A 12V car battery has a capacity of 100Ah. what is the energy stored in the battery? If a car's light needs 100 W. Find how long the battery can keep the lights burning when the engine is not running?
Answers
Further to the other posted calculations, consider the age of the battery and the nature of the load.
A battery rated at 100Ah will have less capacity as it gets older. After two or three years’ use, its capacity will be reduced by 20–50% depending on how it has been used and recharged. A battery tester will measure capacity. Cheap testers cost about $20.
If the load includes a motor, the starting current of the motor will momentarily reduce the voltage. As the battery weakens (by discharge or age), the starting current will cause a voltage drop below the minimum level required by the inverter.
For example, a 100Ah battery that is three years old might have only 75 Ah capacity. If it is half discharged (I.e. showing about 12.1 volts) its capacity is then 37 Ah. When a 10 amp motor, say, in a refrigerator’s compressor starts up, its momentary draw can be 30–50 amps. This can cause the battery’s voltage to drop below 11 volts. Most inverters will stop working below some threshold (often 11.0 volts) even though the battery is not fully discharged.
Capacity ratings are usually made by a resistive load wired directly to the battery. Commonly, the capacity is measured by timing how long the battery can produce two amps before its voltage drops to 10.5 volts. Using a 20 amp load instead of a 2 amp load will reduce the measured capacity significantly.
If your inverter is drawing 2 amps and your battery is new, you can expect to get ~40 hours’ use. If it is older and you are drawing 20 amps continuously, your inverter will likely cut out after 2–3 hours. If you are averaging twenty amps but need to start a motor, the inverter will stop working much sooner than 2–3 hours.