensure all is clean and tight, then use a dedicated charger to charge, trying to charge a second battery with a voltage sense relay does not really work.
The voltage sense relay onlt joins the two batteries together, so if one battery has a bad cell, the other battery will never get a proper charge, the alternator will just put out the power as asked for by the regulator, it doesnt care where it goes, if you are running two matched batteries, same type, CCA capacity, in the same condition and age, they can work, but once one battery starts to die, the system will fail.
If you try to run a deep cycle for Aux and a lead acid for main, these batteries require a different charge rate and voltage, they dont like being hooked together, so a dedicated charger is required.
It would be possible with a dual pole voltage sense relay, that switched between batteries, not charging both at the same time to run two different battery capacities anf charge from your alternator, but only one battery can be charged at a time, the voltage sense relay will ensure your main battery is full before switching to the other.
I had a manual set-up of the above run through two 100amp constant load/300 peak load, 100% duty cycle relays, feed from alternator went to both relays, the seperate output from each relay then when to a diffrent battery.
a twin volts gauge (two voltages displayed at all times) and a 3 pos switch with slave relays switched the other relays in as desired, I will try and attach a drawing of the above, it sounds complicated, but works very well.
You can choose which battery get charged, keep an eye on voltage of both, you could setup lights to warn you of a low battery voltage (idiot light) if you like, it depends on what you want your Auxiliary battery to do, winch? fridge? lights??? then you can work out what charge rate you will need to give it as a max and set up your system to suit.
