I bought a deWalt cordless drill dc727 about 8 years ago. Only used 10 times or so and not at all for past 5 years. Batteries recharged bu drill not working. What can I do to repair, please?

I will interpret this as "can I get replacement batteries for my 12V cordless shop source drill?" probably not. Most manufacturers phased out 12 volt tools years ago, replacing them with 18 or 20 volt tools , though a few people still sell dewalt 12 volt batteries.

What is the relationship of the drill, radio, and battery? Is it a rechargeable battery for a cordless drill, or what? You are leaving out a lot a details here. Perhaps the voltage and polarity of the battery is correct for the drill but not the radio.

Here is the manual for the charger - note on page 2 the meaning of the problem you describe - time to replace batteries (depends how old they are)
http://download.sears.com/own/00925737e.pdf

That being said, check the videos out, perhaps can get it to work after all for a while longer.

The charger for the dewalt 18V battery.
http://www.ezbuybatteries.com/dewalt-72v-18v-battery-charger-universal-replacement-13821.html

rechargeable batteries can build a memory, if you use it say for 5 minutes and then put it back on the charger - then the next time you use it for 5 screws and put it back on the charge - see where I am going
Over time the rechargeable batteries "learn" they only have to work for 5 minutes ( dang my son was that way)
You need to fully drain them and recharge them fully several times might take 10 times.
We have a program in our shop that when you are done with a battery it goes in the flashlight until dead, dead, dead then to the charger. I have batteries that are 10 years old and still work almost like new.
You may have to replace yours but it is worth a try

Not necessay to run the batteries dead at this point. Chrge them fully as they are. What occurs as time and use go by, is that the batteries retain a memory of fixed levels and begin resisting the movement of charge past a certain point. To break this memory, you need to have the batteries discharge fully, then recharge fully. But only need to do this once every 20to 30 charges. Or maybe every 10-15 charges when the batteries are over 3years. Enjoy, Macgivor

Your battery could say its good by just testing voltage,but when a load is put on it , its a different story. My dewalt batt. did the same thing I opened up the batt. case and there was 12 "C" sized batteries inside linked together with little metal strips. I bought 12 rechargeable "C" batts. and carefully drilled where the metal strips are soldered, changed out batt. and re solder. It's been over a year now and still drilling . Also you should do this one batt. at a time so you don't put one in the wrong order .

Newer rechargable batteries aren't like the older ones back from 5-10 years ago. They do not have a "memory" like old cell phone batteries and cordless tools used to have. So charge it whenever.

Stick it in the charger and when it's charged, the light will stop flashing. Usually takes a few hours for a full charge.

It's not recommended to leave it in the charger for extended periods. Not only does it siphon off your power bill, but it's harder for the battery I believe.

I've had mine for a few years and they done excellent. Just as powerful as the day i bought them.

If they ever do give out, be sure to recycle them, don't throw them away. Lowes and Home Depot have rechargeable battery recycling at the front desks.

Are batteries for the dewalt cordless drill interchangeable? Will an 18 volt battery fit in a 12 volt drill?

it's the depends on the type of battery it is some you should never let run all the way down and some you should.