An important influence on puck design (especially considering desire for low profile) is the type of battery we use. Currently we have been using Duracell Alkaline AA with a capacity of 1500mAh (the iPod mini batteries have 850mAh capacity). We expect the new puck to draw power at a faster rate than the old puck due to the force sensitive resistor interface. It will be important to know the average current draw rate of the new puck so that we can weigh that against battery time capacity and weight/size. A very useful metric would be battery energy density in terms of volume (eg, mAh/cm^3). I don’t think that this property is already tabulated somewhere out there, but if we know what kind of time capacity we would like our batteries to have (and the rate at which the batteries are drained), we can start narrowing down choices and get a general idea of what size options we will have.
I have done some quick calculations to get us in the ballpark of how fast we will be using juice in the new puck:
The mote user guide lists nominal and maximum current draws for a number of states — the three that of are interest to me are “MCU on, Tx/Rx,” “MCU idle, radio off,” and “MCU standby.” This information is available in the user guide, but I have made the following assumptions:
- The mote will draw 8.5μA (or 100μA, depending on what “sleep” means…)
- The mote will draw 21mA when it broadcasts
- The mote will be awake for 500ms when it broadcasts, 100 times a day
- The FSR conglomeration will idly draw an average of 1.5μA
I say that the mote has a duty of 500ms*100/(24*60*60 sec/day)=0.0006, so the average draw of the mote is Duty*(Tx/Rx current)+(1-Duty)*(Sleep current)=21μA. The time capacity (how long it takes for the batteries to run out) is given as Battery Capacity (mAh)/Current Draw (mA). With an iPod mini battery, this comes out to 850mAh/(21+1.5μA)=about 4 years. Maybe sleep is actually “MCU idle, radio off” for a draw off 54.5μA nominal (rounded out to 100μA just in case), in which case the time capacity is about a year. So there’s something not quite right here. Likely as the battery nears the end of its capacity, it’s voltage will drop, rendering it unusable. Maybe this method of calculation is too rough. But I’m still at least an order of magnitude off.
Anyway, as we prototype the FSR interface circuit, it may a good idea to get an empirical measurement of the rate at which the whole thing will draw current.
We just measured a test circuit for the FSR and saw that it drew 850 microAmps when idle. Tim redid the calculations and found that it will last 40 days and 40 nights, which in the bible means a long time.
Comment by gthomas — April 22, 2010 @ 5:02 pm
Geb and I tested a puck (1st gen) today and found that it actually draws around 20 micro amps in “idle,” and (a max of) about 2 milli amps when it broadcasts. This behavior was a bit strange though: immediately after the mote is powered it draws about 25uA and then settles to about 23uA after a second or less; sometimes, the mote would settle to about 5uA, sometimes it wouldn’t. To be conservative, I will estimate 20uA average idle draw. Also, the broadcast duty is probably on the order of tens of milliseconds. To be conservative, I will still estimate the duty at 500ms, 100 times a day. Under these calculations, the average current draw of the puck and the FSR together is 871.2uA. To achieve one month capacity, we require a battery capacity of about 630mAh.
Comment by tdecker — April 27, 2010 @ 4:05 pm
[...] 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. [...]
Pingback by New puck power / batteries « Hand Hygiene — April 27, 2010 @ 6:10 pm