HP-15C CE often freezes

[Divasson]

Before deciding that the unit is defective, I would exchange the batteries and test again.

The programs that run over a long time drain the batteries extremely fast, since the machine is running at full speed (48Mhz) which consumes several orders of magnitude more energy than at idle. And at idle the battery voltage level is high enough to not lit the low battery indicator, but this voltage level drops fast when running a program or a solve/integrate procedure due to the battery's high internal impedance.

Your unit's behavior is very much like when I had to change batteries (after 1 year of use, and with a back up of the memory!)