I wrote this email on 6/14/13:
I think the best way to measure power consumption is to implement the Linear Technology fuel gauge / coulomb counter IC that we have on TS05b. Once we know the number of coulombs that have been consumed then we have to look at the current draw to de-rate the battery because of the power lost through the battery’s internal resistance. Hopefully battery life will be long enough that it’s not practical just to wait for the battery to run down to see how long it lasted (hopefully it will be many months).
I want to look into using a regulator to draw 0.19mA from the coin cell for longer periods to charge a big capacitor so that the capacitor is what powers devices when they come on. There would have to be a second regulator to buffer the supercapacitor output. It’s hard for me to believe that somebody isn’t already making an all-in-one IC to do this but I can’t find it. There are several very interesting energy harvesting IC’s from Linear.
This device might work really well: http://cds.linear.com/docs/en/datasheet/3105fa.pdf. The drawback is 80% efficiency at 0.2mA which might approach the loss from the battery’s internal resistance, but having regulators will make the design a lot more stable than just running off a battery, and the battery can be used even when its voltage gets very low. This device has a power adjust pin (MPPC) that can be used to prevent too much current from being drawn from the battery. The boost voltage would be set to maximum (5.25V) to charge a supercapacitor and then that voltage would drive a buck-boost regulator (maybe http://cds.linear.com/docs/en/datasheet/3534f.pdf) to output the system voltage (maybe 1.8V or 2.5V). Since the LTC3105 will work with an input as low as 225mV we can completely drain the coin cell. The absolute maximum input voltage of the LTC3105 is 6V so putting two CR2032 in series (to increase capacity) is questionable. At 20 degrees C the output of a CR2032 is 2.9V but at 60 degrees it is 3.2V.
Or maybe it’s best just to put a big ceramic cap across the battery to minimize pulsed current and have no regulator. That’s probably what everybody else is doing.
Now there are more energy harvesting products from LT. Here’s the portfolio: http://www.linear.com/parametric/Energy_Harvesting
Article: How much energy can you really get from a coin cell?
The article references this white paper from TI entitled Coin Cells and Peak Current Draw: http://www.ti.com/lit/wp/swra349/swra349.pdf
I assume for now that the way to achieving the longest battery life possible is to extract the most energy possible from the battery, and therefore the highest priority is to limit the amount of current drawn from the battery. The regulator chosen must limit the current drawn from the battery to 200uA or less, but must be able to deliver 300mA in short bursts.
The acronym to look for when selecting a device is MPPC – Maximum Power Point Control. To use the MPPC feature in a regulator, I think the coin cell must be used as the the energy harvesting source. The “battery” powering the regulator will be a large capacitor (“supercap”).
To use an energy harvesting source, such as a thermo-electric generator (TEG), multiple regulators would have to be used.
TI bq25504 (datasheet http://www.ti.com/lit/ds/symlink/bq25504.pdf) looks to be a flexible device. The startup voltage is 330mV but it will run down to 80mV. However, it can only supply about 80mA. A capacitor on the output might be able to absorb the 300mA spikes.
TI paper: Indoor Light Energy Harvesting Reference Design for Bluetooth® Low Energy (BLE) Beacon Subsystem. It uses a bq25505 and CC2541.
Papers that might contain relevant information:
Development of a Wearable HRV Telemetry System to be Operated by Non-Experts in Daily Life
Using a Supercapacitor to Power Wireless Nodes from a Low Power Source such as a 3V Button Battery
Parallel Battery Configuration for Coin Cell Operated Wireless Sensor Networks
A 700-uW Wireless Sensor Node SoC for Continuous Real-Time Health Monitoring
Battery Capacity Measurement and Analysis using Lithium Coin Cell Battery
Ultra Low-Power Smart Medical Sensor Node for In-Body Biomonitoring
Power Management Subsystem with Bi-directional DC to DC Converter for μ-Power Biomedical Applications
To be continued…