I'm running into some issues powering a test board using a LoRa chip (RFM95W) with a CR2032 battery. I'm well aware of the peak current limitation of such battery and I can see a major voltage drop across the battery when too much current is asked during LoRa TX (around 80mA), even using a 100uF capacitor in parallel; the voltage drops down below 1.5v which make my MCU brown-out and reset. Thus entering an infinite loop as my MCU is setup to send a measurement on start-up. I'm using a power gating timer (TPL5110) in order to minimise the current consumption between measurements, 1 mesure every two hours.
The question is: how do people manage to use CR2032 coin cell battery alongside LoRa RFM95W as I assume the peak TX current would be fairly similar in all these other IOT boards I can see around internet. I'm using default firmware mode of communication under LoRa-WAN standard of The Things Network and my support hardware around the RFM95W does not account for more than 5mA of current.
I'm not sure my question if well formulated, but mainly I'm looking for any advices on how to use a CR battery and still manage to power my device. In the meanwhile I'm also looking at replacing by 2x AAA battery, but space is precious.
Thank you for your time you might take to answer.
Best Answer
To be blunt, that's really not a great idea, as the current demand is far in excess of what a coin cell can supply.
Typically sensors are designed to run for a long time, and so will use higher capacity cells, which are typically able to support proportionately higher peak loads as well.
If you really want to try to design something that can achieve a very short number of transmissions off an undersized battery, you probably need to use a large capacitor to provide the transmit current. But realize that packets of hundred of milliseconds are common (or well over a second in Europe), so that's going to be a fairly large capacitor. You may even want to go so far as to to have MCU logic which pre-charges the capacitor only in preparation for a transmission - typically the packet duration ends up being calculated anyway, so you could anticipate the overall energy requirement and factor that into the preparation.