Bought 4 days ago,but the battery do'st stay chARGED very long.why?

I DON'T KNOW. COULD BE UNDER WARRANTY FOR FREE REPAIR OR REPLACEMENT. I HAVE RYOBI AND UPDATED ITEMS. THEY HAVE 3 CHARGERS 2 SMALLER RATED CHARGERS WOULD NOT GIVE ME
'FULL CHARGED' LIGHT. OLD BATTERIES WOULD NOT CHARGE ON OLDER CHARGER. FROM INTERNET ADVICE, I REPEATEDLY PLUG AND UNPLUG CHARGER BY HAVING PLUG LOOSE WHEN DOING THIS. SOMETIMES IT TAKES 4 TIMES TO CHANGE FROM DEFECTIVE TO CHARGE. SOMETIMES IT TAKES 20 TIMES TO SEE THE CHARGE LIGHT.

Since the charger says that the battery is not faulty, but it does not charge the battery, then the charger is faulty. The same happened to my client. The dewalt chargers do have 1 year warranty.

sounds like the charger is bad

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.

May read instructions and see if charger is compatible of charging the two types of batteries you are charging ,,,,I have 32 dewalt 18v dc9096 batteries and 1d dewalt 18v dc9099 battery

first have bought an original battery replacement? if no then buy original, 2) do not recharge the battery before it drains complately,3) do not over charge the battery like leaving it plugged to electricity all night ,the life of battery goes down, recharging should be done for a max1.30hrs at a time

your alternater could be bad and not charging the battery, get it cranked then remove a battery cable. If it stays running, alternater is good if it dies alternater is bad

The warranty is 90 days. The expected lifespan is always longer than what you get.

if you use it every day, it would be normal for the battery to be at the end of its life. ni cads are usually good for 300 charges/discharges