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, I recommend not matching what you have or buying knockoffs: scraping the NiCd setup and replacing the problematic NiCd system is the better play. Sometimes, the old NiCd packs were recovered via a jump, but they never lasted because the packs are toast once it's THAT bad. You can usually wait for a sale on the midrange brushless 20V drill to come up with 2 batteries and end up with the batteries at a lower cost, albeit 1.5Ah or 2Ah, depending on the deal. The original NiCds were 1.2Ah at the base, so 1.5Ah is still an upgrade.

If you need them NOW, try to jump them. Sometimes, they can be saved for one or two final runs, but don't expect them to take that kind of abuse much longer before completely dying out. They don't take abuse like that well after 11+ years; that's in addition to the charger zapping them to drain them quickly, which was done to kill the NiCd memory effect problem. For all intents and purposes, due to the risk of permanent damage on frail batteries, it's better to treat the use of the jumped packs as their final run.

When they go bad and do their last job, I usually recommend that these older DeWalts get the DCA1820 adapter and the 20V batteries; 1.5Ah or 2Ah works as both are upgrades from the stock 1.2Ah DeWalt shipped back then in the non-XR configuration. AND they lose the memory effect problem. However, the rub is this is the dream setup (especially if you can buy a drill with TWO pack-in batteries), and it's more cost-effective then bare batteries alone*. If you want to reduce your guilt about buying a tool for batteries, look for a reconditioned drill from a shop like CPO Outlet; you will probably get batteries with some wear, but it's not as wasteful. The kits with dual batteries always go on sale, so ¯\_(ツ)_/¯. I'd take it if I didn't own CR tools and 5Ah batteries from battery sales without a second thought.

DO NOT BUY BRUSHED TOOLS; BUY BRUSHLESS ONLY. The brushless stuff lasts a lot longer and has far more power. Brushed is old news outside of previously owned tools. Your best bet to buy the DCA1820 is piecemeal from Home Depot or Amazon.

DeWalt also sells the parts needed as a kit, but it is not truly as cost-effective UNLESS IT IS ON SALE (DCA2203C). If the tool can't be converted (which is rare), consider it a dead tool and buy a 20V equivalent (yes, I have two of the hand saws because of the free tool deals, and one of them is an "XR" tool (brushless).

I have this info because I own a NiCd drill where I lost both batteries to wear and tear once and for all, and the only feasible way to put it in service I saw was to swap to LiOn this way, so I've done the research to make conversion economical.

The advantage of the DCA2203C kit is that the packs are safe for 100% of the tool lineup, even the infamous DC970, which is not able to use the premium batteries. However, these packs are weak compared to others. The MASSIVE DOWNSIDE is that it is never discounted. Because of that, the more economical conversion comes when you buy a mid-tier Brushless drill with two batteries in the package and then source out the DCA1820 separately.

For the green ones who don't want to buy extra items but need new batteries: This is (probably) a bit more wasteful upfront because you also purchased a tool and charger, especially if you have them on hand (as I do with the XR tools). However, the math still tells me buying a lower-end brushless drill kit on sale is the cheapest way to put my DC970 in service again.

That said, in SOME cases there may be a slight price differential in favor of the DCA2203C kit; but this is usually only +/- ~$40-50 if it's all you can find. Either way even with the "waste", my opinion is you might as well buy the drill with 2 batteries, especially when they get put on same. These NiCd tools are brushed, and brushed tools have a lifespan based on the wear level on the brushes. This is generally not the case given how often DeWalt subsidizes their tools so the hardware stores can use them as a "loss leader" to get you into the store for little things you probably needed anyway.

*I picked up some Powerstack batteries and these may be a nice candidate, but the downside is you need to own the charger otherwise this makes no sense as a single 1.7Ah Powerstack (safe for all tools) is $120. No thanks, too expensive :/... Buy the drill with the 1.5Ah or 2Ah packs and adapt it. If you own some of these, you own all you need short of the adapter if you need to buy the parts.

While I still have my NiCd charger in storage, it's a doormat until I can get a deal on a 2-battery drill bundle. DeWalt discontinued the NiCd batteries, likely due to cell availability issues (either way, GOOD RIDDANCE). But that's fine because the downsides of keeping the NiCd packs in service are worse than upgrading to Lion and buying new tools when there's an official fix.