Why won't my battery charge?

These days, it’s due to tired original NiCd batteries. I have a DC970, and both of my batteries are done. However, my recommendation here is NOT to match what you have, or buy knockoffs; scrap the NiCd setup, even if you buy a new drill with 2 batteries in the box. They may come back if jumped, if you don't fry the pack permanently (it can help, but you're more likely to kill it after 11+ years of abuse, use and memory effect shock removal).

What I usually recommend for these older DeWalts is the DCA1820 adapter and the 20V batteries (even basic 1.5A will do, since it's capable of more capacity then NiCd will ever be, with zero memory effect). The rub is this dream setup is more than a new drill, so it's more cost effective to buy a drill on sale new, or a refurb 2-battery combo from a place like CPO Outlet (read: only buy Brushless, not brushed; brushed is old tech and the original NiCds are brushed with age) and buy the DCA1820 from a place like Home Depot as CPO does not sell it piecemeal. DeWalt also sells the parts needed as a kit, but it is not truly as cost effective UNLESS IT IS ON SALE.

Being as I own one of these NiCd drills too and I need batteries, I did my research on this and when I looked at the DCA2203C vs a 2-battery bundle CPO refurb and a Home Depot sourced DCA1820, it's about the same both ways. NOTE: The drill kit and adapter setup may be slightly more expensive in some cases, but it's within the $40-50 range most of the time -- if there is one with how often the kits get discounted.

My NiCd charger is in a box since I do not intend to bother with any new NiCd batteries -- the downsides are worse than upgrading to LiOn and buying new tools when there's an official fix.