Charging a cordless drill - Page 1
Forum section: electronics / projects

author: rdl
author_rank: Super Contributor
timestamp: April 04, 2017, 07:35:36 pm
content: The AC adapter/charger for my cordless drill stopped working. The drill is 7.2 volts (NiCd I guess). The AC adapter was 9 vac at 200 or maybe 300 mA. Could I charge it with my bench power supply?

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2017, 08:09:32 pm
content: Sure, provided it has current limiting. Some cordless tool chargers are considerably more advanced than others. Yours sounds like it may be the sort that has nothing more than a diode in the charger base and relies on a "leaky" transformer in the wall wart to limit the charging current.

author: Ian.M
author_rank: Super Contributor
timestamp: April 04, 2017, 08:17:12 pm
content: The usual reason those cheap & nasty chargers die is because the battery has failed. If a cell has shorted out, the charger is overloaded and cooks itself to death.

author: rdl
author_rank: Super Contributor
timestamp: April 04, 2017, 08:25:34 pm
content: It's an Agilent E3610A power supply. The drill is a not terribly old B&D 9909KC cheapie from Walmart with probably less than 20 minutes run time on it so I didn't want to toss it, but I didn't want to spend as much as it originally cost for a replacement charger either.I had the right size DC barrel plug (2.1mm) with banana plugs to the other end, so I set the voltage to 7.2 and current limit to 200 mA and plugged it up. The power supply went into current limit mode at around 4.5 volts, but the drill spins pretty good and after 15 minutes there's no smoke and nothing is even getting warm. I guess I'll leave that way for a while and see how it goes.edit:Yeah, it could still be a shorted cell or something. I vaguely remember maybe burning out another adapter trying to charge it last year. I started to take it apart to get a look inside, but what looked like a separate battery pack, isn't. The whole thing splits in half and I figured no telling what kinds of springs and clips and other tiny parts might come flying out if I kept going, so I stopped before it was beyond repair.

author: IanB
author_rank: Super Contributor
timestamp: April 04, 2017, 08:47:27 pm
content: A 7.2 V NiCd pack is going to be 6 cells in series, which is a fairly fragile configuration. Basically, if you run the drill down until it starts showing signs of weakness then one of the cells is potentially going to have its polarity reversed, which will damage it. A golden rule with NiCd and NiMH packs is never run them to empty, always recharge early.If you do want to recharge a NiCd pack from a bench power supply, then low rate current limited charging will work fine. The peak voltage will go up to about 1.4-1.5 V per cell, so set the voltage to at least 6 x 1.5 = 9 V to get a full charge. If the pack doesn't reach about 9 V when fully charged then you will know it is damaged.If your drill is one that can also take a lithium battery pack, then buying one of those (plus charger) could be a good plan. The lithium batteries generally work way better than NiCd batteries. The NiCd battery that came with my Ryobi drill was useless. Never held a charge and wore out quickly.

author: grifftech
author_rank: Frequent Contributor
timestamp: April 05, 2017, 02:55:32 pm
content: I used to think cordless drills were junk, but when I got a 12 volt lithium ion one that drove screws better than a heavy corded drill, I changed my mind

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2017, 03:30:27 pm
content: There's a big difference between the cheap ones that are little more than a cordless screwdriver in drill form and the better cordless drills. I've had a 14V Bosch drill for years that I use for everything, it's more powerful than my corded drill. A couple years ago I had the battery packs rebuilt and it's still going strong. The drill is made in Switzerland which is unusual, most are made in China.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 05, 2017, 03:59:41 pm
content: When using a good lab supply for charging it is a good idea to have something like a fuse or at least a diode in series. The supply might have an crow bar circuit, that might engage during power down if the battery is still connected.Even with 1 cell shorted, it might be still useful for occasional use in a drill. I have used a 9.6 V pack with 1 dead cell for quite a while.Charging is more like setting the current limit to 200 mA and voltage to something like 10 V. The voltage setting on the supply is not really suitable to detect end of charging. If you are in a hurry and the battery is rather empty one might start with something like 2 A and a low voltage limit (e.g. 8 V) to do a fast charge for the initial part. One should still watch it and stop after 1/2 hour.

author: rdl
author_rank: Super Contributor
timestamp: April 05, 2017, 06:59:14 pm
content: With 200 mA current limit and 7.2 volts, it just sat there at under 5 volts for hours with no change. What I did was increase the current to 500 mA and lower the voltage limit to 6 volts.At that setting, the current draw would decrease slowly to almost nothing while the voltage would finally climb to the limit I had it set at. From then on I could increase the voltage setting slightly and the current would also increase a bit. By repeating this until I hit 9 volts I was able to get the thing fully charged.It now spins faster with more power than it ever did with the dinky factory "charger".

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2017, 07:55:03 pm
content: Sounds like you cleared out the short, it will probably come back but you may get reasonable life out of it.A lot of times you can buy inexpensive aftermarket packs, I got a 12V pack for an older DeWalt drill gave my mom, it was under $20 shipped and so far has been working fine.

author: Ian.M
author_rank: Super Contributor
timestamp: April 05, 2017, 08:27:14 pm
content: You need to try to get the cells in the pack re-balanced.NiCds and NiMH batteries can withstand trickle charging at rates below C/20, but rates below C/50 may not keep up with the self-discharge.48H constant current charging at a C/30 rate should do it, if the cells are salvageable, and the pack is initially charged.N.B NiMH batteries should NOT be permanently trickle charged.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 05, 2017, 09:04:50 pm
content: Quote from: grifftech on April 05, 2017, 02:55:32 pmI just had my eyes opened the same way. Replaced the mains powered Bosch that I bought as a student, with a top of the line 18V DeWalt cordless drill.Good grief, that thing's powerful.

author: metrologist
author_rank: Super Contributor
timestamp: April 05, 2017, 09:10:25 pm
content: not my horrible freights... the drills are weak and the 400ma charger is wired straight to the batteries, no fancy diode!