After making conclusions reached in my previous post on puck batteries (see http://groklab.org/handhygiene/2010/04/20/puck-current-draw-battery-capacity/ and its comments), I’ve taken a look at the battery selection from Digikey and Mouser. Here’s what I dug up (prices are per case of fifty batteries):
- Digikey PN P340-ND – CR2450, 620mAh, 24.5x5mm, $45
- Digikey PN P121-ND – CR3032, 500mAh, 30×3.2mm, $98
- Digikey PN SY340-ND - CR2450, 610mAh, 24.5x5mm, $58
- Mouser PN 658-CR2354 – CR2354, 560mAh, 23×5.4mm, $73
I’ve done a quick search for other suppliers and none can seem to beat Digikey’s prices. But these are standard packages, so I’m sure there is someone out there somewhere dying to sell really cheap batteries.
Two problems that we may run into with these types of batteries: internal resistance and standard current ratings. These issues are inter related. The former may shorten the life of the batteries by allowing them to dip below the mote’s threshold when it is broadcasting, probably after some of its capacity has already been used. Actual data is not spec’ed well in datasheets. The latter may imply that the battery chemically won’t be able to supply current at a fast enough rate to power the radio. Typical spec’s for standard current is around 0.2-0.5mA, while the radio seems like it draws around 20mA. It seems like a vague term, but I am guessing that this is the current draw that will provide nominal voltage and capacity. Overdrawing from the battery may cause voltage drop or decrease in life, or we simply may not be able to pull power out at that rate. Another related issue is that if the battery has a limit to the power supply, this limit might affect the radio Tx/Rx power, (thus RSSI?).
Deepti and I ran some quick tests: turns out that a CR2430 (3V, 0.2mA standard current) will run the mote, and it will allow the mote to broadcast (at least with the RadioExp program). We used the same mote, alternately with two AAAs, and with one CR2430 and compared RSSI at set distances. The first test was performed with the mote horizontal, and after rough (eyeball) analysis of RSSI data, the AAA gave RSSI of around -8, while the CR240 gave RSSI of around -20. The second test was performed with the mote vertical, facing the receivers. This test gave similar (around -9) RSSI values for both batteries. Hmm…. Our test setup was on-the-fly and not very thorough, so I am guessing that the discrepancy is due to poor setup. I will plan on repeating the test in the next couple of days with a better experiment.