Quote:
Originally Posted by Mobando
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A COUPLE of points.
I'm a retired ASE mechanic AND I worked in a transit department servicing (and later as a driver) E350 based buses.
Every upbuilder does things a little differently but I suspect that the underneath battery was for loads on the bus body and independent of the main starting battery except for a connection to charge it. I would remove any wiring (cables) between the two and use a DC-DC charger to charge the second battery. No relays, isolators, etc. Just the DC-DC charger. This is the best way to ensure your starter battery is protected from the house system and the house battery is properly maintained (as long as you're running often enough).
That said, I myself have used a regular (and well used before I got it) car battery to run the loads in the 8 1/2 slide in camper I had while in the military. Then again, my only loads back then were a few lights and an occasional 12 V fan. It was connected directly to the battery with a relay and a switch inside the cab. When the engine wasn't running the relay disconnected the camper battery circuit. When it was running I could break the circuit by flipping a toggle switch. I'd start in the morning with the relay disabled until I'd driven for half an hour or so and then energize the relay to send some juice to the battery. It worked for my needs but those were traveling cross country and in a hurry unless I was in a camp ground with full hookups.
And again, with that said......
I would NEVER put a WalMart battery into anything other than yard equipment, a kids toy, or something like that camper I had.
When comparing deep cycle batteries WalMart sold a 101 Ah 12V unit for 99.99. The competitor was at least $25 more at the time. However, the WalMart 101 Ah unit was only rated at 1 A draw to get to 101 Ah while the competitor used the industry standard of a 20 A draw. That is a MAJOR difference in useful power....and how WalMart beats the competitions prices on a lot of things. If you were to put a 20 A draw and that WalMart battery it would never get close to 101 Ah. And being lead acid, anything more than 50 Ah would damage the battery.
Not hating on WalMart, just pointing out that you have to be really careful and read ALL the specs. Their SuperTech oil for instance is made by a name brand manufacturer and perfectly acceptable (as long as you buy the right spec oil).
AND if ALL the spec's aren't available.... RUN to the competitor.
Finally, you're planning to put a significant load on your battery. Watts are the unit of measure of actual work and what everything should be converted to as it doesn't change with voltage.
To get 500W (ignoring the inefficiency of inverters) from 12V you will need to draw 41A from your battery so if you get anywhere near pulling that 500W you'll exhaust your battery in an hour and half.
When you said "...weekender vehicle and it is a 12v fridge that runs on <5amps and 180 watts..." something doesn't add up. 12V and 5A is only 60W, no where near 180W. 180W at 12V is 15A. An industry standard deep cycle of 120Ah rating is useable for 50% of that 120 (or 60A) before damaging the battery. NO, it won't kill it but the next charge discharge cycle will not give as good a performance and it's cumulative damage. So, 12V, 15A to run the fridge will give you 4 hours before you're battery is at 50%.
I suspect however that the 180W for your fridge is the high draw figure and only applicable when it's trying to cool things down. Once they're cooled it will "cycle" between a much lower, perhaps even close to zero, draw and the 180W needed when it begins the next cooling cycle.
While the same is not true for the 500W inverter, if you're only using it to charge something like a laptop, it will not pull the full 500W.
Remember OHM's LAW E=I/R and P=I/E
Also remember that inverters and converters have an efficiency factor in that 100W in is less than 100W out, perhaps lower than 90W. So you'd need to use over 550W to get 500W of AC power from that inverter.
I'm working on a calculator for skoolie folks to calculate their electrical needs and perhaps I'll include an Ohm's Law calculator that stands alone though there are plenty of APP's for smart phones to do the same.... it's simple math.
over a weekend with the fridge and a 500W inverter (which is unlikely to be pure sine wave and should therefore not be used for any sensitive electronics like phones, tablets, laptops, etc.)
Linear Mode
