Can a hub motor on ebike be started with battery charger ?

Make sure the battery is fully charged and firmly connected. Also check the display Settings.

https://www.google.com/search?q=no+display+on+instrument+panel%2C+ebike

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If the battery is down, use a battery charger and get charged up. Check battery connections. If you can get it running, check your charging system, and applicable trouble codes.
If hood release is electric, sometimes you use battery charger at starter motor bat terminal, to charge the battery? If you can get at the starter motor?
I don't know model year?

Clean connectors.

Make sure the battery has a full charge. If you use a digital multimeter across battery posts, everything off, the generic spec for a full charge is around 12.6 volts.
If your using a jump box or charger, hopefully the gage on the tool will tell you something?
If you want to check the starter motor or wiring circuits at the starter motor, we can do that. If you try that, make sure tranny is in park or neutral and functional parking brake is set.

I can't find any reference to a Homelite model 720 generator but most battery charging systems do not supply enough amps to run a car starter. Battery chargers usually work in the two (trickle) to ten amp range. Car starters need way more amps to have enough power to turn an engine over, depending on the size and number of cylinders in the motor. Some chargers have a switch with a 60 amp 'start' position to help the battery turn the motor over, but without that there is little chance the charger side of your generator will supply enough amps to help your battery.

probably your battery cant conform minimum requirements of digital circuits used to drive starter motor in high tech cars. that is why it works only with external charger. measure your abttery. it should be showing at least 12.4 volts. normally battery should show 12.5. to 12.65 volts. probably your battery has sulfation. end of its life.

Behind the glove box. Please see my tip at http://www.fixya.com/cars/r6234839-doesn_t_heater_or_air_conditioner_blow . These are generic instructions. If you get stuck, please get back to me.

See instructions from autozone.com below.

REMOVAL & INSTALLATION

With Air Conditioning
See Figures 1, 2 and 3

1. Disconnect the negative battery cable.
2. Remove the glove compartment door and glove compartment.
3. Disconnect the blower motor wires from the blower motor resistor.

Fig. Fig. 1: Disconnect the blower motor wires from the blower motor resistor-1988 Escort

1. Loosen the instrument panel at the lower right-hand side prior to removing the motor through the glove compartment opening.
2. Remove the blower motor and mounting plate from the evaporator case.
3. Rotate the motor until the mounting plate flat clears the edge of the glove compartment opening and remove the motor.
4. Remove the hub clamp spring from the blower wheel hub. Then remove the blower wheel from the motor shaft.

Fig. Fig. 2: Unbolt the mounting plate from the case

Fig. Fig. 3: The motor can now be removed from the case

To install:

1. If removed, assemble the blower wheel to the motor shaft and install the hub clamp.
2. Install the motor and blower wheel assembly.
3. Secure the instrument panel at the lower right-hand side.
4. Connect the blower motor wires at the blower motor resistor.
5. Install the glove compartment door and the glove compartment.
6. Connect the negative battery cable.
7. Check the system for proper operation.

Without Air Conditioning

1. Disconnect the negative battery cable.
2. Remove the air inlet duct assembly.
3. Remove the hub clamp spring from the blower wheel hub. Pull the blower wheel from the blower motor shaft.
4. Remove the blower motor flange attaching screws located inside the blower housing.
5. Pull the blower motor out from the blower housing (heater case) and disconnect the blower motor wires from the motor.

To install:

1. Connect the wires to the blower motor and position the motor in the blower housing.
2. Install the blower motor attaching screws.
3. Position the blower wheel on the motor shaft and install the hub clamp spring.
4. Install the air inlet duct assembly and the right ventilator assembly.
5. Connect negative battery cable.
6. Check the system for proper operation.

with a trickle charger you can connect to any wire that goes to the battery as long as there is no circuit between the charger and the batt

provided you disconnected the battery before you charged it, its probably just a new battery you need, but if you did not diconnect it you have probably done damage to the starter motor and maybe the alternator and god knows what else