What voltage does a charger need to be to charge a 21.5v battery?

The voltage in the battery is too low to “excite” the charger. The charger needs to read some sort of voltage to know it is connected to a battery and connected the right way otherwise it will not begin charging. If you have another battery and jump leads you can connect it to the flat battery to excite the charger. Once the charger commences charging you can remove the jump battery and leave the flat battery to be charged.

PLEASE NOTE: The Low Voltage Protection feature on this battery charger will not allow the charger to activate unless it is connected to a battery pack that has a minimum voltage of 16 volts. Battery packs with voltages below that level of charge cannot be recovered or recharged. This is a safety feature to prevent overheating in the event of a faulty battery. This battery charger will not output any voltage to the XLR connector until connected to a battery pack meeting this minimum voltage requirement.

John the red flashing light indicates a low battery When you charge the battery your charger should be showing a red light for power and a yellow light. When the batteries are fully charged you should show a red and green light on the charger. Now if you have a voltmeter plug the leads into the two outter holes of the charger port located under the front of the joystick.Without the charger plugged in you will be reading the battery voltage. Fully charged it should read 27-30 volts if you are only reading 20-24 volts then the batteries are low. If you have an onboard charger plug it in and you should read the charger voltage of 29-32 + volts If you aren't getting that voltage then you have a bad charger if you are getting the charger voltage then you have bad batteries and they need to be replaced Hope this helps you

If your bike works when you have the charger connected but doesn't whith the charger disconnected and just the battery then your battery is not charged or you have a bad connection between the battery and ebike.
If the battery charger is showing green then it normally means that charging has stopped charging and there is no current flow to battery. This could be because the battery is fully charged, OR there is an open circuit (broken wire, etc.) between the charger and battery, OR your battery is so flat the voltage is below the low voltage cut-off point of the charger so the charger is not switching on.
You need to check the voltage out of your battery and compare it to the LVC of your charger (should be specified in the user manual), but from what you have described I'd say you need to replace your battery.

You will need to check the voltage of the battery before and after charging , If the voltage is not coming up to 1.2V then the battery is defective. even if the voltage is present but does not drive the unit then the charge is not retained. If so the battery can be defective.
check the terminals of the charge for the charging voltage, use a Ammeter with a load to test the current. If these test prove good then the charger is working fine.

The measurements below are for a 12 volt charger or across one battery at a time. In a 24 volt charger or across both batteries at once, double the values.
The charger will not show any output unless connected to a battery. To test the charger, attach the charger to the battery but do not plug in the charger. Connect a meter to the battery and read the voltage (9.8v - 11.6v).
Now plug in the charger. Read the battery again (13.6 - 14.2v).
The voltage must not be higher than 14.8 on the battery or the battery will over-charge - the charger needs service. If the voltage does not come up to 13.2v, the charger is not running. Again, the charger needs service.
Be patient. You can wait ten minutes and measure again. Batteries respond slowly to the charger.

You didn't mention what brand & model of wheelchair/scooter you have. To properly check to see if the charger is working correctly is testing the total voltage of both batteries together while the charger is plugged in to the controller joystick. First check the total voltage of both the batteries together before you plug the charger in. The total voltage of both fully charged batteries together should be around 24.5 to 26.5 volts. Plug the charger into the charging port on the powerchair/scooter. Now with the charger on, the voltage should read at the very least 1+ volts more than the total voltage measured earlier. If less the charger is bad and will need to be replaced.
Hope this helps, if so please rate this solution which will help others who may have the same answer and thank you for using FixYa.
Flavadave75

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.

You are charging. The meter corresponds to the level of charge in the battery. Try setting to 10 A for 4 hours, battery should be half-way up. To determine if battery id dead.. leave battery off charger once it is fully charged...test with a voltmeter after 10 min.....battery should still be above 12v...after1 hour it should still be above 12v...if not battery is spent.

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