Help needed in calculating battery life

[espruino-discuss3]

Posted at 2018-04-04 by BootySnorkeler

Hey guys,
I'm making a very power-critical project(a wristwatch) and I want to get the most battery life from the smallest battery.
I have an Espruino Pico, two 16 segment LED displays, 4 SMD leds(2 on the pico 2 on the pcb), 5 tactile switches, and 220 ohm resistors for every LED, including those on the 7-segment display.
I've roughly estimated the total power requirement to be about 100mah. However, other similar diy watch kits I've seen on the web like https://www.sparkfun.com/products/11734 gets a full TWO YEARS from a single coin cell! Am I doing something wrong? I too would like to get several years of power from a coin cell.

Thanks!

[espruino-discuss3]

Posted at 2018-04-05 by @gfwilliams

Are you using setDeepSleep(1) on the Pico? When not doing anything that should put the power usage down to 0.02mA - which would be around a year's life on a coin cell.

However powering LEDs will drastically increase the power usage - that SparkFun watch will spend all its time with the display powered off, only turning it on when you press the button - which might explain some of the difference in power draw you're seeing?

Do you have an oscilloscope? One thing I sometimes do is power the device via a low value resistor (1 Ohm?) and then measure the voltage drop across that resistor with an oscilloscope. You can then get a pretty good idea of when and how power is being drawn.

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Posted at 2018-04-05 by BootySnorkeler

If I make the LEDs turn on only when a button is pressed, how much will it improve the battery life?
I also heard that the moment an LED lights up it sucks a ton of power for a fraction of a second and then stabilizes. is it true?

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Posted at 2018-04-05 by Wilberforce

Have you considered using a puck? You might have enough io pins and it has the coin cell as is already round like a watch?

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Posted at 2018-04-06 by @gfwilliams

If I make the LEDs turn on only when a button is pressed, how much will it improve the battery life?

It depends on how many LEDs you have. Realistically though, it's a massive amount. It'd take maybe 50mA to light up 10 normal LEDs at modest brightness, but with them off and the Pico sleeping it'd only be 0.02mA.

I also heard that the moment an LED lights up it sucks a ton of power for a fraction of a second and then stabilizes. is it true?

Honestly I'm not sure, I think it acts a bit like a capacitor. I know that sometimes to get the most efficiency/brightness LEDs are actually driven with Pulses (like those really annoying brake lights on cars that leave a dotty trail on your retina)

Puck

I'm not sure I've seen one but there were a few people looking at adding an OLED display to a Puck.js. There's also a 3D printable case that includes holes for watch straps. It's a bit different, but it could make quite a neat little device.

The same would apply with the display though - you'd need to turn it off most of the time.