Convert them to lithium,
Motor technology has been the same since 20 years ago and it's still the same in all new drills "DC Brushed Motors"
Up on opening my "Old" drill and having a look at my motor my motor looked very new and the brushes ware far from worn if not still would go strong for many years to come.
On the other end the battery's ware totally on the end of its life.
Step 1: Opening Your Battery Pack
Order a smaller battery then the dimension as it will allow you to move the battery around.
Step 2: Soldering The Power Connector
Always do one lead at a time solder the positive wire then heat stink the connector and then the negative wire and then heat shrink it.
Also helps to add a deans connector to make charging very simple.
Step 3: Closing The Battery Pack
And if your like me then you want a battery pack with no loose wires hanging out then the next step is for you.
Step 4: Soldering DIN Connector
male for the charger and the female end for the battery as their are glued to the battery cover for a nice flush finish
I'm always throwing my tools around and this is the best method as it doesn't interfere with any thing or get pulled.
Step 5: Enjoy The Power Of Lithium
What old drill ? Lithium power baby
The power gains I received was very amazing and even tested it with a power meter my battery's are well within the power the motors requires
As along as you don't let your voltage drop low then your all fine
Step 6: Charging Your New Lithium Battery
I dint think in this day and age that I will still think people will charge lithium battery's on a NI-CD/ NI-MH
Lithium battery's are no longer new and I'm guessing if your on this site then I'm sure every one knows about the charging procedure.
But you still get a few saying its a bad idea because in your low IQ head you think I'm using the original charger think why would I go through the trouble of fitting a DIN connector as a balancing connector. any ways.;
You must use a lithium charger to charge your battery packs!
The set up I'm using is a very simple LI-PO charger made by Hobby King HKE4.
Which iv set the charge rate to about 1.8 amps and removed the dial.
ATM I'm running the charger off a 12v battery in my shed but an ac adaptor is the best option for taking it to site.