John N:
Regarding point one, I do agree with you. So let's look at what the capacity might be closer to for the current and power drain we are planning. For this, I'll like to refer you to the same data sheets from the Energizer website. For convenience, these are the links
for the CR123 and
for the Lithium AA.
If we only look at the description at the top right of the data sheet, we will get those fantastic values, which are for conditions nowhere near our actual. However, there are tables lower down on both data sheets that show data for higher discharge rates. The EL123 table is titled "Simulated Application Test". (This does make me wonder how was it simulated exactly. Was it actual discharge rate to simulate a device drawing that much current, or was it merely data extrapolated from lower discharge rates to indicate how long a higher current might be sustained?) Similarly for the L91 which has two tables of interest, one showing "Constant Current Discharge, Typical Service", the other showing "Constant Power Discharge".
If the EL123 actually does perform as the table shows, then it provided 1000mA for 1hr. That is 1000mAh.
Similarly, for the L91, it delivered 1000mA for 3hrs, that is 3000mAh. The constant power discharge table shows only 2hrs for 1500mW drain, most likely because constant power means that as voltage dropped, current drain would be increased.
So it seems that for 1A current drain, the EL123 is 500mAh or 1/3 off the mark, whereas the L91 seems to still be on the money. Even at 1900mA drain (which is as high as the table shows) the L91 is still pretty close to the mark, running for about 1.5hrs, which gives 2850mAh.
And yet, this is not exactly earth-shattering news, since we knew that the closer the discharge rate is to the capacity of the cell, the less power we would be able to extract. For the EL123, 1000mA is 2/3C. For the L91, 1000mA is 1/3C.
And yet, after all that, we also know that the EL123 only needs to provide half the current of the L91 to deliver the same power. Thus, it would only need to provide 500mA instead of 1000mA. That is 1/3C. If all the relationships hold true, then we can expect the EL123 to perform in a similar fashion to the L91, which means that it should deliver close to its rated capacity. (And 500mA vs 1000mA is closer to what we we looking at to provide 1200mW from either cell.)
It's a pity that the power and current discharge rates which are of interest to us are not explicitly stated. All we can go on is extrapolation and conjecture, which we know may actually be quite different from reality.
However, the calculations in my above post should hold quite true because we are looking at ratios and relationships in 2 cells of similar build and chemistry. Thus the internal resistance should start at a similar value and increase in similar proportion. The current in the L91 will be twice that of the EL123, so losses due to internal resistance should follow the ratios mentioned above.
At the end of the day though, it will probably be the boost circuit that determines the majority of power loss. And generally, it seems that boosting from a lower voltage source tends to be less efficient.
Of course, as you have pointed out too, the cells in question are actually from Batterystation, not Energizer. We don't have enough experience or performance data from this cell manufacturer and all we have said so far is mere speculation.
If it performs as well as the Energizer stuff, which it doesn't always seem to be, all is well. But if it doesn't, then all the above exposition is wasted. Then, enough hot air from me! /ubbthreads/images/graemlins/grin.gif
