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I opened a bad old cordless NiCad drill battery last week. The battery has three terminals. The tow terminals on the sides are labeled + and - and as expected are connected onto the ends of the series cells of the batteries, but there is another smaller connection which doesn't state anything. Its not used by the drill but by the charger and when opening the battery, a small,thin, rectangular object reading  ""Klixon 7-S 4MM45B2-21V Z3J" was on it. I haven't been able to find any datasheets for it.

Could someone tell me what that rectangular thing is and what it is used for? I found a similar one i another NiCad drill battery.

Thanx in advance... Have a great day
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A lot of the more sophisticated Ni-cad chargers could charge ni-cads at higher current levels thus performing 'quick charge' Vs the older C/20 trickle charge method. However for this to be safe the charger really needed to be able to monitor the temperature of the Ni-Cad pack being charged, and that is most likely what that third wire/connection is a raw temp sensors that the charger reads to determine if pack is getting too hot to continue fast charging.

At least that is my guess of what you have.

Lefty
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Another possible reason a battery pack might have extra wires is for balancing charge between cells in the pack - you'd expect
a total of N+1 wires for an N-cell pack in that case.  This is common for LiPo packs for instance.

More advanced cases are lithium ion packs for laptops and similar - these often have a battery-management system built in and
a (proprietry?) serial data link to interface to it.

The temperature sensor idea is a likely candidate, in which case the extra wire should read as being at the same voltage
as one of the main terminals, and a temperature-dependent resistance between it and that terminal.  Probably.

[caution:  never try and measure resistance with a multimeter without first checking the absence of a voltage across
the part being tested]
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Thanx for the help people i appreciate it a lot. I just discovered what that tiny thing is. I forgot to mention it only has two leads.

It is basically a battery thermal switch a.k.a. thermostat. When the battery gets too warm, the contacts inside separate and the charging process stops since the circuit is open. Once the temperature climbs back down to safe levels, the contacts touch and the charging procedure resumes. As i said previously it was found in the heart of a Ni-Cad drill battery not Lipo.



 
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