How much power does a car battery contain, and for how long? I heard that batteries give off a massive amount of power for the first few minutes, then drop off. Let's say this is a new battery, fully charged and ready to go.
Unless you give the make and model of your battery, it's not possible to give you specifics. However, most of the info should be printed on the label, and you can also generally go to the battery manufacturer's web site and look at the actual specs.
Most car batteries are "12V" (not really exact, and also depending on remaining charge and current draw) but there are many systems out there so there's not really a general answer.
Plus even if we choose to say "12V is a safe bet" for voltage, the general capacity, max current, reserve capacity, CCA, ability of battery to perform in cold conditions, etc., can vary a lot.
So, you'll have to go look up your battery on the mfr site or check the label. Here are some of the typical stats (as well as definitions take from here):
- Cranking Amps (CA) - A rating that is used to define the battery's ability to start an engine in moderate temperature conditions. BCI defines it as "the discharge load in amps that a new, fully-charged battery at 32ºF (0ºC) can continuously deliver for 30 seconds while maintaining a terminal voltage equal to or higher than 1.20 volts per cell." This artificially high rating should not be confused with CCA, which is conducted at 0ºF (-17.8ºC).
- Cold-Cranking Amps (CCA) - A rating that is used to define the battery's ability to start an engine under low-temperature conditions. BCI defines it as "the number of amps a lead-acid battery at 0ºF (-17.8ºC) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt lead-acid battery).
- Nominal Voltage - The rated voltage of a battery
- Closed-Circuit Voltage (CCV) - The voltage of a battery when the cell or battery is under a specific discharge load and time interval. See also Open-Circuit Voltage.
- Reserve Capacity (RC) - BCI defines it as "the number of minutes a new, fully-charged battery at 80ºF (27ºC) can be discharged at 25 amps and maintain a voltage equal to or higher than 1.75 volts per cell" (i.e., 10.5 volts for a 12-volt battery). This rating represents the time the battery will continue to operate essential accessories in the event of a charging system failure.
- Capacity - The ability of a fully charged battery to deliver a specified quantity of electricity (AH) at a given rate (amps) over a definite period of time (hours).
- Rated Capacity (Flooded) - The CCA, RC or amp-hours that a battery can deliver at a given rate of discharge, end voltage and temperature. These ratings are often displayed on the outside of the battery
A "standard" battery will deliver 12.6 volts when fully charged. When charging (e.g. when the engine is running and the alternator is charging the voltage will rise to around 14.2 volts (there is a normal range from around 13.5 to 14.5 volts – don't be to picky about the actual voltage while charging unless you have specific specs).
Different batteries will have different capacities, measured in amp hours (Ah), the actual amount of power the battery can deliver also depends on the rate and the construction of the battery. The amp hour rating of a battery is based on a standard rate – 1 A – higher draws will result in a lower number of amp hours and a lower draw will extend the life of the battery. For example (made up, not based on any real battery) a 100 Ah battery might be capable of delivering 400 A for 6 minutes (40 Ah), but 0.1 A for 2000 hours (200 Ah).