Battery voltage circuit check, whats the circuit on the battery side?

I have an iPod Shuffle 2G torn down to the open circuit and am checking voltages before spending the money replacing the battery. I want to make sure it isn't something else wrong with the Shuffle.

Between the battery and the main logic circuit board, there is an intermediary circuit (part of the battery assembly). I get a reading off of the battery directly before this board, but not when reading off of the other side of it (on the logic circuit board side). What is this intermediary board for? And why is it not passing voltage through?

Part numbers:

  • Left black chip on intermediary board:
  • M 6093 W7L1
  • Right black chip on intermediary board:
  • 8410100 B01-0
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That's the battery protection circuit -- you can actually buy these separately (see e.g.

In general, these circuits are fairly simple; if anything goes wrong (undercharge, overcharge, short circuit, etc), it stops passing voltage through. Otherwise, the voltage on both sides should be almost identical (and between 3.0-4.2V). If they get tripped, they will usually reset themselves if you apply a normal charging voltage on the logic-board side; if not, just buy a new battery :)

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