batteries

ok, i think I'm getting this. as i reread it it makes more sense to me. How about this, what exactly do you have set up for your electrical set up in your vehicle?

I have a HUGE 4x45W CD radio with a pair of 6" speakers in the doors and 2 pairs of 2" speakers in the headrests. A 3-digit voltmeter (across the main battery) added to the dash. A MASSIVE 3.5" GPS, a 3 speed heater and no reversing lights. And lots of relays.... For a few years I've been running a spare battery in the tray. Mainly for camping, partly for running a fridge, and partly just in case I leave headlights on etc. Originally I used a voltage sensing isolator (a $22 80A kit; adjustable voltage; latching relay; normally under 0.5mA consumption as I recall) both front (to isolate when not charging) and rear (to isolate load when the rear (or camp) battery was too discharged). After washing the front isolator (it was not water-isolated), I replaced it with the UIBI - a 30A relay and 20A fuses. Later that was upgraded to 50A self-resetting circuit breakers and a 70A relay. Batteries have varied. Generally a 40AH wet up front, with the same else 70AH wet else 38AH AGM in the rear. After my two 40AH wets retired after 8 years, I starter using the 38AH AGM as the cranker. Although I expected it to be unsuitable for cranking, it's still going strong after nearly 2 years. (I suspect my 140A reduction starter in place of the original 240A starter helps, though my brother has used the same battery (history & size) for over 3 years on his van which probably has an ~250A starter. (I gave him the battery for emergency cranking use!) The rear battery has since generally been another 38AH AGM since my mum stole my 70AH wet. Generally alternators have varied from 70A - 100A (originally an ~20A generator and later an externally regulated ~25A-35A alternator). A few weeks ago I replaced the 70A alt with a 90A unit. The only hardships the newer AGM set up has seen was some extreme winching about a year ago (the alternator got so hot its internal sensing diode unsoldered itself), a few "forget to reconnect the alternator" drives, and an insistence on cranking the car up ramps, out of bogs, etc. The UIBI has worked well except for my last alternator which had a faulty charge-lamp circuit. The interlink 50A breakers have worked well. Since finding that the 38AH AGM can absorb 40A upon initial recharge, it was not surprising the original 20A and later 30A fuses occasionally blew. (The fridge load then was ~8A. Now it peaks at 2.5A being a real fridge - not the original Peltier-device cooler/heater. Occasionally I have also manually triggered the UIBI for paralleled batteries during cranking. AFAIK, the breakers haven't yet tripped (probably to date only ~15-20 seconds of cranking max). So no big audio loads in my case. (But if I did, I'd run the amp's HV from the front!) Probably headlights were my biggest load - 4x100W. My 4WD will have at least dual batteries plus a winch up front. That should be one the road before 2005. Er, 2006. Maybe 2015.