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Can batteries of different A.h (ampere hour) ratings be used together in series?

The ampere hour ratings of a battery indicate the rate at which the battery can be loaded. For Example a 20Ah battery indicates that indicates that the battery can supply a current of 1 ampere for 20 hours.
Smaller batteries have their discharge rates indicated in mAh (milli ampere hour)

When batteries of different ampere hour ratings are connected in series, it is necessary to ensure that none of the batteries is discharged beyond its capacity. This is because if a battery connected in series in a battery bank is discharged to its maximum, the voltage of this battery will become zero. This will cause a voltage from the other batteries to be applied across the battery in the wrong direction. This can cause damage to the battery