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Power Source Selection

A project log for My Very Own Geiger Counter

Fulfilling a childhood dream

jon-kunkeeJon Kunkee 09/26/2019 at 04:540 Comments

I debated with myself quite a bit about what kind of power source to use for this project.

I do apologize for the rambling nature of this, but it does reflect my thinking. It was not a clean-cut engineering problem, but a mix of personal preferences and decisions made in the thick of learning the topics and discovering available solutions.

AC

Wall outlet power is nice when the power is on and can't be overcharged, but it is not useful wherever power is not available--whether during a blackout or hike.

Batteries

Batteries are mobile, right?

Well, here's my train of thought. As I wrote it out, I noticed that it's chock-full of received wisdom, folklore, and places where actual numbers could be referenced instead of rough hand-waving. Still, here you have it documented so perhaps you won't make my mistakes. :)

Alkaline batteries are ubiquitous, but they aren't reusable. They keep alright--if they're not rechargeable, they can't be overcharged, right?

NiCad batteries lose capacity if kept charged.

Lead-acid batteries have fairly low energy density and, depending on the design, venting issues. They tend to be large and don't handle deep discharge well, but they're incredibly well-studied and well-tested. They don't tend to come smaller than lantern batteries, and that's close to the size of the rest of the project combined.

NiMH batteries don't handle overcharging well and, well, I don't like the chargers and usage patterns I've had experience with. If I could find a good charger+monitor circuit (I haven't looked), that combined with the easy availability of NiMH cells would make this my second choice.

Lithium Ion and Lithium Polymer chemistries have incredible energy density, but I've had pouch cells swell before, they're known for capacity degradation on the order of 100s of cycles, and also don't handle staying charged very well.

LiFePO4 batteries have a little more than half of the energy density of their LIon and LiPo cousins, but they don't have the safety issues and they handle constant top-offs (being plugged in for weeks at a time) well. Once I found a good charger circuit, the LiFePOwered/18650, this became my first choice.

Power Requirements

The highest sustained current the GK Plus kit (with display and even GPS) is stated in the kit assembly instructions as about 90 mA. Since it's a little unclear what voltage this was at, that doc later says that a 600mAh battery will yield about six hours of runtime, and the kit runs at 5V internally, I figured it was 90 mA at 5 V. That isn't a hard peak current to supply.

The trick is that I am building in the GK WiFi kit with an ESP8266 on board. My rule of thumb for WiFi is peak 1A current draw, though one blog measured the ESP8266 peak current during startup at 480mA or so. Even if the battery won't last more than a few hours, the charger and chemistry and cell should all be able to handle that much current continuously.

I do plan to be able to display the WiFi board when off AC, so it's more of the on-AC steady-state situation that governs my continuous 1A requirement.

With this in mind, I figured something around 2000mAh would yield >12h of runtime without AC and with WiFI off, while it would still be ~2h with WiFi on. This was partially driven by LiFePO4 cell sizes, with 18650s being about this size. The next size smaller felt too small (too short of a runtime) and the next size up gets expensive, large, and beyond the convenience of the LiFePOwered kits' holder brackets.

So...some technical reasons, many not-so-technical reasons. That seems to me to be much like commercial hardware and software development.

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