How Long will my battery Last?

Main Question or Discussion Point

Hi All

I know this is fairly elementary but my brain doesn't want to play ball at the moment.

I have a battery pack rated

Voltage: 12 V dc
Nominal Current: 500 A
Peak Current: 900 A
Capacity: 12 Ah

And a portable coolbox/Fridge rated

Input Voltage: 12 V
Power Consumption: 47 W

If I use this combination on a camping trip, how do I calculate how long the setup will last before recharge is required?

TSP

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scottdave
Homework Helper
For ease of calculations in my head, say it's 48 watts. 1 watt = (1volt)(1amp), so 48 watts / 12V = 4 amps.

Now take the 12 amp-hours divide by 4 A and you are left with hours. But does this rating mean it will deliver the 12 Volts for that amount of time? It probably is not stamped on the battery, but if it is a brand name perhaps you can go to their website and download a spec sheet. Hopefully it has a voltage graph showing how output voltage changes with useage.

anorlunda
Mentor
Nominal Current: 500 A
Peak Current: 900 A
Those are almost certainly wrong. Not A for amps but mA for milliamps. Fortunately, that is irrelevant to your question.

12V*12Ah=144 watt-hours

144 watt-hours/47 watts = 3.06 hours.

Allowing some margins for inefficiencies, says 2.5 hours.

But here is the problem. The 12V battery voltage will be decreasing as it is discharged. Not all the way to zero, but perhaps to 11V. The fridge says 12V in, but it does not say at what voltage it stops working. The point is you may get less than 2.5 hours depending on how fussy the fridge is.

scottdave
Homework Helper
Those are almost certainly wrong. Not A for amps but mA for milliamps. Fortunately, that is irrelevant to your question.

12V*12Ah=144 watt-hours

144 watt-hours/47 watts = 3.06 hours.

Allowing some margins for inefficiencies, says 2.5 hours.

But here is the problem. The 12V battery voltage will be decreasing as it is discharged. Not all the way to zero, but perhaps to 11V. The fridge says 12V in, but it does not say at what voltage it stops working. The point is you may get less than 2.5 hours depending on how fussy the fridge is.
I was thinking the same thing about the milliamps. If it is not a car battery, then it may be milliamps, which means the fridge is more load than the battery can even handle.

Wow Guys

Thanks very much for the quick responses, very simple when explained.

Looks like I'm gonna have to use some other method to keep those beers cool lol

The current figures are taken directly from the back of the battery pack.

It is a battery booster pack, primarily for use as a car jump starter, but has the cigar lighter socket which the coolbox can plug into

scottdave
Homework Helper
The current figures are taken directly from the back of the battery pack.

It is a battery booster pack, primarily for use as a car jump starter, but has the cigar lighter socket which the coolbox can plug into
If it is the small pack, like maybe 2 packs of cigarettes size(but much heavier), those you plug into power socket of car and let it deliver some charge to the car battery over several minutes. There is no way something like that would deliver 500 to 900 Amperes. So it's likely 500 to 900 mA, which is 0.5 to 0.9 A (much less than the 4 Amps fridge). Maybe a motorcycle battery could work, or a cheap car battery. I've seen some metal recycling places sell reconditioned car batteries for \$45

jbriggs444
Homework Helper
2019 Award
It is a battery booster pack, primarily for use as a car jump starter
That wound indicate that the 900 amps is real, e.g. Jump Starter from Walmart

davenn
Gold Member
2019 Award
I have a battery pack rated

Voltage: 12 V dc
Nominal Current: 500 A
Peak Current: 900 A
Capacity: 12 Ah

It's well past time that you supply a make and model of this thing … preferably a www site

you have everyone just guessing answers to what you may have and what it is capable of doing, it's just wasting time

Dave

anorlunda
Mentor
That wound indicate that the 900 amps is real, e.g. Jump Starter from Walmart
On second thought, that could be right. 12 AH/900A= 0.8 minutes. Let's say, 40 seconds of cranking. That's plenty to start a car in most cases. People even use supercapacitors to start cars in less than 1 second. But what about the thickness of the lead wires?

But the answer to the OP stands. 12 AH is too little for the fridge application, and a starting battery is the wrong kind, you need deep discharge. The fridge on my boat used 40 AH/day, I used 2x200AH deep discharge batteries for that.

If it is the small pack, like maybe 2 packs of cigarettes size(but much heavier), those you plug into power socket of car and let it deliver some charge to the car battery over several minutes. There is no way something like that would deliver 500 to 900 Amperes.
You'd be surprised how much current even a small lithium ion battery can output.
Take a look at this video that shows the insides of a small lithium jump starter and what happens when you short it out.