Can u charge a 18 volt battery on a 24 volt charger

No you can't. You really need the charger for the scooter itself. They are often a funny voltage, 18 or 24 volts.

18 volt battery charger will give out 20 to 22 volts. and drop off while charging. to around 19 volts. its just finding the right amps. as this is the most important part. these can very from 3 to 20 amps. some cheap chargers are 1 or 2 amps and can take up to 8 hrs to charge a battery. as an even a charger that says 18 volts at 8 amps or 6 amps should be fine. really depends on the manufacture setup.

During the initial state of the charge, the charger operates in a fixed-current mode. This means that the current will be held at a constant rate until the battery is charged to a bit over 18 Volts. During this phase, the output of the charger will not be 18 Volts. This is done for safety and battery preservation; forcing 18 Volts on a battery discharged down to 15 Volts or less could cause the battery to overheat. If no battery is connected, the charge controller may hold the output low until a battery insertion is detected.

The exact behavior of the charger depends on the chemistry for which it is designed, whether Nickel-Cadmium, Nickel-Metal Hydride, or Lithium ion. If the output of a known-good battery is 18 Volts after charging, and it has normal capacity, the charger is working properly.

However, a bad cell in the battery changes things. If you run a Ni-Cd battery down too far, the weakest cell in the battery may begin to reverse charge from the current coming from the other cells. This usually causes an internal short circuit, and the cell will be stuck at 0 Volts. A battery pack with one shorted cell will read 16.7 -16.9 Volts when fully charged, and the drill will not have the peak power it should. The charger will give you a battery fault indication if it is equipped to do so.

Bad connections on the battery posts is most likely. If the circuit breaker was bad you'd get nothing at the battery . The charger voltage is 29 volts when not connected to the batteries. 26 or more when connected and on.

I have used a DieHard portable battery charger (model 200.71222) for several years to recharge my 24-volt, 18-year old Craftsman mulcher, model 315.370270. This is after the on-board charger died. I just disconnect the link between the two 12-volt batteries and remove all other battery connections. Then connect the charger leads to the positive and negative terminals on one of the batteries before plugging in the charger. I put the settings on the charger at conventional low maintenance and 2 amps. Charge one battery and then the other (assuming you have two 12-volt sealed lead acid batteries like my mower). I use binder clips from an office supply store to hold the battery wires on the terminals when I'm mowing...makes it simpler to remove and reattach for charging instead of the nut-and-bolt connectors that came with the mower.

No. It will probably charge but you will significantly reduce the life of your battery.

Nope. It takes a different charger.
Farmerbear at FixYa.com

check output voltage on charger label most chargers are not regulated so if it says 18 volt output it may put out 24 volts but if its putting out 51 volts it may have a short batteries will get warm but not hot replace charger

If the battery is not damaged or totally dead, you can attempt to charge it with any
24-Volt DC charger. The charging voltage rises to about 28 Volts when nothing is
connected to the charger. You must first connect (with jumper leads) the positive
charger output to the positive battery terminal. Use a test meter to determine the
polarity of the battery. Also test the charger output to determine the polarity. Be
sure to connect positive to positive and negative to negative. Do this before turning
on the power to the charger. If the battery is a "smart" battery; that is with built-in
electronic control circuit, the ordinary charger may not work. Normally, a battery
will accept a charge until it is nearly completely charged and then the battery
voltage will slowly "buck" the charging voltage. When the battery voltage and the
charging voltage are equal, no more charging current will flow. Amperes, that is
coulombs of current, charge the battery; when the battery is at (approximately) 24
volts and the charger is putting out 24 volts, NO current flows. The battery is then
fully charged.