# Will a car battery be able to run this?

1. Oct 30, 2013

### syst3mboot

Im trying to calculate if a standard car battery will be able to run 2 computers, 2 monitors, and some small hardware, and if so, for how long.

The entire configuration should be no more than 1000watts. (400 each computer, 50 each monitor, and 100 for small hardware). I cant seem to find how many watts a car battery will output.

Since car batteries are 12v, Ill have to run everything through a transformer (12v -> 120v, 1000watts stable).

Any help would be strongly appreciated.

2. Oct 30, 2013

### gmax137

Batteries produce dc current; this is no good for a transformer. To make AC current from dc you need an inverter or a motor-generator, which is a good bit more complex than a transformer.

1000 watts is a lot , I don't think a normal car battery would last very long.

3. Oct 30, 2013

### syst3mboot

Sorry, yes its a 12V DC -> 120V AC inverter (1000 watts). What if the car is running, would it be able to charge the battery faster than the system will discharge it?

4. Oct 30, 2013

### AlephZero

By Ohm's law, 1000W at 12V means a current of 83 amps. A typical car battery has a capacity of about 40 amp-hours, so it would run the system for less than half an hour. Given the high current involved and the losses in the inverter, maybe only 15 minutes.

If the car engine is running, the output from the alternator would probably be of the order of 100 amps, but only if the engine was running faster than idling speed. But if you compare the the cost of fuel with the cost of mains electricity, that would be a very expensive way to power the system. If you want to go that way, buy a portable generator with a small IC engine that is designed for the job. That would be much more efficient, and reliable, in the long run, than using a car engine and alternator that were designed to do something very different.

5. Oct 30, 2013

### syst3mboot

I see, I was calculating 1000W at 120V, I see where my mistake was. The problem is I need these to run for 12 hours continuously, and I need them to be portable, (inside a car).

6. Oct 30, 2013

### syst3mboot

It's like that, would that be 1000watts at 120v or at 12v?
if its at 120v then its 8.3 amps which is very little and a battery should be charged faster than depleted. If it's at 12v then its 83Amps which is too much.

7. Oct 30, 2013

### Khashishi

Computers also run on DC. The power supply typically converts the wall AC to something like +-12V, +-5V. So you may be able to avoid the inverter if you hack your own power supply.

8. Oct 30, 2013

### gmax137

Neglecting inefficiency in the inverter, the battery supplies 1000 watt at 12 volt.

Sorry for any confusion, I deleted my bad post but maybe not fast enough ;)

9. Oct 30, 2013

### Staff: Mentor

The load is 1000 watts at 120V, but that means the battery also supplies 1000 watts, just at 12V. Plus losses, of course.

Dropping back a bit, 400W is a lot for a computer. Is that the PSU rating? Have you actually measured the power draw? I bet its half that.

10. Oct 30, 2013

### syst3mboot

I havent measured, I just checked that the max is 500watts, the problem is that both computers will be running processor and graphic intensive applications which might actually push the wattage to 400. I will try and get something to measure the power draw soon to check for sure but i dont know. :(

11. Oct 30, 2013

### rcgldr

Some UPS (uninterruptable power supplies) have a display that shows an average wattage, but not the peak wattage.

12. Oct 30, 2013

### syst3mboot

Ill just get something like "kill a watt", and by the looks of this I guess ill have to go with a power generator, a 2000watt one should be enough right?

13. Apr 24, 2015

### Drakkith

Staff Emeritus
This thread is over 2 years old. Let's leave it to rest in peace. Thread locked.