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I'm designing a frost alarm for farmers. The basic idea is that the farmer puts a sensor somewhere in his field. This sensor reports the temperature every 10 minutes to a base station with a wireless link. When a temperature near 0 degree Celsius is detected an alarm will sound and the farmer can save his crops from the frost.

This sensor needs to be very reliable and work at least 6 months without intervention. (So a low self discharge battery is required.) I'm thinking about using a low power AVR microcontroller and a HopeRF RFM12BP 500mW (max) wireless module.

What is the best battery (technology) for this sensor? Most batteries perform badly under low temperatures like -10 degrees celcius. My sensor should still work under such condition. I prefer a rechargeable battery, size and weight are not very important.

This is what I already found about battery performance at low temperatures:

  • NiMH: Very poor
  • NiCd: ??
  • VRLA (sealed lead-acid) ??
  • Li-ion: Poor
  • Li-Po: Better
  • LiFePO4: ??
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Quick: I'd have a good look at GP ReCyko cells as a good starting point


Some assumptions:

  • For temperature >> zero C a lower reporting rate may be OK. Presumably the station could transmit say once per hour as a confidence check BUT drop into "fast" mode whenever a temperature drop threatened.
  • Assume total TX cycle at 500 mW is 1 second per 10 minutes.
  • 1s/10mins x 500 mW = 5/6 mW. Assume 1 milliWatt means draw. Adjust as required.
  • Assume ~~10V pack for convenience --> 0.1 mA mean draw.
  • 6 months ~= 4000 hours or ~400 mAh delivered capacity required.
  • So worst case 6 months of late Autumn - Winter - Spring a 3 times over provision of battery capacity to allow for temperature effects requires about 1200 mAh. 4x = 1600 mAh. 5 x = 2000 mAh.

So AA cells of 1200 mAh in many chemistries would probably work (see below).

Primary cells are not a terrible idea. AA Alkalines at 2500+ mAh and a degradation factor of say 3x average would last about 2 years.

Solar recharging looks highly attractive. 1 mW mean power = 24 mW.hour / day. ie 1 hour of charging at say 50 mW would suffice to keep the battery always topped up. That's about a 1 square inch mono or poly crystalline solar panel exposed to one "sunshine hour" of sunshine per day on average. As a guide, that would be more than adequate in NY NY in January (worst month) , and 3 square inches would work in Moscow in Russia in mid winter.

In all matters to do with batteries YMMV widely - experience is unfortunately the best guide as to how good claims are. Results can be very dependent on manufacturer. In many cases, if you don't have the volume to do your own investigations (and few do), choosing a reputable brand label which has been in the business for a substantial period and which is liable to have researched the product they sell and stand behind the results.

I would accept as likely to be approximately true, technical claims by Chinese makers BYD, BPI and GP (GoldPeak). Also mainstream labels such as Sanyo (make their own cells, usually very competent) etc. Most others I'd treat with far more care. Note that GP are so successful that there are Chinese clones of their products.

Note that for 6 months + lifetimes the self discharge rate of the battery used becomes relevant. NimH is very poor, NiCd is poor, lead acid are good and Lithium Ion and LiFePO4 are very good. Low Self Discharge (LSD) Nimh are very good. The latter are available as eg Sanyo Eneloop and GP ReCyko. Also now many more.

For general use the GP ReCyko are excellent. I have not yet found data on low temperature operation but I guesstimate that, based on other NimH data they'd be OK to say -20C at a sensible derating of their capacity - say 33% of nominal.

LiFePO4 are usually specified as operating to -20C. They will have substantial capacity loss at this temperature. Here's one manufacturers example. Reputable manufacturers are generally happy to provide detailed data to genuine inquirers.

enter image description here Capacity loss That graph is from Hi Power group which seems to be a typical Chinese manufacturer. All such information should be regarded as a starting point and "due diligence" is definitely required for anything regarding batteries.

GP rate their NimH batteries as operating to -20C. I have no data on this but its probably available. Here's a sample GPn1500 mAh NimH datasheet I'd expect lower capacity batteries within a given size range to have somewhat improved low temperature operation all else being equal. eg 1500 mAh AA better that 2500 mAh AA. But the increased initial capacity may cancel this out. (Larger capacity batteries squeeze in all possible active material at the expense of electrolyte volume etc).

