Tuesday, August 12, 2014

Dewalt 18V LiPo Battery Conversion

Many moons ago I purchased a Dewalt 18V drill and trim saw combo. They served me well for a long time but the Ni-Cad batteries crapped out on me. Why not just buy a new battery? Well, a new battery is 89$. And if that wasn't bad enough, a new drill with two batteries and a charger is...$89! New drills with lithium batteries are just so much nicer. They last longer, charge faster, and are much lighter. Let's see if we can't update this old workhorse with an inexpensive lithium polymer battery.

dead DeWalt Ni-Cad battery

The Battery

I started by disassembling the battery and taking some measurements of the inside. I have seen some people put large LiPo packs in by having it stick through the battery case. It may work and give long run times, but it looks like crap. I want my LiPo to be concealed inside the original battery case. After measuring and searching online, I figured I could fit a five cell 2250 mAh 130C pack.  A 5S pack has a nominal voltage of 18.5 volts.  Fully charged it puts out 21 volts.  The drill should be able to handle the extra volts but you could always under charge the battery if you are concerned about it.

disassembled pack

Five torx screws hold the pack together. I removed them and pulled the halves apart. The end with the copper contacts must be pushed down from the top to get the battery bundle completely out. It's just a tight friction fit; no glue.

push down on the contacts with a blunt object

Once the battery bundle is out, desolder one of the battery contacts and carefully pry up the other contact that is spot welded to the battery.

save this piece.

Now we need to solder some silicone wire to the battery contacts. The contact that was soldered is positive. The spot welded contact is negative. Solder your red and black wires to the corresponding contacts. I placed an XT60 connector on the other end of the wires. That way, the LiPo battery can be easily removed.

new LiPo battery

I cut a small slot in the base of the pack so that I could access the balance lead.  Balance charging keeps your cells even and the battery healthy. During reassembly, I noticed the plastic screw holes were starting to crack so I reinforced them with some epoxy.

perfect fit


finished pack

The Charger

With the pack done, we need a way to charge it. The original Ni-Cad charger will not work with LiPo batteries. Doesn't matter anyway because my charger got fried when a pipe burst near it. Rather than jury rig some alligator clips from my LiPo charger to the battery pack, I decided to re-purpose the DeWalt charger. I pulled out the circuit board and cut off all of the components except the battery clips.

Charger circuit board. I don't think this works anymore.

bare circuit board

Next I cut the circuit traces going to the battery clips so that they are electrically isolated from the rest of the board. After that I cleaned the board.

Battery clips are left center. Note the cut traces.

Then I sanded the insulation away with some emery paper and soldered on some 14 ga silicone wire.

I sealed everything back up in the original case and soldered an XT60 connector to the end of the wires.

finished charge adapter

To charge, I just plug the base into my battery charger. Charging takes about 1 hour with a conservative 2.3 amp charge. The battery can handle an 18 amp charge but my charger maxes out at 6 amps. So at 6 amps I can charge the pack in about 23 minutes.

charging the pack

The new battery works great. It has plenty of power, doesn't bog down a bit even under heavy load. It runs at full power until the pack is dead instead of winding down as you use it like with Ni-Cad batteries. And as a bonus, the new pack is much lighter than the original. The old pack made using the drill feel like lifting weights. Extended drilling is now much easier, especially when drilling over your head. Check out the pack in action in the video below. It easily drills the long way through a 2x4 with a 1/2" bit. Looks like I've coaxed a few more years out of this old drill.