Computing stuff tied to the physical world

Bluetooth power consumption

In Hardware on Nov 8, 2012 at 00:01

The Bluetooth module in yesterday’s setup has a nasty power consumption profile:

SCR78

The yellow line is total power consumption, which to over 60 mA at times, and the smaRelay.ino sketch is querying the SMA inverter roughly every 10.5 seconds. The drop in baseline is the ATmega going to sleep as it waits for the next period, so you can clearly see what the Bluetooth module is doing – while kept on and connected to the SMA in fact.

I’m not sure that the Hameg’s math integral function is up to summing such fast-changing values, but it’s the best I’ve got to measure power consumption here at JeeLabs at the moment (well, either this or tracking the discharge on a hefty electrolytic capacitor).

Note the baseline consumption of about 5 mA, and the frequent but highly irregular brief power consumption pulses. That’s BT doing its frequency hopping thing, I assume.

Anyway, my goal was to get an estimate of the average power consumption, so here we go:

  • two cursors were used to peg the integral (summed) value over one cycle
  • charge usage over one 10.5 second period turns out to be 134 millicoulombs
  • that’s 134 / 10.5 ≈ 12.75 mC per second, i.e. 12.75 mA average

Whoa… not much of a candidate for battery-power this way!

That leaves a couple of options:

  • just power it via a USB adapter and move on
  • explore the RN-42’s low-power mode, which is claimed to go as low as 300 µA
  • completely turn off power to the RN-42

I’m inclined to go for the latter. I don’t really need solar PV readings that often, since the SMA accumulates its daily and total generated power anyway. And during the night, all this reading and reporting activity is also quite useless.

That would also solve – or rather: work around – the intermittent problems present in the current code, in which the sketch stops relaying after a few minutes. It always seems to get stuck after a while, waiting for incoming data from the Bluetooth module.

One readout every 10 minutes would probably be plenty for me, and since the SMA has a time-of-day clock which can be read out over BT, I can stop readouts during the night (or even simpler: add an LDR and switch off when it’s dark).

It looks like powering up, establishing a connection, and reading out one set of values can be done in under 6 seconds, so that leads to a 1% duty cycle. Let’s say 200 µA on average – this ought to run a year on 3x AA Eneloops.

Sometimes, sophisticated solutions are just distractions – whatever works, right?