Leisure Battery Charging

[Barry Wilkins]

Hi I want to change the way my leisure battery is charged in my motor caravan, the starter battery is 88 Ah and the leisure battery is 110Ah.
I can charge either battery from the mains so they are fully charged ,but when you start the engine a relay connects the batteries in parallel,any extra capacity in the leisure battery will be used up so you end up with only 88Ah in the leisure battery.
My idea is to remove or disconnect the split charge relay and charge the battery independently from the alternator ,but I dont think this is going to work very well because of the maximum voltage output of the alternator. I think I need a minimum voltage of 16 volts to regulate this charge even if I use a high power mosfet with a very low rds of 0.014 ohms.
This means I need a buck boost circuit to raise the voltage from the alternator so I could operate a charging regulator.
Does any body have any other ideas I could use ?

thank

Barry Wilkins

 

[crutschow]

Why do you think the extra capacity in the leisure battery is used up? Connecting the two batteries in parallel equalizes the voltage which equalizes the charge level as percent of total charge, but it doesn't necessarily discharge a battery with the higher Ah rating. The battery voltage determines the percent of the total charge capacity, thus at 12.6V it is 100% of the battery Ah capacity, independent of the absolute Ah rating.

So I don't see that what you are trying to do makes any sense. Either way you don't need a separate regulator to charge a battery from the alternator, it already has a regulator which adjusts the charge voltage to nominally somewhat over 14V.

 

[squishy36]

One thing you might do is visit an RV store and see what equipment they have. We used a tent camper for a number of years and I bought an electrical device that isolated my tent camper's battery from the car's battery. This let the tent camper's battery be charged while I was towing the camper without ever letting the car's battery be discharged. It was easy to install too (although it required some 6 or 8 AWG cable). I got 10 years of use out of the thing before giving the camper to an offspring unit.