Threaded Mode | Linear Mode

jwr · 05-14-2021, 07:53 AM

(05-13-2021 08:35 PM)Maximilian Hohmann Wrote: Hello!

(05-13-2021 07:31 PM)jwr Wrote: If you plan to rig something like this on your own, you will need to add a voltage regulator to get the LiPo voltage down from 3-4.2V to 2.4-2.5V which the calculator expects.

I was thinking of two silicon diodes in series between the battery and the calculator to achieve that. I don't really have much use for my calculators, I mainly collect them - if this will cost an hour of operation on each battery charge then I can live with that.

That's not likely to work well — LiPo voltage varies a lot, the usable range is roughly 3.0-4.2V. With two diodes, you'd be getting 3.0V when the battery is fully charged (or charging) and 1.8V when it's nearly depleted.

(05-13-2021 07:31 PM)jwr Wrote: This kind of pack could definitely be designed for other HP series, but only the schematic would be the same — every pack would need a different PCB and obviously a different case.

(05-13-2021 08:35 PM)Maximilian Hohmann Wrote: And don't forget that most classic HP calculators need three batteries, so even a fully charged 3,7V LiPo may just not be sufficient. So one would either need a 2-cell LiPo or a voltage booster. More complications...

You're right, I actually forgot about that — it's not a huge problem (one would need a buck-boost instead of a buck converter), but it does require a redesign.

(05-13-2021 07:31 PM)jwr Wrote: Please be careful with those "wireless charger receivers" from AliExpress — the ones I saw have USB connectors, so they produce 5V. Definitely not what your calculator wants to see on its battery terminals, and this will not charge a LiPo, you need a charge controller for that.

(05-13-2021 08:35 PM)Maximilian Hohmann Wrote: I ordered some which are based on the TI BQ51013 chip and seem to be clones (or vice-versa?) of this one here from Adafruit: https://www.adafruit.com/product/1901
The Aliexpress sellers claim that the receiver can charge LiPo cells directly (there are pictures of a LiPo connected to the module) whereas Lady Ada writes that a LiPo charger module should be inserted in the circuit. I will carefully try it out - outside the calculator and outside the house, at work they keep showing us what LiPo fires look like - and see how it goes. I am 95% certain that somewhere in one of my univentorised boxes with stuff there must be some LiPo charging modules :-)

bq51013 is a wireless receiver, with no charger. What it does is produce a constant voltage on the other end. The Adafruit device is connected to a LiPo charger in their demo. I use the bq51050b, which has an integrated LiPo charger (though I placed another charger in there as well, to be able to also easily charge from USB if needed).

To be clear, the circuit I built is not rocket science, but power management is tricky, especially with LiPo batteries. I built this because it turned out that I couldn't easily assemble it from available off-the-shelf components.