UPDATED! : I’ve added a photo to the battery terminals, see below.
Recently my trusty Black & Decker Quatro drill/driver/saw/sander started misbehaving. It’s been great for years and gotten us through a lot of DIY and IKEA modification. But now it could barely manage to cut a 15cm length of 1cm chipboard.
Time for a bit of investigation.
I wondered if this was due to the charger not actually charging, it seemed ok, the LEDs lit up but on closer inspection there was a suspiciously low voltage at the charging pins and a distinct smell of “magic-smoke“. So after a quick trip to the Ferreteria to buy some star drivers (why do they use these annoying types of screws) I could see that there was a very hot and rather charred looking transistor. Looks like the charger was dead. I thought about replacing the transistor, but unfortunately the part wasn’t marked and after a lot of Googling I couldn’t find any schematics. It’s quite an old model and not available any more and plus there’s no knowing what else might have died, so I bit the bullet and bought a new fast charger.
Turns out that the battery pack had died too, it probably died first and took the charger with it in some sort of suicide pact. A quick search of some online DIY forums shows that this is a common “feature”… The new charger has a diagnostic “blinky light” that alerts you to a dead pack.
The pack is a 14.4v NiCad (12 cells of 1.2v) which should have a terminal voltage of around 14.4v (not surprisingly). Mine had dropped to just a little over 6v. Bad. at nearly 60 Euros for a new pack… Very bad. Especially as I had just shelled out on a new charger.
I remembered from my childhood RC car days that NiCads have this problem called crystal dendrite growth. What happens is that inside the battery crystals form, the crystal conduct electricity so when enough of them have formed they basically short out the battery. What we used to do in those days was whack the battery across a car battery for a second or two and that’d sort it out. It burns away the crystals and brings the battery back to life. Also gets quite hot too! I remember that it works quite well, not quite as good as a brand new battery but better than you might think.
That’s easy when the battery pack is 7.2 volts and you have a 12v car battery with effectively infinite current to zap with. Not so easy when you’re living in an apartment with no car and the pack voltage is 14.4volts.
I tried with an 18v 2A spare laptop PSU but it made no difference so it was time for something a bit more adventurous.
Some people have had a lot of luck using disposable camera flash electronics to zap their NiCads. Here’s an instructable. Others have used welders…
Just so happens I had a disposable camera that I had saved for a project just like this. It’s a Kodak, not sure what model it is as they don’t print much on the outside. They’re easy to open, especially as I’m not bothered with re-using it, at least not as a camera.
After ripping it apart the I had the circuit out and made a note of the polarity of the AAA battery (positive end is the spring on the circuit board). After a quick test to make sure it still worked, using a wooden chopstick to press the buttons, I de-soldered the flash bulb and soldered in a couple of extension wires across the main capacitor terminals.
Now I was ready to go.
Note: This could be quite dangerous. The capacitor is charged up to around 300V (measured) and when triggered the circuit boosts the voltage again to around 1000V (not that I measured it). So be careful if you intend to try this. Do not short this across your body, it will hurt and might be life threatening.
Charge up the circuit, listen for the whine, then when the light comes on I disconnect the battery and short the extension wires across the terminals of the B&D NiCad pack, positive to positive, negative to negative.
It makes a loud crack and a small spark!
The voltage at the main capacitor is around 300V. There’s no need to actually trigger the flash as such, and I think that it’s probably best not to as that would mean the output voltage would be much higher as it dumps the capacitor voltage into a coil to raise the voltage to around 1000V.
After two tries my battery voltage had risen to nearly 7 volts. So I plonked the battery on the new charger for 10 minutes and the pack voltage had risen to 12 volts.
One more zap and another ten minutes on the charger and the pack voltage had risen to 15V Hurray! After an hour on the charger the pack was fully charged and back to normal. The light even went green when the pack was fully charged!
I’ve been using the drill quite a bit since, putting up curtain rails, and have recharged that pack a couple of times. It definitely seems to be back to normal. The capacity seems to be up to the same level as before the problem.
Not bad for free and certainly better than 60 Euros for a new pack!