The drill has stopped charging up. No obvious

Could be stuck brush (depends on the model) otherwise a loose wire in the switch could be the reason.

Could be that the carbon brushes have prematurely worn or you have faulty wiring. If it had a previous owner maybe they worked it to hard. You can usually get carbon brushes on Ebay

Your problem states you have a Makita tool and your question is about a Dewalt lithium battery. In either case if the battery won't take a charge or if the charger is showing the battery is defective the only fix is to replace the battery. Some causes for defective batteries or batteries that won't take a charge are bad cells in the battery, bad or shorted IC board in hte battery, broken or burnt connections between the many cells or between the cells and IC board or even bad or old cells that will no longer charge up. The time and effort needed to find out what the problem is then finding another battery to scavenge parts from (individual battery parts are not available) is never worth the cost of just buying another battery and going back to work with the tool(s).

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.

I think your battery is died. I suggest you can replace a new of this battery for your drill.

http://www.drillbattery.com.au/buybattery.php/makita_1822_power+tool+battery.html

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.

Depending on how old or more to the point how many times you have already charged the battery, it may be worn out. All rechargeable batteries do reach a point where they won't charge up anymore. If you have fully discharged the battery and tried to fully charge it up again and it won't take a charge or only lasts a few minutes on a charge it may be time to recycle it and buy a new one.

Here is the link to Skil. http://www.skiltools.com/en/CustomerService/ServiceCenters/
Call them and see what they recommend that you do. They will want to keep you as a valued customer. Regards, Joe

Does the drill do anything? Check in the cavity where the battery goes and make sure there is nothing there to prevent the battery contacts from connecting with switch. I take it the charger indicates the batterys are fully charged. If you cant see anything obvious I would take it back and get a replacement.
Dave