Battery will not charge, charger flashes red and green

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.
My charger is flashing red when i place the battery in it - Fixya
Dec 7, 2010
Ridgid temp delay light flashing on charger - Fixya
Sep 2, 2019
My RIDGID Rapid Max Twin 18 volt charger, all it - Fixya
Aug 31, 2019
18v ridged rapid max charger flashes red and green same
Mar 12, 2023
More results from www.fixya.com

..

All I can see is that your charger is flashing red. There wasn't any brand mentioned. Most battery chargers flash green when in charging mode, solid green when charging is complete and flashing red when there is an issue with the battery/batteries being charged. Rechargeable batteries don't last forever (usually 200-300 charges) and perhaps this is the case with yours. Also, if you are using imported batteries (China), that would be a major red flag!

If it changes from red to green and stays green while being charged, it is just a sign that it is being charged and there is nothing wrong.
If it flashes back and forth between red and green while charging, there might be something wrong with your charger's connection.

A red light indicates the charger is charging the battery, an amber light means it has switched from fast rate charging to trickle to keep the battery topped off, and green would mean the battery is completely charged. But don't be fooled waiting for green, many times the charger will only be on green for a moment before changing to amber and keeping the battery topped off.

Flashing can be a sign of a problem on some chargers.

A rapid flashing red can on some chargers indicate a bad or shorted battery, or a bad battery contact in the charger itself. If in question, try a different battery on the charger or compare with another charger to determine if the blinking is normal.

Best regards.

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.

The battery is gone for sure... You must get new one... take a look on amazon (my recommendation): Amazon com nailers paslode battery

Ridgid charger lights read as follows: Solid red; charger has power. Flashing red; charger is evaluating the battery, checking if it's cool enough to charge and if it is in chargeable condition. If the battery is hot, i.e. just came off the tool after extended use, it won't charge the battery until it cools down. Flashing red & green; the battery is defective and the charger will not charge it. Solid red & flashing green; battery is charging. Solid green; battery is full.

When the charger light is changing from red to green that is a indication that it is charging the battery. When the charge is complete the light will stay green until the battery is removed or the charger is unplugged.

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.

Have you activated phone yet? When phone is activated and in service it flashes green. It's hard to believe it would flash green without a SIM card, as there is no service...unless AT&T is providing emergency service only and you're getting a flashing green which means you're in range of their towers.