I have a 18v battery charger for my great neck drill but the red light insant lighting up when I go to charge my my battery, but the green one is lit up.

Seems batteries have completed their normal life and require replacement. Keep in mind that when batteries are exhausted, their voltage remain normal but loose current. It seems that batteries do not sustain current i.e. have become un usable.

The Ryobi charger will flash green as it charges and then remain steady or slowly flashing green when fully charged. Flashing red means testing and if it remains steady or slowly flashing red it is defective.

Either the charging adapter is faulty or the battery is worn out.

If the recharageable battery is more than 3 - 4 years old then the battery could be worn out.Rechargeable batteries have a finite number of charge and discharge cycles andwill lose their charge capacity over time, i.e. won't charge to 100% andgradually the charge reduces until the battery won't charge up at all. OR Thebattery shows a 100% charge but when the adapter is disconnected the batterydrops off to zero capacity in a very short time. If the battery drops to anunacceptable charge level then the battery needs to be replaced.

Battery is shot. Replacements are $75 - $100. If its new you have a 1 yr warranty.

Batteries are only good for about a year, if they are older than that, take them back to the store and see if they are guaranteed at all, (all Ryobi tools and batteries have a date code on them, and they are all guaranteed for 2 years, a little plug for Ryobi,) that is all I buy now days, they work great. So, to check your charger, you will need a volt meter set on 12 or 20 v dc, and see if the charger is putting out, if not, then, the cost of a new ryobi might be the same as the fix for the black and decker. That is if you use the thing, we use our cordless tools every day and charge them every night, hope this helps.

Chargers will out last batteries by a long time.You will need to get a new battery. Black & Decker is a cheap household brand and does not hold up for long periods of time. I have better brands and buy several new batteries every year.

This is not right. They should last much longer then that. I have one of the more expensive 18V lithium batteries (the one with the button on it that tells you if it needs charging or not) and I can use mine almost all day before it needs to be recharged. I do construction work and I use my Ryobi products almost all day. Either both batteries are bad (I don't believe that is the case) or the charger is not charging them all the way to a full charge. With this charger, you do need to make sure that you remove the batteries as soon as the green light comes on. It is not good on these batteries to stay charging after they are fully charged. You also should not leave the charger plugged in without any batteries charging.Please tell me more about the charger. When you plug in the charger, does the red light come on? When you plug in a battery, does the red light stay bright and the green light blink? Does the red light go off and the green light stay on bright when you remove it from the charger?
If the charger is not working correctly, then return it for a new one.

This isn't true because I have a second charger and it works fine

The light will turn green once it is fully charged.