Why battery charger has rated amp hours

Question

I have gotten a car battery charger as a present which is rated at 6A max and is also labeled up to 65Ah max.

I'm not understanding why there is a battery capacity indicated on the charger when the battery will keep getting charged for as long as the charging voltage is greater than the battery's voltage.

Say I have a 100Ah battery which requires around 10A to get charged, with this charger it should take roughly twice as much time, but the charger should still be able to charge the battery.

Why is there a capacity label on the charger?

Answer 1

A 100Ah battery will likely have a lower internal resistance than a 65Ah battery. That will mean that the 100Ah batter will try to draw more current when charging than the 65Ah battery.

A good quality charger should be able to limit its maximum current to say for example 6A without any harm to itself or the battery. In that case it will take almost twice as long to charge a 100Ah battery than a 65Ah battery.

A cheap charger may not have over current protection and if a battery tries to take more than 6A for a prolonged period, the charger may overheat and possibly break.

Answer 2

The only reason the charger is rated in Ah is the ability to correctly determine the end of charging.

Generally, the charger limits the current until the battery gets to some voltage, then limits the voltage and monitors the current. The battery is considered charged when the current falls below a certain value.

A larger battery will draw more current in the voltage-limited stage and depending on how good it is, it may never fall below the threshold to be considered "charged" by the charger. The charger will stay "on" and overcharge the battery. (Yes, the charger will overcharge the battery because the battery is too big!)

Some chargers are protected from such a mishap by also cutting off after certain time passed or certain amp-hours inserted into the battery. This way you may also end up with a partially charged battery.

You are pretty much OK to occasionaly charge a large battery with a small charger that is rated for smaller battery. Just be sure to not leave it connected for days and weeks or you may end up with a dead battery.