How long will a battery charge last?

[ozziemick]

Hi,
I am trying to work out how long a 12 volt car battery will last before recharging is required? My basic understanding is that a 150 amp/hour battery will supply 150 amps for one hour, or 15 amps for ten hours. But what i need to know is how much charge can be taken from a battery safely and still be able to start a car; that is before it goes flat. Example: I would like to run a fridge which draws 3 amps and I have a 300 A/H battery.
Thanks in advance for any help or links.

 

[BillJ]

If you are referring to 300 cold cranking amps, the battery is probably only a 33 amp hour battery (approximately). At 3 amps (.09C rate) a new battery would be dead in about 10 hours. I probably wouldn’t want to go more then 2 hours if I wanted to start my car. A lot depends on how much current your car needs to start and what the temperature is. See the Power Sonic “Sealed Lead-Acid Batteries Technical Handbook” available on the Web.

 

[jbartos]

Suggestion: It appears to be too risky to use the car battery for such a big load like fridge (3A). Perhaps, it may be better to have a dedicated battery for your fridge and use the common charger from your car to charge the fridge battery. This way the fridge will have much more time to run and your car will always start safely. It could ultimately also pay off for you handsomely.