# Car battery, how much can it do

1. Feb 22, 2009

### gumpfer

I am looking for extremely basic information that anyone could understand, as I know nothing of electricity.

My friend wants to use two automotive alternators to charge two 12-volt car batteries. These batteries are not in a car. He just wants to run a few shop lights and a compressor. I told him two car batteries will not run a compressor of any size but I don't know the math to back it up.

He is sure he can run a three-stage compressor intermittently with two car batteries. Assuming that his alternators are up to the task of keeping the batteries charged, it seems to me there is not enough horsepower in two plastic boxes full of lead and acid to run a three stage compressor bigger than my hand. He says his compressor is a cube about 13 inches by 13 inches.

I know how to size the compressor once I get the info on what a battery can do. I just don't know how to answer this question: what is the power available from a pair of car batteries to run a mechanical device like a compressor, with a 24 volt DC motor, assuming that the compressor is running about 1/10th of the time?

All I know is that a hp is 746 watts and watts is voltage x amperage. I just can't visualize electricity doing something, it must be a trick. Respondents: I need numbers. I already told him it can't work but I have to prove it with numbers.

Thanks.

2. Feb 22, 2009

### mgb_phys

Electrical power is current * volts.
So 1 hp = 746 W = 12V * 63A or 24V * 32A

Then you have a couple of things to consider, how much current you can pull from the battery long term without damaging it. This depends on the battery design and temperature but typically around 25A

Then there is the capacity, this is given in Amp hours - so in theory a 100Amp-Hour battery can supply 1Amp for 100Hours, or 10Amps for 10Hours. In practice a car battery can typically only deliver about half the rated capacity before being damaged.
Marine batteries / leisure batteries / deep discharge batteries can deliver the full power but cost more. These can't deliver the huge startup current of a car battery.

Last edited: Feb 22, 2009
3. Feb 23, 2009

### DaleSwanson

It really depends on the amps of the compressor, if you can get that, then you can figure the exact amount of power it will take to run it. This page has some 24v compressors for sale, that range from 10-17 amps.
Assuming 25 amps a typical car battery might do a 1-2 hours. Really without a certain amps draw for your compressor it's just guesses. As mgb_phys said, normal car batteries are not designed to be drained dead and recharged, they are meant to deliver a huge amount of power for a very short time and then be recharged. A deep cycle battery would be more suited for something like this.

4. Feb 23, 2009

### brewnog

What drives his alternators?

5. Feb 23, 2009

### mgb_phys

I'm guessing he has a workshop/garage with no power so he is going to charge a couple of batteries with a car + jumpleads and then use them to run tools.

IMPORTANT - if you are thinking of running a car in a garage to charge some batteries make sure you have enough ventilation. Having the garage door open might not be enough to prevent Carbon Monoxide build up.

6. Feb 23, 2009

### brewnog

Given that the OP stated the batteries weren't in a car, and he wants to charge them with two alternators, I was suspecting he wasn't charging them with a car. Car batteries don't like being deep cycled so may not be appropriate depending on the application.

Gumpfer, what drives these alternators?