I charged a 36v pack with a 48v charger to keep me on the road. A 24v pack with 48v charger >>> NOT<<<< I had to watch the voltage on battery the whole time which is mega boring.
This is very dangerous to your batteries. Some chargers can do this and some chargers will break, catch fire.
So do not be tempted to do this. Muzza gives good advice, label them so you can not make a mistake.
The optional charger is for more amps.
The optional charger with bigger amps is for a fast charge. They charge the battery faster and is safe to your battery as long as it is the right volts. It is not as good for your battery to charge faster but it is fine to do occasionally when you need it.
People buy the optional charger and carry it in a bag with them. They can charge fast when out on a ride and use the slow charger at night..
There are parks in our area that have electricity supplied in places for people to use there lap tops and electric cookers. Sneak some power from the socket at the mall near where you park the bike.
You would be surprised where you can find some free power.