Is there a mathematical formula for calculating the time it would take a 300 Watt device to drain a 12 volt battery 1 volt?

I have a 300 Watt (12 volt) heater that does not indicate the amp usage. I was wondering how long I could keep the heater on (without the engine running) before there would NOT be enough juice to start the Runner.

BTW, the battery is a 1000 CCA unit.

Thanks for any help.

 

I think the formula is amps x volts = watts. A 300 watt 12 volt heater draws 25 amps.

The rest of your question deals with watt/hours. I'm not so sure how to figure watt/hours in relation to battery life.

I'm also not sure how many amps a starter motor draws. Its probably around 400 but that's just a guess. We need to know that before we have a real answer.

 

Yep, the basic formula for current draw is Amps = Watts / Voltage

Given that it's a car battery, I'd suggest 300 / 13.5 = 22.22 Amps
but you're always best to double check with a multimeter if you can.

As for your battery, it has a rating of 1000CCA (Cold Cranking Amps). This means it can supply 1000 Amps for 30 seconds (at 0 degrees F) before the voltage drops below a certain threshold (7.2 volts says one manufacturer).

Compare this to a deep-cycle battery with a rating of 300AH (Amp/Hours). This means it can supply 300 Amps for 1 hour, or similiarly, say, 10 Amps for 30 hours.

It isn't actually possible to convert between the two ratings as each battery has a different application.
I wouldn't suggest you hook up your heater to the battery and find out, as cheaper 'cranking' batteries only need 2-3 discharges before they call it quits.

But if you were to use that same deep-cycle battery (300AH), it could theoretically run your heater for around 13.5 hours!

Personally I would only use a cranking battery for exactly that, and to ensure I don't get stuck in the bush, connect all the creature comforts and accessories to a 2nd (or 3rd!) battery

Dave.