Hi,
here are some things that may explain your observations.
The time taken may not be a true indication of the total Ah put into the battery. Due to different battery characteristics (different brand) and different age, the time taken to reach full charge may be different. How are you actually determining when the battery is fully charged?
The only way is to measure the SOC voltage which would involve removing all loads and charging sources, waiting for about an hour to ensure the surface charge on the plates has been removed.
The time taken during charge to reach a certain voltage is only an indication of the battery being charged and may or may not be an indication of the true SOC.
As an example, taking two batteries, both discharged by the same Ah, a 100Ah battery will reach 14.4V sooner than a 120Ah battery given that the charging current is identical. The difficulty is that chargers seldom deliver a constant current unless the battery is discharged low enough to ensure the charge is running at maximum current.
take care
Mike
A good answer Mike
@mikerezny . As you say, if we go by the SOC voltage in battery to approximate remaining capacity, 75% of a 100ah battery is an indication that only 25ah has been used, whilst the same 75% SOC on a 120ah battery would indicate that 30ah has been used. Obviously with a 10 amp charger, it will take longer to put 30ah back than 25ah.
It also depends on the algorithm that is programmed into the "smart" charger too.
Just an added point about charging - should a 100ah battery be discharged by say 30ah, and you have a 10 amp charger, it will take more than just 3 hours to replace that amount of discharge. The old rough rule of thumb is amp hours to be replaced divided by charger current (averaged), equals approx time (in hours), but you have to
add an extra 10%!! This extra time needed, is owing to resistance to charging current and an allowance for any heat generated, disregarding that different battery chemistry and peak charging voltage may differ. Hence an AGM is able to charge faster than a Flooded lead acid type battery as internal resistance is lower (higher purity lead plates) which also means less heat in charging. Gel batteries are not able to accept as higher charge current as an AGM, as this tends to dry out the paste (electrolyte) between the positive and negative plates. Lithium batteries have even less internal resistance and can accept an even faster charge without problems.