Early on I figured I'd need to test some CR2477 cells to see how much energy they could practically supply in a short time. I finally got around to building the tester and putting it to work (not to spoil the surprise, but things don't look great so far). Here's the tester:
The circuit is basically what I described in an earlier log, except with a few R's and a C added for stability.
The first low-offset op-amp I found in the parts bin was an MCP6V01 chopper. It's overkill for this application but contributes a ridiculously small error to the result. The cell is discharged at a constant current set by the potentiometer, while serial-port-enabled DMMs log the current and cell voltage (not shown). I didn't have any 1% 10-ohm resistors with a decent power rating, so I paralleled 10x 100-ohms. The MOSFET and op-amp are mounted on #Ugly SMD Adapters.
My initial plan was to discharge the CR2477 at a constant 40mA. This represents a C/25 discharge rate, which is completely reasonable for any battery other than a coin cell. For the CR2477, it turns out, this is an enormous drain that wastes most of the energy in the cell. Here is a plot of the voltage during the 40mA discharge as well as the total energy (in this case dissipated as heat in the 10-ohm resistor):
After 10 minutes of discharge, the cell voltage has decreased below 1.5V, which I think is as low as I can reasonably expect to go using a PIC12LF1571 in the DC-DC converter (the PIC is specified down to 1.8, but I will push it). At the 10-minute mark, the energy extracted from the cell is around 55J. Terrible. Integrating the current to this point yields 6.6 mAh, which is 0.66% of the cells capacity. Awful.
I let the drain continue for about 9 hours, even though after 25 minutes, the cell voltage dropped below the 400mV required to maintain a 40mA discharge in the load resistor. At the end, 157J had been extracted. Ghastly.
Lesson: don't discharge CR2477 cells at 40mA.
What about 20 mA?
Undaunted, I discharged a CR2477 cell at 20mA (note the different time scale).
In this case, around 260J and 35 mAh have been extracted from the cell before the voltage drops below 1.5V (lousy). This 5x as much as the 40 mA discharge, but still represents a small fraction (3.5%) of the capacity of the cell. Atrocious. When I finally stopped this test after about 8 hours, 392 J had been extracted from the cell. Dreadful.
Lesson: don't discharge CR2477 cells at 20mA.
Are you seeing the pattern yet? Lower currents will be required to extract more energy from the cell. Of course, these discharge curves don't come anywhere near the 25-hour estimate I originally used for charging (the 20mA discharge is essentially done in 3 hours). Still, the total amount of energy I have been able to obtain from the cells is very low. Abysmal.
What I Got Wrong
So, in my initial estimates of current drain, I assumed that a 1000mAh CR2477 was like a 225mAh CR2032 scaled up by 4x, so I could draw 4x the current (I found data fro CR2032s on the web). The differences between the cells are more subtle than that. The maximum current you can draw is proportional to the electrode area, and assuming the electrode size is proportional to the cell diameter, a CR2477 only has 24^2/20^2 = 1.44x the electrode area. The total amount of reactive chemicals inside may be 4x, but the maximum current you can reasonably draw is likely to be around 1.4x.
I also based my initial estimates on numbers in this article, which says you can use 88% of the available energy in a CR2032 when draining it at 10 mA. Unless CR2032s are much better at higher currents than CR2477s, I just don't see it. I should test a CR2032 for comparison.
There is a trade-off between the length of time taken to drain the cell and the self-discharge of the supercapacitors. The longer you take to drain the cell, the more energy you get, while the more you lose due to self-discharge in the cap. Somewhere in-between, there is a maximum amount of energy you can get into the capacitor.
I'm currently running two more tests. First, discharging a CR2477 cell at 10mA. So far, this is looking better, already having extracted 845 J in about 10 hours, and the voltage is still about 1.5V.
The second test is to get a better estimate of the self-discharge of the 400F supercapacitors. I charged one to 2.33V, and am logging the voltage as it self-discharges. At first, the selft-discharge was around 5.5mA, which is absolutely terrible, but it is dropping steadily now. This is probably due to some of the charge being a surface or shallow charge in the capacitor.
Data from these tests should give an indication of the optimum duration for charging the capacitor from the cell, and how much energy can be expected.
Bootstrapping the PIC
One more idea occurs to me. It seems that under "heavy" drains like these, there can still be a significant amount of energy left in the cell even when the voltage drops to 1.5V or lower. So, why not have the PIC bootstrap its own power supply? Once the supercapacitor is charged to more than the cell voltage, the PIC can be run from the output voltage (a pair of BAT54 Schottky diodes can switch the PICs Vdd). A low-quiescent regulator might be necessary, but the scheme should work, and will allow more energy to be drained from the cell.
Bootstrapping also gives me an idea for running TritiLEDs from single AA cells (lithium or otherwise)...