When a lithium battery in the Pocket3 is dead, the operating system is disappearance because it supported by a lithium battery. Then nothing is done. A system card is necessary to restore.
Fortunately, the system does not disappear until AA battery runs out, even if a built-in battery runs out. Therefore I keep on usually putting AA batteries.
NiCd battery is the rechargeable battery that acquisition is easy.
The voltage is 1.2 volts. Out of battery is easy to detect because divergence with the threshold is small.
To avoid this situation, three NiCd batteries are connected in series, and supplied by a external power jack.
The voltage is 3.6 volts by this way. Nominal voltage of Pocket3 is 3 volts, and the danger by an overvoltage is little.
I have not tried this way actually.
I heard that three NiCd batteries of 700mAh in available time is almost the same as the two alkaline batteries.
Note that when the NiCd batteries dead, the remaining amount warning alarm of the battery is not displayed.
This is because the voltage of the NiCd cell drops suddenly compared to alkaline batteries.
Instruction manual is describing that NiCd battery can be used at a Pocket3. And initially the launch of Pocket3, NiMH battery was not launch. Therefore there is no mentioning about NiMH battery on instructions. The NiMH battery is barely available for a Pocket3, there is a case that the battery dies in about 10 minutes. This reason is unclear, maybe individual difference of Pocket3 and/or NiMH battery. When measuring the dead battery, it shows a value as new.
In such case, I recommend that batteries connect a external power jack with four NiMH batteries connected in series, and use the zener diode or the 3 terminal regulator.