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Is it a current limiter or what??? Answered

when rebuilding a battery pack for a Sears cordless drill,I encountered a tiny diode or fuse or thermistor of sorts,in the wire coming from the negative pole.It appears this is only active during charging. At any rate,what is it and where do you find a new one??



8 years ago

Your device is likely to be a diode,used as a protection device should a short circuit occur within the charging circuit. Here's a clue, diodes have a polarity band around one end ONLY (cathode), if this is so the unmarked end will be connected to the negative battery terminal. Take a close look for any identification markings (letters & numbers) Most of the ones I've come across are standard silicon types in which case a type 1N5401 would be suitable... you must be sure that you can positively identify it as a diode of course!

backwards.... 1. getting a new one means figuring out what it is and what it's characteristics are. Do you have a "good pack" we can use for testing? 2. could be a rectifying diode to prevent neg transition voltages form reaching the battery on catastrophic failure of the charging circuit. diode test 3. Could be thermal fuse as indicated below....inlines often are, 4. Could be a standard fuse to avoid overcurrent failures (usually presented by internal short in the battery cells or over voltage in the charging circuit) 5. Could be a thermistor or better, polyresettable fuse. if so, it should not have failed. So, if it has failed, it's most likely either a standard fuse or thermal fuse, but it could be a diode still. second reason for the "good pack"...Is the charger verified ok?

Digikey, mouser, and a host of other distributors have loads of these parts in a huge variety of working values, so finding a replacement is primarily a task of figuring out the chars of this device, or of its load and charging parameters. Lead acid cells? if so, this will make life easier. You have a DMM? You have product docs (battery and charger)? Alt You have access to docs online?

Yep, if it's strapped to the barreries it's probably a thermal cutout to shut off the charge current if things get too hot. It'll be hard to find a replacement other than from the original manufacturer.

It's most likely a temperature sensor for automatic shutoff on overheat. As to where you can find one, not so sure.