16.8 Battery Charger Electronics - needed

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?
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On Thu, 07 Apr 2005 16:55:37 GMT, "David Martel"

All well and good, but click on the item I need and you will see that it is NOT available at this time.
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I think I will remove that component that is heat sensitive and epoxy it to a cpu heat sink and hook it back up with wire. That should help.
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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 drill.
Battery Charger Circuits: http://www.discovercircuits.com/B/batt-chrg.htm
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PJx wrote:

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.
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Thanks, Good idea. I think I can wedge the component (which looks like a resistor) in between the fins of a cpu fan heat sink.
JimL/PJX
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Is that a peak charger, you may be defeating the peak function on 2 bad packs. It could be the batteries.
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On Thu, 7 Apr 2005 13:53:17 -0500, snipped-for-privacy@webtv.net (m Ransley) wrote:

It's a sears. $69 for the two batteries and drill.
I love it.
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Look here: .. http://www.bobvila.com/ProductServices/SmartBuys/SmartDirectory/Tools-PowerToolAccessories-CordlessToolsBatteriesChargers_and_Flashlights-2.html
Dave M.
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Isn't Sears cordless stuff usually rebranded Ryobi? Easiest solution might be to pick up a used charger on fleabay.
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Epoxy's a lousy heat conductor. Typically, you want some thermal paste with some mechanical clamping. Without a picture of the board and components, I'm just whistling in the dark beyond that...
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Andy Hill wrote:

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:
www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid'6-1368
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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