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

[Massimo Gnerucci]

(12-11-2020 06:39 PM)Hans Brueggemann Wrote:  with its anemic energy content, utterly long charging time, short lifespan, and inability to supply the card reader about just every time you would need reading a magnetic card, the HP 82120A Hurricane truly belongs into the "most disappointing HP accessory of all times" category. imho, of course.

when reviving my HP-41CL's configuration for the umpteenth time after an unforseen power loss (or getting an unresponsive HP-41C just because CLONIX is a tad picky when it comes to inrush current), i was no longer having it and decided to do my own rechargeable battery pack. here's my take on it.

instead of using an LiIon type pack, i opted for three ENELOOP AAA cells that would just so fit into the battery space of the calculator and still have some room for the converter circuitry. being at it, i opted for an USB-C connector that allows me to use any USB-charger that i have in my household.

rechargeable battery packs for the HP-41 have been done before, so why re-invent the wheel? the one thing that i was really missing in existing LiIon battery packs was the fact that they by design disable the LOW BAT warning on the HP-41. also, they would require some extra circuitry to take care of the LiIon's characteristcs in order to play safe. hence, i decided to implement a converter that would actually boost the ENELOOP's voltage to the desired level that the calculator requires and at the same time would track the ENELOOP's voltage during discharge. thus, the HP-41 is still able to report a LOW BAT condition to remind you of connecting an USB charger.
the charging circuitry itself clamps the max charging current to very low levels, so it takes quite a long time (approx. 8 hrs) to recharge the pack. at that point, the charging circuitry goes into a kind of "super low trickle charge" mode that allows you to have the charger permanently connected without damiging the battery cells. the boost circuitry is in fact quite beefy and has no problems feeding the high inrush current of the card reader, even if the card gets stuck for some reason.
so, finally, say bye-bye to your HP-41 falling into coma just because you didn't get notified on LOW BAT in time!

for now, this is for information only, as you can imagine i'm currently busy with testing something else. however, i'm eager to hear what you think about this approach!

I'd say it's very promising and really interesting!
Keep us posted about development and availability, thank you.