Both green and red lights flash and audible sound from charger

Question edited for clarity and correct spelling of the maker.

Here is the manual section on the LED Lighting sequence. Both Red and Green flashing is not there. Your charger has a model number on the bottom of it and you didn't supply that, so this may be the wrong charger!
http://powertool.manualsonline.com/manuals/mfg/ridgid/140276002.html?p=5

The question was mentioned on Fixya before.

I have a ridgid rapid twin 18 V charger that flashes red and

Fixya https://www.fixya.com > Forum > Drills > Ridgid
Nov 21, 2019 - Red flashing means the charger or battery is to hot and must cool down before it will charge. The charger will not charge a battery if the lights are flashing.
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according to manualslib type site for the wg540, solid red means charging for the wa3840 and wa3838. After 30 minutes if battery has not reached 10 volts (of 18), the red will begin flashing meaning defective.
Just green? Ready to go. Green flashing? Overheat.
Tangent; sometimes a discharged battery needs a little coaxing - have to get a little voltage into the battery for charger to try to charge. Different company, electric drill. I plead ignorance if this applies for the 3840 since they mention the half an hour time.

Phil after a full charge you should be reading more than 24.8 volts on the batteries I'm still leaning toward replacing the batteries of course thats not cheap but if you can locate 2 batteries from a provider that will let you try them on the scooter to see if that fixes the flashes you are getting I don't have too many more ideas to give you sorry

If both lights are on and both are flashing the charger is reading the battery as defective and the charger won't charge the battery. If the Red light (yellow if plastic is faded) is on and the green is flashing the charger is trying to charge the battery. If the battery won't charge replace it.

Flashing red light usually means low batteries. Check the batteries after charging all night with a volt meter through the charger port you should read 26-24 volts dc. If you get a lower reading and you are sure the charger is working correctly then you will need to replace both batteries Hope this helps

Disconnect the - lead from that battery, Repower the charger. The LED should be green. If it stays RED, charger defective.
Reconnect the - lead to the battery, measure voltage when charger is on. Should be between 11.5 and 13 volts. If OK, replace the battery. If lower than 11.5, check for charger cable connections OR for a connected load ( power drawing ) of more than 10 amps.

it doesn't change to green, the solid red light means it is fully charged, there should be a legend on the front of the charger to help you with this. If it flashes real fast the battery is malfunctioning and slow flashing is charging and again solid is fully charger.

Dust getting in the charger is a problem, Try blowing it out with a moisture-free dust remover, found at most any store. Green flashing means improper power, due to dust or power from the outlet, try a differant wall outlet. Red flashing means the charger or battery is to hot and must cool down before it will charge. The charger will not charge a battery if the lights are flashing. Solid Red light means it is charging and solid green light means it is charged. Plug the charger in first before putting the battery on the charger. If the charger has been droped or hit the floor the coil inside could have broke loose and dammaged the charger requiring it to be replaced.

I hooked a 12 volt car battery charger to the battery for about two min. Then I put it in it's own charger,the yellow & green lights came on for about 5 sec. then the red light came on . About 30 min. later the green light came on and the battery is fine.