How Long would a 100 Ah battery last



The Amp Hour rating of a battery simply describes the battery capacity. The Ah rating shows the amount of energy(charge) that the battery can store. Common examples include 65Ah, 75Ah, 90Ah, 100Ah, or 200Ah.

The Ampere hour (Ah) rating tells us the amount of current a fully charged battery would deliver for a certain period of time. In essence, a battery rated for 100Ah should be able to supply 100 Amps for 1 hour or 1 Amp for 100 hours.

Similarly, a
  • 65Ah battery should deliver 1 Amp for 65Hrs or 65Amps for 1hr.
  • 200Ah should deliver 1 Amp for 200Hrs
  • 100Ah battery should deliver 1 Amp for 100 hours,
  • 100Ah battery should deliver 2 Amps for 50 hours,
  • 100Ah battery should 3 Amps for 33 hours etc.

It would be nice if this equation held true all the way up to 100 amps for 1 hour, but there are some limits on the maximum amount of current you can draw from a battery without destroying the battery.

Batteries however have a rating called C-rate. This rating informs us of the maximum current at which it is safe to discharge the battery, it also informs us of the safest current that can be used to charge the battery. In simple words, the charge and discharge rate of a battery are governed by c-rate. The c-rate suggests the shortest time (in hours) the battery can be safely and completely charged and discharged.

For example;

A 100Ah battery with a C rating of C/10; simply means that it is recommended that the battery is discharged over 10hrs so the maximum current the battery is expected to deliver is 100Ah /10hrs = 10 Amps.

A 100Ah battery with a C rating of C/8; simply means that it is recommended that the battery is discharged over 8hrs so the maximum current the battery is expected to deliver is 100Ah /8hrs = 12.5 Amps.

The most common type of battery used in cars, the Lead-acid battery rated for 100 Ah is usually rated for that capacity when discharged over a period of 20 hrs because Lead-Acid batteries are best on slow rate discharge. C/20 is the informal industry standard when no C-rate is specified. In other words, 100AH / 20hr = 5 amp load for 20 hours.

So the maximum rate of current drawn from the battery should not exceed 5A for a Lead Acid Battery. If you are powering a device that would demand 10A two batteries should be connected in parallel to supply the required load.

As a side note,
The C-rating is written in two different formats depending on the manufacturer. The Decimal format or the fractional format could be used. For example a 100 Ah battery with a C rate of C/2 is the same as a 100 Ah battery with a C rate of 0.5C

For a battery with a c-rate of C/2, we are informed that we can discharge the battery fast within 2 hours without damaging the battery. Our calculation gives us 100AH/2hrs = 50 Amps max

Similarly, for a battery with a c-rate of 0.5C, we are informed that the battery cannot deliver more than 50A continuously without overheating and damaging the battery. 50A is gotten by 0.5 * 100AH

Other examples are 0.2C is the same as C/5 and 0.05C is the same as C/20.

 The decimal format tells you to multiply the battery capacity by the decimal to get the max charge or discharge current while the fractional format tells you to divide the battery capacity by the denominator to get the max charge or discharge current.

In Summary.
  1. The amount of time a battery would last depends on how much current a battery is asked to deliver or the discharge rate.
  2. The amount of current a battery can deliver without overheating and damaging is limited.
  3. C-rating is the safest continuous discharge rate the battery will support.
  4. C/5, C/10, C/20 suggest the recommended time in hours over which the battery should be discharged.



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