It's imperative you know what chemistry those cells are, and the output voltage of the charger before buying ANY other cells to use in it:
A few words of wisdom before any money is wasted:
Their are currently 3 popular configurations of Li-Ion RCR123 cells and chargers:
1. LiCo02: the "standard" in Li-ion, charges to 4.2V, nominally rated at 3.7V, discharges to about 3.0V under a load. Must have charger terminated at 4.2V for a full and safe charge. Label ratings range from ~550mAH to ~1000mAH but anything over ~650mAH is an inflated claim with no basis in reality, so buying a "900mAH" cell is not going to give you anything that a "750mAH" cell doesn't have.
2. LiCo02 + Voltage regulation: These are often sold as 3.0V cells, but are in fact 3.7V cells as described above with a built in voltage regulator to pull the voltage down to come close to matching that of CR123 primary cells. These cells require about a 4.4V charging voltage to overcome the voltage regulator in reverse. It's so important to NEVER put a regular LiCo02 cell in one of these chargers as it will overcharge it. You could put one of these cells in a standard 4.2V charger, but it would only receive about an 80% charge. Label ratings are all over the map on these, but in reality, you will never get much more than about 500mAH of usable power out of them after the lost space from the regulator, and the lost efficiencies from the regulator.
3. LiFeP04: This is a completely different internal chemistry, they have a charge voltage of about 3.6-3.8V depending on the manufacture, but they are pretty tolerant to slight overcharging, (so a 3.8V charger would in most cases be fine for a cell that is rated for 3.6V charging, and vice versa), they settle to around 3.4V and run around 3V into light loads, they are "dead" around 2V. Label ratings range from about 400mAH to about 800mAH but in reality, they are all about 300-400mAH regardless of label ratings. A charger designed for these cells will not hurt a LiCo02 cell, but it will not charge it fully, (in fact, it will only charge it to about 20% give or take).
So. it's very important to match the right cells to the right charger, in some cases, for safety, in others, simply because it won't be charged all the way on the wrong charger.
Eric
