12v 1.5amp fully auto, alwyas worked, hook up to battery 12v 8.5amph, alwys red light flashed, went solid while charging, then off, or flashed, and off cause was already fully charged. now red light just flashes on and off.Other batteries (same) I change out about every two years and they are still good. This is a new one only 7 mo old, and is showing still good, batt has gone from 12.5 to 9.1v when charger is hooked up, and flashes red led. what does this flash mean?

When the light is solid red, the batteries are charging.
A flashing red LED light indicates that the charger detects abnormal batteries or the battery temperature is abnormal.
When the light is a solid green, the batteries have completed their charge.

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

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.

Let batteries cool down before charging, too much heat from the battery tricks the charger into thinking the battery is fully charged or is broken

According to the owners manual for these swimming pool lights, this is what each colored indicator means. Hope this helps.


Solid green
The battery pack is fully charged.
Slow flashing green
The battery pack is almost fully charged.
Slow flashing yellow
The battery pack is half charged.
Medium flashing yellow
The battery pack is less than half charged.
Fast flashing yellow
The battery pack has almost no charge.
Fast flashing red
A charging error has occurred and the light should be unplugged and then plugged back in. If the Fast Flashing Red persists after several attempts, the battery condition is questionable.



Also in the trouble shooting guide, it mentions this:


Problem: Red Indicator Light Is Flashing While Charging

Solution:
A red flashing indicator light when charging indicates a selftest
error condition. Unplug the product and plug it in again.
Try this several times; if the indicator light continues to flash
red, unplug the GlowBuoy and take it to an authorized service
depot.

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.

Ok the lightning bolt is your power indicator: solid green-charging, Flashing green-fully charged, flashing red-low battery, flashing red three times the once green-bad battery. the "trash can" looking one is your hard drive activity indicator it will flash while your hard drive is reading and writing.