My charger for my cordless drill recently stopped working. My drill is
dead as a doornail, and I've got projects pending. It's an off-brand
(Master Mechanic), and I've had some difficulty locating the
manufacturer. Efforts to purchase a new charger have gone nowhere.
So...I was able to determine that the fault lies with the AC/DC
converter ("wall-wart"). It's rated at 200mA and 14.5VDC, but at the
moment it's putting out essentially 0VDC (verified with my
multitester). I went to Radio Shack and purchased a 15VDC 1A wall wart,
with the intent of cutting and splicing it to the charger, but when I
tested it's output myself, it's actually kicking out 20.33VDC. This
What happens if I solder a 20.33VDC output to a charger that's used to
operating with 14.5VDC? Will it damage the battery? Cause an explosion?
Other deleterious effects?
Radio Shack also had a (rated) 12VDC wall-wart. If it runs hot like the
15VDC version does, I might actually get pretty close to 14.5VDC. Would
that be a better idea? Do the outputs from these low-grade AC/DC
converters always vary that much from their rated output? And does
anyone sell converters that will match the OEM one fairly closely?
Good point. The 20.33VDC figure was with no load. With a load (charger,
with battery pack inserted) the voltage comes down to 17.8VDC. So for a
12V battery...I'm 5, almost 6 volts high. Maybe the 12VDC converter is
a better idea.
A 12v pack of nicads is actualy discharged at 12v, 13.2 v 13.5 or so
would be 100% charge. A 12v input would never charge the pack. Using a
higher V is ok the real issue with any pack is not over charging it. Its
a Master Mechancrap, who ever that is, it likely does not have a peak
detection charger , standard with quality equipment that stops charging
when battery voltage begins to drop and chemical conversion of electrons
goes to heating and ruining a pack. 2 ways to test a fully charged nicad
pack is 1 when pack warms, but being in a charger producing its own heat
this is impossible, pack must be charged by wire leads. 2 is voltage
drop. packs increase in voltage till 100% charge is met, then actualy go
down in V as heat is now the conversion of electrons. Go by time or hook
up a V meter and monitor. I still have Makita packs from 93 that work
but have ruined 1 yr old packs on non peak chargers Tool manufactrers
dont tell you this as they make their big money on selling you a new
pack. Time is another fairly accurate way to keep from ruining a pack,
usualy 1 hr , but all chargers are different, never leave it in a non
peak charger, it just cooks the cells slowly.
Good to know. I was thinking of exchanging the 15VDC converter for the
12VDC one this evening. I thought the 12VDC might under-charge the
battery, and was wondering which is the greater of two evils.
I'll keep the 15VDC converter and use it. Then, in a year, when I cook
my battery accidently, I'll go buy a DeWalt cordless drill.
Thanks to everyone who responded, this thread has been a great help.
(Trots off to splice wires.)
Before it was broken you could have measureed the output of the
original converter, with nothing attached, and it too would have been
a lot higher than the rating. (If you really need convincing, you
could buy another one with a drill and later return it, or even just
plug one in at the store, and measure it there.)
The output, the secondary winding of the transformer in the converter
itself has its own resistance -- I guess it would be called internal
resistance -- and when current flows through that resistance, there is
a voltage drop, even when measured before it reaches the wire that
goes to the charger.
If all it is is that the cord is broken inside, it's also possible
that you can repair the old one. Take a hat-pin or two** and shove
it into the wire until it hits the copper. Then check with your meter
**Or even a small pin or needle. Hat pins are better because they
have "handles". There are occasional short pins that come with "big"
knobs on the end. They come with corsages and various other things.
When you get these save them for this purpose. Mine are in my sewing
Remove NOPSAM to email me. Please let
me know if you have posted also.
The wall warts are not usually regulated. They will drop under load. Look
for one that has a current rating (mA) of the battery you are charging. The
voltage can be rated a volt or two more. The higher current rated chargers
will not drop the voltage to as low a level as the lower current rated ones.
This will tend to charge the battery faster. For short times this is not
too bad, just don't leave the charger on the battery all the time.
Master Mechanic is TRUE VALUE HARDWARE.
Generally those chargers fail because of a shorted battery pack, but
can fail due to surges in the power line or other things.
Be careful, I had one of those batteries explode once. Not in the
charger, but in the drill. This is not just a story, the thing just
went bang, a chunk of plastic was embedded in my arm, and pieces of
the battery were all over my living room. I was only drilling a 1/8"
hole in a piece of angle iron (leg on my wood burner). I recently
read on the web about someone who had their cellphone start on fire
from the battery. Nicads are dangerous. I have not used a cordless
device since that happened. Skil sent me a new drill after I
threatened them with a lawsuit, but I sold it. I'll stick to tools
with cords. Thats what extension cords are for. Not to mention that
they dont suddenly run out of power mid job
The one amp is power available -- doesn't guarantee that any particular
device will use the entire amp. Like a 15 amp circuit in the house doesn't
guarantee that your plug in night light will take all 15 amps.
I'm not sure about charging nicads, but it's possible they will fast charge
if the power is available. Which it would be, with a 1 amp plug. I'd suggest
rather brief charging times, instead of overnight.
Second though. American Science and Surplus www.sciplus.com if I remember
the URL, has wall warts in assorted size and type.
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