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.
Open the battery compartment.
Set your voltmeter's range such that you can measure 3.2V DC or more (e.g 10V).
Connect the probes of the voltmeter as shown below.
Replace the batteries if the voltmeter shows 2.7V or less.
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
It should be enough to perform this test every 2-3 month.