paul replied:

On a metal plate on every appliance, including your fridge, there will be a metal plate with power and current details, it will either say "X" watts at 12v or "x" amps at 12v. If it gives your amp rating then when it is going, that is the motor is going, that is about how many amps it will draw. If the plate is rated in watts then use the variation of ohm's law amps = Watts / Volts to give you the current (measured in amps) draw.

Based on the amount of amps the unit _Affordable_Storage_Drawers.aspx you will then have a fair idea of how much power (measured in watts) it uses and can determine a suitable battery.

But you may as well take into account the temperature and conditions of where you travel and that once a fridge (dunno bout a cooler) reaches it objective temperature the compressor closes down until the internal temperature falls below the objective temperature. That is like on a cold night if a fridge is closed for the last time at 11pm and cycles down to objective temperature by 12pm then it may not need to start again for qute a few hours. I have had a fridge use 30amps on a hot day and the same fridge use about 6 amps at night.

paul replied:

Just to clarify that spiel, when you find out how many amps it _Affordable_Storage_Drawers.aspx then that is a "per hour" rating. Deep cycle batteries have their capacity expressed in A/h - which is amps per hour. As voltage on a battery drops as it approaches its level of full discharge you will probably find that you can use only about 80% of your battery capacity (though ideally for longevity don't use more than 50% of the battery). So if your cooler uses 2 amps/hr and is on 100% of the time then on a 70 amp/hr battery used down to 20% of its capacity it is 56 (80% 0f 70) divided by 2 = 28 hours. So a 70amp hour battery would give you 28 hours running time of the cooler if it runs the whole time it is on.

Scott replied:

Trevor, you will find that your cooler should use about 5-6 amps. As coolers do not generally cycle like a fridge/freezer, your total amphr usage is higher than a cycling fridge/freezer. As a result you need to be more conscious of leaving it on the whole time. I used to own a Waeco 32litre cooler and found if I froze a bottle of water and left it in the cooler, then the inside would remain very cold, even if the cooler was switched off. As you can probably see, these thermo-electric coolers are ok on 12 volt whilst driving (better if the outlet is hard wired to the battery) but due to the high power consumption are not ideal when stopped or camping (using frozen water will help). However, I found that everything would keep cold for about 2 days whilst stopped and "manually" cycling the cooler. Sounds too hard? Save up and buy a proper fridge (especially if you are looking at a second battery). Hope this info helps.