difference in charging battery of different Ah
This can be done in theory, but won't be very useful in practice.

First, you can't just connect batteries together.  The charging battery needs the right combination of volts and amps.  Even if the two batteries are of the same chemistry, and therefore will have the same voltage at the same state of charge, just connecting them is still a bad idea.  The current can be very high, dependent only on the internal resistance of the batteries.

To do this correctly would require a switching regulator that can convert the source battery's volts and amps to what it takes to charge the destination battery.  That is doable enough, but will lose some energy in the transfer.

However, the bigger issue here is that there just isn't that much energy in a 20 Ah battery compared to what a 110 Ah battery (assuming same types) can hold.  You'd have to use a bunch of smaller batteries to charge up the larger one.  I suppose this could be done if the larger battery is someplace fixed and remote, so you shuttle power there by using a set of smaller batteries.  Again, this works in theory, although there really should be easier ways to get this accomplished in practice.  This is like filling the gas tank in your car by walking back and forth between the car and gas station with a 1 liter bottle.  You'd eventually get there, but other methods will be easier.