Houston we have a problem…

I have been doing run tests on the newest flow sensor builds, and this morning I had a little red error-state led waiting for me on a sensor that I had assembled the night before. I assumed I had botched a solder joint, or broken a wire somewhere while loading everything into the housing, but instead it turned out to be another Duracell battery failure.

The dead power module

Normally I would ignore this but this is the third Duracell battery failure I have had in a week. One is ‘meh’, two is just ‘bad luck’, but three…that’s a problem that requires attention.  My first though was a bad batch, but the batteries are not all from the same package, each being a slightly different variant of the marque.  Interestingly I have a log of the voltage curve during the start of this problem as I track the power supply via the internal 1.1v reading trick on the Atmel processor:     (mV on the vertical axis)

The logger recorded about 500 samples before going into shut-down mode.  My overnight runs race along at one sample recorded every 15 seconds just to give them a workout, so this graph only represents about two hours of operation. But still, that is one weird looking voltage curve (…especially with those ‘gaps’, and the voltage was increasing??). Since I have not seen one like this before, I thought I would post it here. The power module has 2 banks of 3AA batteries, isolated from each other by shottky diodes, so this probably represents one set trying to keep the logger alive while the other bank went down hill (while venting internal pressure?) Then when the bad battery leaked enough, the alkali affected the others, and the whole power supply pooped out.

Another from this week

I don’t know why the logger stopped at this point either, as all of these voltages were within the units operating range, so I have to suspect that some kind of “burst” occurred around record 500, that pulled the voltage down below the 2800mV sleep trigger in the code. Certainly there was battery goo sprayed all over the inside of the housing.  The cells that leaked were still delivering 1.3 volts each when I pulled the unit apart in the the morning, so this certainly was not caused by running the cells dry.  My current hypothesis is that the new Keystone Electronics battery holders are so strong, that they are exerting pressure on the battery housing causing some kind of containment failure. Until I get a handle on this,  I will have to go hunting for a replacement brand before the next field deployment.

P.S. The Tinyduino logging platform seems to have survived the event, and is now ticking away in the basement for a proper shake-down run.