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Originally Posted by quantum

I have a 2 battery set up 12V cab set up, auto generator start.

Question:

Anyone that has the 16ft residential fridge/freezer know how many hours on average or how long the cab batteries should provide power for the fridge through the inverter (1800 watt) before the generator starts

I have a Thor ws31- double cab batteries and both are the correct deep cycle. Standard wet cell each 750 amp

Thanks

I assume the "750 amp" is cranking or cold cranking amps? If so, you don't really have depp cycle batteries, you have combination starting/deep cycle batteries.

Battery capacity is measured in amp-hours (AH) usually at the 20 amp rate. So a 100 amp-hour battery can supply 20 amps for 5 hours before it is dead (0% charged). But lead-acid batteries permanently die a little if discharged below 50% so a 100 AH lead-acid battery really only has 50 AH that are usable.

The state of charge of a lead-acid battery can be approximately measured by its no load voltage.


But the voltage under load will be lower depending on the load. Other factors affect the battery voltage as well such as temperature.

When your AGS starts the generator depends on what voltage threshold it is set at. When it reaches that threshold depends on the load the house batteries are supplying and their actual capacity which varies with age as well as state of charge.

How much energy your residential fridge uses varies with many conditions such as ambient temperature, insulation, temperature of the stuff inside, etc.
If you just threw a couple of two liter sodas in there and filled the ice trays it will use more energy than if everything was already at a cooled temperature. Also being Energy Star rated makes a difference.


BTW, a 16 Cubic Foot Fridge is might large for an RV but lets say it uses 500 watts on the average; and it has an average duty cycle of 50%, so 250 watts average running power or 21 DC amps in a 12 VDC system. Say 10% inverter ineffeciency and we're at 23 DC amps average.

If you have two 100 AH house batteries (100 AH usable) you would be at 50% capacity in a little over 4 hours and that's where you would want your AGS to kick in.

But 4 hours is just an educated WAG because there are so many variables and unknowns.