In our previous post, we focused on the definition of a battery’s Voltage and how it affects the battery’s performance. Now we look deeper into another factor which greatly affects our batteries, the Ampere Hour or Amp Hour rating.
An Amp Hour (Ah) is the amount of current a certain battery can supply for a certain period of time. The Amp Hour also has sub-units like the MilliAmpere-Hour (mA-h or mAh), and the MilliAmpere-Second (mA-s) which is the unit of measure used in X-ray diagnostic imaging and radiation therapy.
How to compute Amp Hour
We can compute the amp hour rating of a battery by multiplying the current (amperes) by the discharge time (hours). Or if we have the amp hour rating of the battery, we can compute how long the battery will last while supplying a certain current. We will take our previous example for consistency, thus:
Current (I) = 10 amperes ; Discharge Time (T) = 15 hours
Therefore, this battery would have an amp hour rating of :
Amp Hour = Current (I) x Discharge Time (T)
Amp Hour = 10 amperes x 15 hours
Amp Hour = 150 Ah
Rating = 150 Ah @ 15 hours
Get it?
Now for the example below, we will find how long a battery will last given its AH rating and the current needed by the system to function.
A 12V AGM 35Ah Battery is used to supply power to a system with a current draw of 0.6 Amperes. How long will the battery last?
Using the same equation in the first example we get:
Amp Hour (Ah) = Current (I) x Discharge Time (T)
35Ah = (0.6 Amperes) x T
T = 35Ah / 0.6 A
T = 58.33 Hours
This particular battery will power your system for approximately 58 hours. Other factors may affect the length of service of the battery such as the battery’s age, number of charging and discharging cycles, temperature, etc. But this is the easiest way of getting a pretty good idea of how long your battery will last.
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