I like my Sears 16.8 volt drill kit with two batteries but my charger
has gone south. It works fine when the batteries are only slightly
discharged but if they get too low, it won't work. The cost is
I opened it up and there is a small circuit board with 6 or 7
components on it, and none are marked. One device gets hot and the
charging light goes out so I suspect this device has partially failed.
I can cool it and it charges the batteries longer but I'd like to
replace the circuit with something else. Any ideas?
On Thu, 07 Apr 2005 13:22:57 -0400, "William W. Plummer"
I've been down that path already. I did find a bad cell in one of
the two batteries and replaced it with a cell from an old 9.6 volt
battery. That fixed it but charger only works when the voltage is
above 15.5 volts (on either battery).
I took the bottom off the charger and have a fan blowing on the
electronics and it works great. It charged one battery from 14 volts
to 19 volts without a hiccup. The other one is charging now. I'm
looking for a heat sink as a temporary fix and for replacing the
circuit with a more sophisticated one for a permament fix. I love my
Battery Charger Circuits:
My bet is on the battery rather than the charger. Can you borrow a
battery, run it down and see if your charger will bring it back? Or, if
you really like the tool, buy a replacement battery. That way, even if
your first battery is good, you will end up with two useable batteries.
Epoxy is a perfectly fine heat conductor for an application like this
(the charger was supposedly designed to keep the chip cool enough
without any heatsink), and I do include ordinary epoxy lacking any
particles intended to improve its heat conduction. Furthermore the
chip package itself is epoxy and will impede heat flow much more than
any thin layer of epoxy between it and a heatsink. Even a small
heatsink, like this one:
will likely at least double or triple the heat flow between the chip
package and the surrounding air, whether it's attached with ordinary
epoxy or silicone rubber adhesive (RTV).
Tool chargers typically fail when an electrolytic capacitor dries out
or the switching transistor shorts (MOSFET or bipolar, may be built
into the charging controller chip), and since cooling the hot device
makes your charger work again, I suspect a capacitor. Adding a
heatsink will not fix this but may prevent it from happening in the
first place. Any replacement capacitor must be rated for at least as
much voltage as the original and for approximately the same
capacitance. It's also important that it be designed specifically for
high frequency switching operation (low ESR). www.digikey.com and
www.mouser.com carry suitable replacements. The charging controller
chip is often made by www.maxim-ic.com or www.linear.com (their web
sites have schematics that may help you diagnose your charger), but
many use a general-purpose microcontroller (CPU), which can't be
replaced (but it should always have a separate switching transistor
that can be).
Chargers are not typically transformer isolated from high voltage, so
take great care when operating yours with its cover removed. At the
very least, plug it into a GFCI-protected wall outlet.
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