I have this circuit and I do not understand the purpose of the diode connected to the ground. This is a Bluetooth device (works at 3.3v) that has a 4x 18650 battery pack and a USB charger. When you plug in the USB it supposedly charges the 18650 batteries. (4x 18650 batteries in series) Picture 1 Picture 2

The diode(?) is connected via the yellow wire from battery ground, onto the board, goes passes a few capacitors and arrives at a chip with marking "MPEK 2517 A023" (or AO23) and an inductor right beside that chip.

When no USB is present the end of the yellow wire inside the white socket measures 0V. when USB is plugged in it measures about 2.1 volts steadily decreasing while the voltage steadily increases. (about 0.0001 volt every 2 seconds)

Can someone explain the purpose of this diode? thanks


1 Answer 1


That is not a diode but an thermistor, a resistor that has a it's resistance value dependent on temperature. This is common with Nickel Cadium battery where the temperature of the battery pack its used in the charging algorithm for this battery chemistry


Here a link to digikey showing a NTC thermistor in the same glass package

Also, this is another example of the thermistor in the glass packaging also used for charging on NiCd batteries that I have on hand.

Thermistor in NiCd battery pack

  • \$\begingroup\$ Are you sure? The glass body looks like a small Schottky diode and resistors are not usually packaged in glass. Yes, the yellow wire usually goes to thermistor, but this circuit is weird (look at disconnected black wire on right!) \$\endgroup\$
    – pjc50
    Apr 6, 2015 at 10:57
  • \$\begingroup\$ You are right about the packing but also note that there is no polarity band in the package which is very typical in diodes in this package. The black wire doesn't concern me the circuit has been messed with and the battery pack taken apart in order to expose the thermistor. Also I conveniently happen to have an open battery pack of NiCd battery with the thermistor in the same package. I would be glad to upload a photo I just not clear how to upload photo from my computer to the site. If anyone can point me to the procedure I'll upload a photo \$\endgroup\$
    – Kvegaoro
    Apr 6, 2015 at 14:09
  • \$\begingroup\$ Never mind about how to upload from my computer... I found it... \$\endgroup\$
    – Kvegaoro
    Apr 6, 2015 at 14:10
  • \$\begingroup\$ OK, that's useful to know about the packaging. \$\endgroup\$
    – pjc50
    Apr 6, 2015 at 15:37

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