Draining a 12-V Car Battery

My first comment is to proceed cautiously. When car batteries discharge with such a heavy load as you propose, they generate hydrogen gas. A spark near the battery can cause the battery to explode, spewing sulfuric acid everywhere.

There was a time when batteries were rated in Ampere-hours (AH). This is a useful rating for your purposes. Unfortunately the modern trend is to rate the battery in cold cranking Amps, which is completely useless for your needs. Going from memory, it seems to me that a battery for a 4 cylinder engine was rated around 180 AH and a battery for a V8 engine was rated around 300 AH. A 180 AH battery will completely discharge in 9 hours at 20 Amperes (180/20 = 9). However the battery is destroyed if you run it down to zero. Car batteries can tollerate discharge down to only 10% of their rated voltage. Once you discharge the car-battery down below 10.5 Volts you begin to shorten the life of the battery. This means the useful discharge is only 10% of the AH rating. In the example above, 10% of 9 hours is 0.9 hours, or about 54 minutes at 20 Amperes.

The simplest current limiting device is a resistor placed in series between the battery and the load. Usin Ohms law, 12Volts/20Amperes = 0.6 Ohms. However, the resistor will dissipate 240 Watts (V-squared over R = 144/0.6 = 240) which is a lot of heat. For comparison, a 60 Watt light bulb dissapates only one-fourth the amount of heat that this resistor would. Resistors rated at 240 Watts are big and expensive.

A switching regulator would be more efficient. However the design of a switching regulator for such a large current is no small matter. You'd be better off buying a manufactured solution. What are you are trying to do?

Ken Stewart

Another rating on a battery is the "reserve capacity". This is the amount of time the battery will maintain sufficient voltage to operate 12 volt items with a constant 25 amp load on it.

The current used from the battery will be determined by the device using the power. The standard current limiting device will be a circuit breaker. It will stop current from exceeding the rating on the breaker.

Your circuit should be something like this. Battery to circuit breaker, to switch, to device, back to battery. Tell us what you want to run from the battery, and we will be able to help more.

Hello Guest:
One thing you might want to consider is that standard automotive batteries are not met to be drained. For what it sounds like you want to do you should use a deep cycle battery. Repeatedly running a regular automotive battery close to dead will result in a fairly short lifespan. What will kill it in a hurry is leaving it dead for a prolonged period.

As a previous poster already said if you can provide a little more information I'm sure someone could be of more help. If it's just an emergency backup system you wouldn't necessarily need a deep cycle battery, however if you intend to be cycling this regularly it might be well worth investing in a good deep cycle battery.

PS. I hadn't thoroughly read the first post I see they had already commented about the shortened life, deep cycle batteries are much more tolerant.