EmonTH - voltage reporting

Hi – I have an old EmonTH v1.5.2 (dated Feb 2015) which I have kept in a sealed box in the garden to record outside temperature. It also reports the battery voltage.

It recently stopped working and I assumed that this was because the batteries needed changing. However, when I went to change them, the crystal fell off the board. I have now soldered this back on and replaced the batteries. All seems to be working, except that the battery voltage is reporting as only 1.4 volts. I don’t think that this can be right, because the unit wouldn’t work at that voltage. I have measured the voltage on the pulse counting terminal, and it is showing as 3.3 volts. Does anyone have any suggestions where I might look please?

It can be right, because there’s a step-up dc-dc converter between the battery and the processor precisely so that it can continue to operate as the battery voltage falls.

I suggest checking the state of the battery contacts as a first step. If you have a high resistance contact, that will explain a low voltage.

Circuit diagrams are available on Github, you might need Eagle CAD to view them.

Thanks for the explanation Robert. I’ve checked the voltage of the battery carrier pins on the underside of the board - it was showing 3.1V So I think this is going to be a hard one to crack. As it all seems to be working OK at the moment, I’ll probably leave it and see how long the batteries last.

If you look at the software, the measured battery voltage is multiplied by a constant. I don’t see how that might have changed, but it’s the only thing I can think to look at.