You can get NimH and NiCd in special low temperature versions. Here are some examples from Lionik battery Co another typical looking Chinese manufacturer.

You will be able to get US branded and sold low-temperature batteries. These will almost invariably be Chinese made. Choosing a reputable US label gives you some confidence (or hope) that they have done the due diligence required to ensure that claims meet reality.

Here's a somewhat informal comparison of LiFePO4 with 4 other Lithium battery chemistries. Note that in 3 cases Tmin is given as -20C. Proves little but worth noting.

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  • \$\begingroup\$ Not all US branded batteries are Chinese made. A123 makes top quality well-specified batteries right here in Massachusetts. \$\endgroup\$ – Olin Lathrop Jul 27 '11 at 23:10
  • \$\begingroup\$ The solar idea seems appealing; the disposable alkaline economically and usage pattern pragmatic (though I hate the idea of throwing things away). Most importantly, I think the base station should treat the failure of any field sensor to report as an alarm condition; possibly over-ridden by some sanity rules (ie, if it's far above freezing according to the other sensors and its the middle of the night, you can wait to alarm about the failed communication until morning). Reporting might ordinarily include battery voltage so you can warn about those that are getting marginal. \$\endgroup\$ – Chris Stratton Jul 28 '11 at 7:50
  • \$\begingroup\$ My main experience is at the hot end :-). Above I allowed a degradation of 3 to 4 times over nominal capacity. How does that compare with your experience? What if you had a steady trickle from a PV panel as described above? \$\endgroup\$ – Russell McMahon Jul 28 '11 at 8:47
  • \$\begingroup\$ Thanks for the analysis. I have a lot of experience (measured and real use) with NiMH low self discharge batteries. In my experience the performance of NiMH around 0 deg C is terrible. I think I will go for A123 LiFePO4 with a solar panel, or Lithium AA. The datasheet of the A123 bit.ly/n5NwZE indicates a V range of 3,6 - 2V @ 25 deg C and 3,8 - 0,5V (!) @ 0 deg C so I will need a DC-DC. Does anybody know a simple dual source LiPo charger like the MAX1555 or LM3658 which is compatible with a small solar panel? I'd like to know the chip on this bit.ly/fvK1HZ \$\endgroup\$ – i.amniels Jul 28 '11 at 9:08
  • \$\begingroup\$ During the day temps will be around 0 - 15 deg C (more likely towards the higher end). The real problem is at night when the temps are around -10 - 7 deg C. No solar power then. \$\endgroup\$ – i.amniels Jul 28 '11 at 9:10
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Several things. 6 months is not a long time for self-discharge for most batteries except NiMH. If this is really out in a field, then you should be able to extend the life significantly with a small solar cell.

Some flavor of lithium battery should be able to do this. If you really only need 6 months life, then a primary battery can do this. We just went thru a similar concept-stage analysis and decide that our gizmo could be powered for about 1 year from two AA primary cells. They would power up less frequently and send data over a 802.15 radio link to report on hydrogen sulfide levels eminating from a landfill.

With a solar cell, and oversized battery, the device should be able to live without intervention for the service life of the battery. 6 months actually sounds like a rather short time for something like this.

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one advantage of the LiFePO4 chemistry is its ability to work at lower temperatures than many others. Ever try to use a digital camera with NiMH at 32F/0C? Or start a car (lead acid) below 0F? All chemistries will suffer in some aspect of their performance, but I believe LiFePO4 to be one of the better ones at low temp. That said, with your device plan, you should not really need to draw enough power for any of that to matter much.

I have exactly this sort of sensor arrangement. My device has temp, humidity, baro sensors and an anemometer, which are connected to an AVR and use RFM12B to transmit indoors. Even when the RFM12B is on all the time, the rig draws less than 40mA. I have it sleeping such that the entire device consumes a little less than 2mA most of the time sleeping. It runs on this (chinese) Tenergy LiFePO4, which is recharged by a small solar panel. I realize now that you said rfm12b*P, with higher transmit power. That's different, but not much; you should not have the radio on very much, and should be able to get your power budget very low.

This last winter we had several nights in a row with lows of -18F (-27C). My kludge faithfully reported its data (including nominal battery voltage being constant) without a blink.

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