Part 2 - In which ESP8266’s do HORRIBLE things to batteries
So initially I went with the naive approach, I stuck a pair of CR2032’s in parallel and strapped them to the ESP8266, I was rather disappointed to discover that that only got me about 20 mins of life, repeated the test with a pair of AAA’s for interests sake, I think I got about an hour out of them, time to do some ACTUAL research…
In doing some fishing around online I found some YouTube video around powering ESP8266’s from batteries or somesuch, which alerted me to the horrific power draw behaviours of the ESP8266, turns out a bare module pulls bugger all current, EXCEPT every 100ms or so where when it’s in AP mode, where it suddenly draws 400+mA to beacon, which the CR2032’s (being that their nominal rated current draw is 15mA) were VERY unhappy with, I did a quick lash up on my electronics bench to validate that that was, in fact, what was happening here, the scope confirmed the findings.
Fortunately there exists a simple solution for this, “strap a big-ass capacitor across the power rails”, 1000uF is a good choice, it doesn’t completely eliminate the spikes, but it reduces them to manageable levels and doesn’t add excessive bulk.
So I went and strapped a 1000uF cap across the module and repeated the test with a fresh pair of AAA’s, sadly that was still only good for 6 hours or so (even after I dropped the beacon interval back to 2 sec).
Back down the rabbit hole then.
The ESP8266 supports various “sleep” modes, reading the doco, the only thing that would really work for me is “deep sleep”, next problem, the way deep sleep works is;
- GPIO16 is connected to RESET.
- Sets a timer to fire GPIO16.
- Powers down everything except the timer.
- Timer expires, GPIO16 fires, chip is reset.
Problem is that the ESP-01 doesn’t have GPIO16 broken out… Time for some microsurgery!
Note the jumper wire between the Reset pin and pin 8 on the ESP8266, also note that the power LED has been removed, that alone pulls ~10mA.
Bada bing, bada boom, deep sleep, did some testing managed to get 60 hours out of a 5min sleep 1min wake cycle, this was less than optimal but at least it showed that there was a way forward.
More in part 3 whence we dig into the tools Espressif hath provided.