In 2006 Target had a sale on Oregon Scientific indoor/outdoor thermometers and I found out that they supported up to three different wireless temperature probes although they only came with one each. So I solved that by buying three thermometers and sharing the gauges. Two of the gauges read all three probes, while a third is only designed to read one probe, so I made sure that probe is the one outside (the other two are in my crawlspace and attic). Then in 2007 I got a wireless rainfall gauge which reads a tipping bucket gauge that is on my roof. It also came with a fourth temperature probe which runs at the same frequency as one of the other probes, but will be ignored by the other gauges as long as it is on a staggered 43-second reporting cycle. So I have the fourth probe in one of my closed off bedrooms to see how cold they get. Only the rainfall gauge reports that temperature.
Anyway, each of these items is powered by two batteries. That’s 3 temperature stations, 3 temperature probes, plus the the three components of the rainfall system for a total of 18 batteries. They seem to last about a year (maybe a little longer) on a fresh set of alkalines. It is a lot of trouble setting up the network correctly and if you take the batteries out of any probe, then that messes up its reporting cycle and the stations won’t read it anymore. So you have to take the batteries out of the station so that it will re-read the probes, but you have to make sure the rainfall gauge’s temperature probe isn’t detected and that the station that only reads one probe is reading the outdoor probe instead of one of the others. So there’s a whole procedure and it works best if you take all of the stations down at once to change the batteries. However, the batteries fail at all different times and you don’t want to throw out a battery that is still working. But you don’t want to take the chance of it failing in another month and taking everything back down again (and losing the cumulative rainfall).
So I thought I would try out rechargeable batteries. I know what you are thinking: Due to the self-discharge rate of NiMH batteries, won’t they all be dead in a couple of months? Not if I use low-self-discharge batteries. But low-drain applications aren’t economically viable due to the high up-front cost of the batteries, right? Well, not if you can get them on a really good sale, which I have been able to do recently. Target had 4-packs of Duracell precharged batteries for $6.45 last September and in December Rite-Aid had them for $7.09 after one rebate and down to $5.18 after another. These will retain 80% of their initial charge after a year and, because they have about twice the capacity of alkaline cells, they should be able to make it the whole year. If it works, it will still take 5 years to break even on this system (based on the cheapest price I ever got alkalines, which was 16 AA and 16 AAA batteries for $9.99).
The only problem is the voltage of NiMH batteries is only 1.2V instead of 1.5V for alkalines. Even fully charged, a NiMH is only 1.45V. So some devices think the battery is low even though it is just fine. And after installing 12 NiMH batteries yesterday (I had just installed alkalines a couple of months ago in the other two devices, so I am hoping they will make it for 12 more months), some are already reading LOW BATT. On alkalines, the probes and everything would work for months on low, so I am hoping it won’t be a problem and they can last all year on low.
The other potential problem is the batteries that are outside may not perform very well in cold weather. They will really be tested the next couple of days as temperatures drop into the teens, but they are also fully charged. Then the other question is how well they will do in the summer when temperatures in the attic go up to 120 degrees.