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So, I am building a gps tracker build on raspberry pico and a neo 6m gps module. Now, I want to power this project with a lithium battery that can offer some autonomy. I was thinking of using a 8200mAh, 3.7V battery, connected with my bike dynamo to constantly charge it. Normally I produce 200W in a five or so hours trip, 300W in much less. A commonly quoted efficiency for electrical generation is 80%, but lets assume 75%.

200 W X .75 = 150 W

300 W X .75 = 225 W

Daily I need to use my bike for 15/20 mins, so we can take the higher figure. 15 minutes is .25 hours, so we have

225 W X .25 h = 56.25 Wh

So that would run a 100 W load for half an hour (provided the remaining parts of the system were 90% efficient.) Based on this numbers, I could charge the battery, but I need to understand 3 things:

  1. how can I connect both the dynamo and the pico to the battery
    • and how can I avoid to make the battery eplode
  2. how can I safely connect the battery to the pico in case I dont use a usb shield + port

Any suggestions?

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  • Not a bad idea, but we've got to have more details on your bike's dynamo before we can answer your question. Specifically, we need to know if your dynamo produces ac voltage - or dc voltage. You may want to peruse this article, and then comment on the difference between this quote from the article: ["Most dynamo hubs produce six volts at a typical maximum power of three watts."]), and your stmt: ["Normally I produce 200W in a five or so hours trip, 300W in much less."] .
    – Seamus
    Oct 25, 2023 at 7:16

1 Answer 1

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Your idea is interesting; let's review the calculations:

AFAIK: Bicycle dynamometers are low output alternators which produce ~3W (according to this reference). Since you gave no specs on your dynamometer, we'll rely on these specs for this answer, and ignore the statement re ... produce 200W in a five or so hours....

First order of business is to distinguish between power (instantaneous; no element of time), and energy (1), 2 (power applied over a period of time). This is necessary to build an energy budget. Let's calculate that budget from the following items of data:

  1. The "8200mAh, 3.7V battery" stated in your Q

  2. Max power consumption of the "neo 6m gps module" - taken from this data sheet:

Imax supply = 67 ma @ 3.6V; i.e. (.241 watts)

  1. RPi Pico power consumption. This figure depends largely on the use-case, but for this exercise we'll use a value of 90mA based on figures in this reference.

Based on the above, we can calculate the demand-side of our energy budget. Begin with the power requirement (which, for now, we'll assume is constant):

  • P = (67mA x 3.6V) + (90mA x 3.7V) = 0.57 watts

The energy we are interested in may be calculated by including the time over which this power is to be applied. From your question:

Daily I need to use my bike for 15/20 mins...

Which now gives us the demand side of our energy budget:

  • E = P x t = 0.57 watts x 0.33 hr = 0.19 watt-hrs. (or, 190 milliwatt-hrs)

Now, compare this figure against the supply-side of our energy budget (the battery), and expressed in comparable units:

  • E = 8.2 amp-hrs x 3.7* V = 30.34 watt-hrs
    *Note that battery won't maintain 3.7V over entire discharge cycle, so this is an optimistic estimate
Supply-Side Demand-Side
30.34 watt-hrs 0.19 watt-hrs

One might draw the conclusion that this is far more battery capacity than required, but such a conclusion assumes much. I will opine that if this were my project, I'd consider eliminating the complexity associated with using a dynamometer as a charging source for the battery - or use a smaller battery.

OTOH, comparing the dynamometer power output of 3 watts (according to the cited reference) against the calculated power demands of the Pico and GPS of 0.57 watts suggests that this project could be powered from the dynamometer alone. Operating in this mode will likely require other design changes (hardware & software), but gives some food for thought.

Consideration of these options are left to the OP, but feel free to ask additional (separate/new) questions if you're interested.

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  • This make so much more sense now, thanks. Now tho I have another question: could it be possible to charge some supercapacitors over a 30/40min ride and then use that stored energy to charge a bit the batteries (not completely, but bit by bit everyday)? My math is this: 40min * (3W/60min) = 2Wh even tho this isnt even close to the 30.34Wh, it could be added to the battery to keep alive the pico and gps module for a bit longer. Oct 26, 2023 at 13:20
  • Yes, you could probably do that, but you'll need to make a decision as to whether or not it's worth the additional hardware & design effort. Please read this.
    – Seamus
    Oct 26, 2023 at 23:36

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