Testing Drill Batteries
Using a multimeter / Volt meter to determine if a rechargeable battery still has enough voltage to hold a charge.
You will have to first charge your battery for at least an hour. Then set a multimeter to the voltage range of your battery. For instance if your battery is 18 volts then use the 20 volt scale on your meter. Your leads should be plugged into the Red and Black or Volts and Common ports and touch the two battery terminals with the probes. Fully charged, an 18 volt battery should test 1 to 2 volts higher than the voltage listed on the battery. In your case, the battery should show at least 19.0 ~ 20 volts and maybe even more. If it’s below 15 volts, its marginal. If it's below 14 volts, the battery needs to be replaced.
If your battery shows a high voltage but still doesn't run your Drill, then you probably have a problem with the Drill. The Drill motor can wear out and may need new brushes, but on most drills these are soldered in and can't be replaced easily. You can burn the motor by using a dull drill bit or non stop drilling for long periods of time.
The main reason that Drill batteries go bad is they’re run down too low before recharging. This can cause a polarity reversal in some cells, which is why they fail to recharge to their operating voltage when recharged. A power tool battery should be charged as soon as it begins to slow down. Let it cool, then put it in the charger at room temperature. You must also maintain the battery by recharging it every 30 to 90 days. It takes that long before it will self discharge down to a voltage that it should be charged. If you let it sit for several months, the battery Will drain completely and you will have to replace it.
Some battery distributors can replace the cells in the battery and with the rising cost of these battery packs, i would recommend going this route. You can specify the amp hour of the cells to use end up with a better battery pack than you started with.

