Let's Make Robots!

Weather station project: death, autopsy and resurrection

On April 2nd a bit after 11:33:22.546588 AM GMT+3 it happened. My weather station's (http://letsmakerobots.com/node/25773) remote unit died. I was having my morning coffee and noticed that weatherlogger hadn't received any updates for last 10 to 15 minutes or so. Last update was at previously mentioned timestamp.

I took a look at remote units voltage data that weatherlogger had been saving for a bit over a month and here's what I found:

Remote unit's battery voltage. Clickety-click for bigger picture.

My first though after seeing the voltage drop was that there's probably some condensed water inside the casing and it's causing shorts in the circuit. Yesterday I did an autopsy and found out that, to my surprise, the casing was completely dry inside. No trace of any water or moisture.

Next thing to check: the battery. And this is how it was:

Yup. That's a dead 6V battery.

Battery's voltage was barely over 2V so it's practically dead. The battery had 4AH capacity and if I calculate 31 days without a glitch I get about 5mA average current. That figure also includes the self-discharge of the battery. Originally I estimated the average current to be in microamp range so either I have calculated it wrong, have a bug in the software (no sleep), battery's self-discharge was bigger than I thought or there's a hardware fault. Or maybe a bit of all of these.

Anyway, I really didn't bother to investigate the mysterious power drain and bought 3000mAh lithium batteries. I'll just try them and see how it works. If the average current really is around 5mA then those batteries should only last 25 days.

Weather station has been resurrected and once again it's logging weather data. Let's see how long it lives this time.



Update Nov 13, 2010: 

PaulJD suggested that the battery might have frozen. So I decided to compare temperature and battery voltage (pics are clickable):

Temperature and battery voltage together.

Close up of last couple of days.

In the end battery voltage seems to follow air temperature after some delay. Delay can be explained by battery's mass. Bigger mass equals slower temperature change in the battery. Right?

So did it freeze? Dunno, may be it did. Those two spikes on 30th and 31st during afternoon certainly suggest that the battery got frozen during the night and "unfrozen" at daytime.


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Well, it wasn't really cold but it did get below -10°C during nights. And because the battery was getting low on charge anyway it might have frozen. I updated the post with couple of nice graphs comparing temperature and voltage.

The battery is rechargeable but my charger requires it to have at least 3.15V (or something like that) before it will start charging. So for that charger the battery is dead.


perhaps there's no mystery at all.   your weather station used all the juice in the battery, and thats it.   it needs to be recharged.   give it 7 or 8 volts anyway you can for 12 hours and your good to go for another 30 days.   if it does not hold the charge then its shot.