Hurricane II: An HP-41C Battery Pack w/ USB

[Hans Brueggemann]

Q1A in the schematics tracks the eneloop voltage and proportionally controls the output voltage by steering the voltage divider R5, R6: the lower the eneloop voltage gets, the higher the feedback voltage at the FB pin of U2 gets.
thus, the output voltage of the battery pack is tracking the three eneloops almost exactly to Vout = 3 x Veneloop + 1 V. hence, for fully charged eneloops, Vout becomes 3 x 1.43 V + 1 V = 5.30 V, while for fully depleted eneloops, Vout = 3 x 0.9 V + 1 V = 3.7 V, which is also the threshold of the BAT indicator.

meanwhile, the HP-41CX w/CLONIX that i'm using for the standby-duration test is starting to collect cobwebs and has clocked approx. 6500 hours of "idleness". total voltage on the three eneloops is still over 3.7 volts and the calculator does power on without the "BAT" indicator coming up.

so, just to confirm, the project is still alive. since march i'm using proto # 2 on a daily basis (HP-41CL) with lots of card reading every now and then (you know, basically every single time when YFNS has left the building. again.), and so far i'm pretty content with its endurance.

for those who are interested in a ready made product: this is probably not going to happen. however, i'll be sharing the design files beginning next year.