Keyz wrote:christer wrote:
Well, yesterday my outdoor sensor started to deliver data without changing the batteries! But. The temp har gone up from -20C to only -5C. Maybee the RF power is dependent of the temp? I will check the batteries later.
Regards
Christer
Hi Christer,
This would tend to suggest to me your batteries have become drained, and the super low temperatures reduced the power available to the point that it could not transmit. The increase in temperature could have enabled the chemistry inside to work enough to send the transmission, but I would suggest the cells are still drained?
It's true that exhausted batteries would be unable to perform at low temperatures, but also, as has been discussed elsewhere here, even good alkalines suffer from a fall off in voltage, especially as low as -20c. A while back, in response to another thread on this subject, I looked up some technical papers on the Energizer site and from memory, the voltage dropped down to about one volt at that sort of temperature. Since any radio transmitter's output of RF is critically dependent upon the voltage applied to the circuit, it would be certain that dropping the voltage from 3v (2xAA in series) to around 2v, would lower the RF power considerably. Reducing the supply voltage by maybe 30%, quite likely at -20C, would reduce the RF output by about 50%, give or take. This would easily make an installation non-viable as the lower rf output might not penetrate walls from outside, even if it could do at the normally higher voltage. Warm up the batteries again, and hey presto - your radio link is miraculously restored.
Of course, it is probably pretty easy to check the voltage on the installed batteries if the transmitter is accessible.
Maybe we should be pleased we are not operating a remote weather station on the surface of Mars... NASA have been doing this stuff for years, and apart from having to send the data rather further than we have to (36 million miles to 250 million miles depending on the relative position of Earth and Mars) and the mean surface temperature is -63c, reducing seasonally down to -140C.... NASA artificially warm the batteries, or they would certainly fail and the machine would never come back to life again. It's much harder there to run out and swap the Energizers too...