Hitachi charger UC18YGSL.

Your charger has failed.

First, tell what charger is it?

EXAMPL
1- RED shows charging and GREEN means complete charging.....on most vacuum cleaners.

2- Blinking RED or GREEN means charging while solid RED or GREEN means charging complete on thigs like standalone battery chargers, like screw/drill driver.

How long have you left battery on charger? It will take about 3 1/2 hrs to fully charge. t is probably not a good idea to leave the battery in the charger for long periods of time though, so once it is fully charged put it back in the drill. When fully charged the green led will turn solid. This link will have more info on charger and battery.
http://powertool.manualsonline.com/manuals/mfg/ryobi/130155001.html?p=2 C

Throw away

please see the commentary. God bless you

• 
  The majority of laptops will display a solid green, blue or purple battery light to indicate that the battery is fully charged. Most manufacturers indicate that a laptop is in the process of charging by displaying a solid or blinking green, orange, blue or purple light. If your battery indicator light is green, blue or purple, the battery is probably functional, regardless of whether the light is blinking. If the light is solid orange, the laptop is probably in the process of charging.
  
  

Battery Error Coneventions

• 
  Many battery manufacturers use rapidly blinking lights to indicate that a battery's charge level is low. In general, a laptop will rapidly blink orange or red when its charge drops below a certain level until the computer is plugged into a power outlet and charged. If your laptop's battery indicator continues to blink rapidly in orange or red or if it remains red even when it is plugged into the charger, it indicates a problem with your battery or the charger.
  
  

Read more: http://www.ehow.com/info_12183593_blinking-battery-light-laptop-mean-charger-not-charging.html#ixzz2fu5qhEWg

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.

These batteries have big problems that Hitachi doesn't address very well. Unlike NiCad batteries, Lith-Ion ones must be periodically used and recharged to retain their battery functions, i.e. if they have been sitting on the shelf at a store for an extended period of time, chances are they will be dead when bought and will not accept a charge. If they go dead i.e. no little red light on the charger when you plug the battery unit in...it's basically junk.

If you can return them or exercise the warranty do so. If / when you get new ones DO NOT completely discharge them like you would NiCads. Lith-Ion batteries do not have the "battery memory" issues that NiCads have which means you don't lose running time if you don't fully discharge the battery unit prior to recharge. Completely discharging before recharge will ultimately kill them. Use the 1815X till almost dead and then charge it. It is also recommended Lith-Ion batteries not be stored fully charged i.e. charge the 1815X fully, drive a couple bolts or screws, remove the battery unit and then put it away. Do not leave battery unit plugged into the tool when you put the tool away. This will discharge and kill the battery unit.

There are non-Hitachi substitute batteries now on the market. I'm guessing the same precautions apply but I don't own one of the replacements so I don't truly know.

The actual batteries in the Hitachi BSL 1815X are Sanyo UR18650SAX lithium ion batteries. Lithium Ion cells aren't all that easy to get commercially because of handling & safety requirements. Not certain if you can take apart the 1815X and swap out the bad Sanyo cells.

As you know, solid red on your charger means that the battery is fully charged and the BLINKING red light means the battery is charging. Unless the light is blinking irradically everything is working the way it is supposed to. If the light is blinking irradically, the battery may just be too hot or too cold to charge ( like when you are driving many lags and your battery dies) and in this case simply wait 15 minutes to charge. If you get a fast blinking light still, the problem probably is with the battery.

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.