A battery test was done using Ted’s new code for the pagers. The following table and graph summarize the data.
Another test was also done with a pager connected to a voltage source to find out the relationship between the battery reading broadcast by the mote and the actual battery voltage. The results are shown below.
As the trendline indicates, the battery reading is about 600 times the battery voltage. The voltage that is acutally being measured is 10/26 of the battery voltage (not 1/2, as it used to be). The code should probably be modified so that the motes shut off when their batteries are around 2.7v, or a battery reading of about 1700.
Here is my question now. Your experiments show that readings of ADC0 will be between, say 2622 and 1604 (but probably there is some variation beyond those figures). Now, reading the internal voltage — the old code — is still meaningful for other, conventionally powered motes using NiMH or CZn types. I suppose (but could be wrong) that the internal voltage measures on a 2.5v reference scale, so that we should expect to see numbers ABOVE the high end of your experiment, since 2622 would correspond to 1.6v on a direct, 2.5v scale (where 4095 is max possible reading, indicating 2.5v or more). My point is this: suppose you get a mix of motes, some from ADC0 and Li-Ion and others from internal voltage only, but you don’t know which is which (later I can explain why you would not know this). Do the numbers indicate we can actually make a good guess about which type of battery/ADC reading is being used? If the answer is yes, code for power monitoring can be simplified.
Comment by tedherman — February 18, 2010 @ 4:22 pm
I’m sick today and not firing on all cylinders. Gregg and I reread your post and are having trouble understanding what you are suggesting. Here’s how I interpreted it:
The old code reads substantially higher ADC values than the new code. We could set up the ADC link with the new code and if we don’t get a reasonable reading (e.g., something between 1600 and 2600), we could that the mote is one of the old ones without a proper voltage divider. In that case, we could run the old ADC code and see if we got some value between 2600 and 4000, indicating a valid internal voltage. We could then monitor the internal voltage to shut the old-style pager off before it runs out power.
I’m pretty sure that I don’t have your idea quite right, but hopefully I’m close.
However, we think that the old code wouldn’t work very well without the voltage divider. When we tested the pagers initially, we discovered that the voltage values were out of range — we thought they were on a 5V scale and they were really on a 2.5V scale. We changed the resister values in the voltage divider and got ADC readings that were in range. Therefore the resister values affect the reading that you’re calling the internal voltage. This result gives me a queezy, cognitive dissonance feeling. I haven’t be able to grasp why something called internal voltage should depend on a wire connected to an external voltage. I have a vague idea that is might have to do with a relative measurement on a mote that is designed to survive with a wide range of external voltage sources, but I’m really not sure.
Just to confirm that we’re not missing something important, Gregg is going to run a test looking for the ADC0 levels on a mote without a voltage divider or anything connected to the ADC0 pin. We’re thinking that he will just see noise.
Comment by gthomas — February 19, 2010 @ 11:12 am