Introduction: Cordless Drill - Improving the Battery

I hate it when my cordless drill gives up in the middle of a job. It seems that the supplied battery packs just don't cut it. So I opened up the battery pack and replaced the NiCads with much longer lasting NiMH batteries. Now I get a much longer use between charges.

Step 1: Disassemble the Battery Pack

My battery pack had 4 Torx screws (I later replaced with regular Phillips screws for easier access). Inside you'll find 10 NiCad batteries. They appear to be just a bit smaller than 'C' cells, and all attached together in series. 1.2 volts X 10 cells = 12 volts. The cells are marked as 1300 mAH which is why they don't last very long. You will probably want to photograph the arrangement of batteries since you will need to make a new pack look just like this. Also notice the thin insulator that keeps the top battery from shorting against the batteries on the bottom.

Step 2: Assemble the New Pack of Batteries

Aquire a new set of identically sized batteries. In my case the size is called 'Sub-C'. I got them at for $24.00. They come with tabs attached. Carefully assemble the new cells into the same arrangement as the original, and solder the tabs appropriately. Be careful not to short out a cell while assembling it - even briefly! You will also need to scratch the surface of the leads before soldering and put a thin coat of solder on each tab before assembly (called tinning). Re-assemble and test. Be sure to re-use the fusable link that came with the original battery pack

Step 3: Notes

Capacity:The new batteries are rated at 3000 mAH, so expect more than twice the run time than the original pack. You may wonder -- will the higher capacity hurt the tool? No: the important thing here is that the voltage is the same, but you may end up working the tool harder with the increased capacity, so don't over-do it. I've been using it for about a year and a half, and it's been excellent.

Charging: Since my old charger is expecting to charge 1300MAH NiCads, it will require a much longer charging time. The spec sheet that comes with your new batteries should help you here. Also checkout for more important detailed info on charging different types of batteries. A better charging method may increase your battery life.

Why do the batter packs die? I've been analyzing the old cells, and I've noticed that several of the cells are bad, but many of them are ok - it only takes one dud to render a battery pack useless.
Since the drill came with two battery packs, I can combine the good ones to make a second working pack. The old cells can be tested as shown below - the important part here is the load. I've used a flashlight bulb as a load which also server as a visual indicator of the battery's charge. Testing without the load will give useless results. I've seen some cells read 1.2volts without a load -- which seems good, but drop to 0.3 volts when the load is added -- clearly a dud.

Economics: A good mod should be economical too! Sears wants $40.00 for a new battery pack - if you can find it (I paid about $75 for the tool with two batteries - so why is a replacement so expensive?) So with this mod, I get a better battery for $24.00

Safety notes: Like many of the projects here on instructables, there are potential safety problems if you don't know what you are doing -- so if you don't know your series circuits from your parallel circuits STOP! don't attempt to perform this mod - it can be potentially dangerous if the cells are mis-wired or short circuited. Also beware any leaking chemicals from bad cells. Also make sure your soldering skills are sufficient before working on this project. A reader from Makezine's blog noted that one of the solder tabs is thinner than the others - this acts as a fuse - make sure you include the fuse in your completed pack.

Environmental issues: dispose used NiCad cells properly. Your new HiMh batteries are more environmentally friendly though some still recommend re-cycling.

Step 4: Soldering to a Battery

Update May 2011:
It's been five years since I posted this instructable - and I have another battery pack to work on. This one is a 19.2 volt pack, but still uses the Sub-C Ni-Cd batteries that my old drill used. Once again, I found that there were a few cells that were bad - making the entire battery pack useless. This time I'm just going to replace the bad cells with identical cells.

By the way... be careful not to short circuit any of the cells while soldering.