Adapting cell battery to HP 41

[Craig Bladow]

I've acquired an MT3068 DC-DC converter. It will hold the output voltage as set until the input voltage falls somewhere below 2 volts. This is not ideal for use in an HP-41 as many lithium-ion batteries have protection circuitry that shuts off their output at 3.0 volts so the 41 would see its input voltage drop from a good level quickly to 0 volts with little or no warning in time to change the battery.

The MT3068 senses the output voltage via a two resistor voltage divider connected to its feedback input. One would need some way to drop the feedback input so the output voltage would go just below the 41's low battery threshold when the lithium-ion battery was mostly discharged, say to 3.3 volts.

I've also printed out the battery cases I linked to in a previous post in this thread. There is not a lot of room in the cases so some smaller DC-DC converter modules are on the way to try out.

With a removable battery case the charging circuit could be external which would save room.