Battery Life Prediction

[bitem2k]

Ive got a portable fridge that will run from 12v. Its says on the back of the fridge that it uses 80w.

I plan on running the fridge from a spare car battery that I have, it says that its capacity is 50A/h.

Does anyone know how i can calculate the amount of time that the fridge can run from this?

My attempt at calculation goes like this:
Watts = V * A

so 12v * 50A = 600W

600W / 80W = 7.5hrs. Is this right?

thanks very much.

 

[picasm]

If it is the type of fridge that also runs on AC mains, then the wattage quoted is probably the maximum when on mains and may be about 10-20W less on DC 12v.
Also, some fridges have a fixed temperature thermostat so may use less power when cool enough.

Your calculation looks ok to me, except that I do it a different way, calculate the current used first then divide into the A/H which gives the same answer.

If your battery has not been used for a while then it may not still be able to achieve that capacity and I bet the manufacturers quoted the capacity as the maximum obtainable in ideal conditions?

 

[ericgibbs]

hi bitem,
Another point to consider is when the fridge is down to temperature and the door is kept closed, the fridge will not be powered 100% of the time.

If its well thermally insulated you can get a say, a 50% duty cycle, so the battery could last twice as long, say 15hrs??

 

[bitem2k]

Thanks for your help picasm and eric.
Ill give it a try, and see how reliable the calculation is.

 

[Hero999]

Your inverter won't be 100% efficient.

Does it have an efficiency rating?

I'd guess about 80% (worst case), so your 80W fridge will draw 80/0.8 = 100W from your battery.

 

[picasm]

Your inverter won't be 100% efficient.

Does it have an efficiency rating?

I'd guess about 80% (worst case), so your 80W fridge will draw 80/0.8 = 100W from your battery.

Hero999,
Many small portable fridges don't need an inverter as they use a 12v DC Peltier cooler and are actually less efficient when powered by the AC mains option.

 

[things]

Hero999,
Many small portable fridges don't need an inverter as they use a 12v DC Peltier cooler and are actually less efficient when powered by the AC mains option.

a peltier fridge?. peltiers aren't very efficient, so unless it uses a very good setup it will probably on all the time anyway. how big is the fridge?

 

[Hero999]

Sorry, I obviously didn't read your post properly.

All those smaller fridges seem to be Peltier coolers. I wonder why because compressor fridges are so much more efficient. Perhaps it's because they're cheaper?

I wonder if a mains fridge powered from an inverter works out more efficient than a Peltier cooler.

 

[Sceadwian]

They're much more physically robust. They're compltley solid state, only moving part is a fan, and even that isn't required depending on the application, a compressor and the coolant lines are much more complex and expensive to make and easy to damage. I'm going to guess (just guess) that a mains fridge is more effcient at the same temperature differential as a peltier can create. Compressor powered friges however are capable of a much higher differential in temperature just by running it longer. It's just the way the basic physics of the two devices work. One pumps heat using electrons, the other uses a gas with a very large compression ability so it can pump heat much better.