HP15c-LE, manual battery test

The low battery indicator of the HP15c-LE (limited edition) does not work (See e.g http://tuxgraphics.org/npa/hp15c-le-power-test/ for more details about this fault). There is however a possibility to test the battery with a voltmeter from time to time. You don't have to take out the batteries to test them.
  1. Open the battery compartment.
  2. Set your voltmeter's range such that you can measure 3.2V DC or more (e.g 10V).
  3. Connect the probes of the voltmeter as shown below.
  4. Replace the batteries if the voltmeter shows 2.7V or less.

[hp15c-le battery test pinout]
The probe for minus goes onto the middle jtag pin on the side towards the batteries (indicated by the blue arrow). The probe for plus can go onto either of the CR2032 coin cells since both cells are connected in parallel (indicated by the red arrow).

Make sure that the probe of your voltmeter for the minus pin has a round ball-point tip. This is to avoid damages and scratches on the jtag connector. You can use very fine (400 grit) sand paper to remove any sharp edges from the tips of your voltmeter's probes.

To replace the batteries take out one at a time and replace them. They are connected in parallel and one cell is enough to supply power to the calculator during the replacement. You must always replace both batteries at the same time and they must be form the same manufacturer and of the same age otherwise there might be a current flow between the cells.

It should be enough to perform this test every 2-3 month.
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© Guido Socher