The capacity of a battery is indicated on it in terms of Voltage rating and the mAH rating. The mAH rating indicates the storage capacity of the battery.
mAH is the product of current and Time. i.e mAH = mA * H
A battery rated at 12V, 3000mAH indicates that,
The battery can supply 12 volts at 100mA for 30 Hours (100mA*30H = 3000mAH) or
The battery can supply 12 volts at 1000mA for 3 Hours (1000mA*3H = 3000mAH)or
The battery can supply 12 volts at 500mA for 6 Hours (500mA*6H = 3000mAH).
So, more the mAH rating of the battery more is the storage capacity.
If there are two batteries with same voltage rating and different mAH rating. The battery with larger mAH rating can supply the rated voltage to the load (drawing fixed current) for longer time than the battery with lower mAH rating.
Replacing batteries :
If you are replacing the battery of a device, you need to choose a battery with same voltage rating and same or larger mAH rating than the older battery.
If you choose a same voltage and lower mAH rating battery as a replacement then the battery backup hours will decrease.
If you choose a same voltage and larger mAH rating battery as a replacement then the battery backup hours will increase.
Choosing a larger voltage battery as a replacement will spoil the device.
Battery make : Always choose the same type of battery make as the old one, If the old battery is a Li-Ion battery, then replace it with a Li-Ion battery only.
The charge and discharge rate of the new battery should be more than or equal to the older battery rates.
Watch this video for more details,
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