Sorry in advance for my question, I am new at this.
I would like to use the AEM10941 e-peas based supercapacitor charger from https://www.tindie.com/products/jaspersikken/solar-harvesting-into-supercapacitors/ to solar power a GPS tracker (Ublox SAM-M8Q) using a ATmega328P. My plan is to get a GPS fix + transmitt using LoraWan and repeat every time the 3.3v regulated output that are enabled when the two * 10F 2.7V AVX has sufficient charge 4.5v.
As far as I understand two 10F 2.7V AVX should be able to power this setup when a ublox gps coldstart draws 45mA for 28s, the ATmega328P 13mA at 3.3v/8Mhz and a few mA for LoraWan transmition. The AEM10941 charges the supercapacitors up to 4.5V and the outputs are enabled down to 3.6V. So if my tracker uses 80 mA the 2x10F*(4.5V-3.6V)/0.08A= 225s giving my plenty of time to run my sketch. Could it be sufficient with one 10F 2.7V AVX supercapasitor?
if you only transmit when you have sufficient charge it will keep on transmitting on a sunny day… and never transmit on a rainy day.
So you must have another form of interval timing,let’s say , check charge every hour during day, if sufficient transmit if not wait till next cycle, preferable use an RTC+lightsensor for interval timing.
At night you have another problem, the charge will be less and less (no Sun … So you need enough energy to transmit at night too.
The chip/module you found is ok I think… 2x 10F should probably work, but your tracker uses to much power (80 mA )
see the 5205 for example
The processor does not need to be running that fast while the GPS is acquiring the satellites and, for that matter, does not need to be running that fast for a basic tracker application. Depending on how long between GPS fixes and how far the device is moving, it will not necessarily require a cold start for each fix and a warm fix can be had in a few seconds.
and only aquire a new position if there has been a movement detected before, if not , the conclusion must be ‘this tracker is stationairy, no need to update position’ … saves energy too .
@Sandgroper you are right, my calculations where based on a ATmega328P running at 5v/16Mhz. Power requirements are a bit lower than my 80mA estimate, probably more like 60mA for 30 seconds to get a gps fix and transmitt. So 2x10F*(4.5V-3.6V)/0.06A gives me a 300s runtime. Does this mean that I could potentially get 10x coordinate positions and LoRaWan transmissions on a single SC discharge? That means I could transmit a couple of positions a night as well. But this would be a best effort GPS tracker, no guarantees.
I was hoping to use a supercapasitor to avoid the lithium ion cycle limitation + operations in cold weather. I also think supercapasitors are supercool and been looking for an excuse to use them for a long time
Say I have a 2V/100mA 79x28mm solar cell that produces 10mA on a
sunny day and maybe 20% 2mA on a cloudy day. How long would it take to harvest enough 30 seconds at 60mA (e-peas chip is 95% efficient)? Trying to use some online solar calculators, but looks like I am having problems converting my W, A and m2
@BoRRoZ in order to detect movement do you recommend 3-axis sensor to check if orientation has changed since last transmission?
I am a bit confused about witch MCU to use on low power projects, I have used pycom + arduino environments but are these really suitable for these kind of applications?
that’s always… LoRaWAN doesn’t have any guarantees that the data is received.
Imagine you are driving around in a city… lots of gateways, no problem.
But then the car is leaving the city… at a certain moment your packet isn’t received, just because there are no gateways within reach … your car is ‘of the radar’
I had a look at the RAK 5205 and in my understanding its power consumption is comparable during GPS fixing + LoraWan transmission. Are you comparing the RAK 5205 15uA during sleep with power during operation?
The UBLOX SAM M8Q will consume circa 26-28mA whilst aquiring a fix. An ATmega328P at 3.3V and 8Mhz will consume around 6mA. Dont know what the current consumption would be at 5V and 16Mhz, never measured it, everything I use for IOT stuff runs on 3.3V.
M8Q hot fix time can be 2-5 seconds, but not if its been some hours since the last fix, it can behave as from cold in these circumstances. If using hot fix over an extended period, as in weeks, you will need to provide an external supply for the GPSs Vbkp, those little batteries or capacitors that are often used dont last very long.
… So you need enough energy to transmit at night too.
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