Summary of tests, see discharge curves below Charge voltage Capacity at 3.65V 3.0V 3.1V 3.2V 3.3V 3.4V 3.5V 3.6V 3.7V 4.2V K Series 2249 mAH 48 mAH 93 174 691 2222 2232 2249 2249 2249 mAH Percentage of 3.65V capacity 2% 4% 8% 31% 99% 99% 100% 100% 100% G Series 2310 mAH 47 mAH 126 168 703 2257 2293 2309 2310 2324 mAH 2% 5% 7% 30% 98% 99% 100% 100% 101% A Series 2136 mAH 57 mAH 101 173 543 2044 2127 2128 2136 2186 mAH 3% 5% 8% 25% 96% 100% 100% 100% 103% H Series 2236 mAH 0 mAH 19 60 440 2153 2175 2196 2236 2238 mAH 0% 1% 3% 20% 96% 97% 98% 100% 100%

Commentary. I include the lithium ion results here to compare. As you can see the iron phosphate results also show an increase in the capacity with charge voltage, but there are some interesting differences. First, the charging starts at a lower voltage than lithium ion, with some charging starting as low as 3V. Second, there is significant charging at 3.3 volts, which opens up some applications for battery backups in 3.3 volt systems. Third, there isn't much difference in capacity when charged at 3.4V or 4.2 volts. Maybe 3% maximum. So surprisingly you can charge one of these cells at 3.4V and get almost the same capacity as charging at the recommended 3.65V.



*Note: Charging above 4.20 volts is bad for the battery cycle life!



Commentary: It looks like the magic number is around 3.3 volts. Below that you don't have significant charging, above that you do. A lithium iron phosphate battery doesn't care if it is never fully charged, so if all you have available is 3.3 volts and you don't mind the loss in capacity you could use the 3.3 volts. This opens up new possibility for a simple but very long life backup battery in 3.3 volt systems.



It looks like the A series cells definitely have some lithium cobalt mixed in with the iron phosphate because when it is charged to 4.2 volts there is higher voltage at the beginning of the discharge, whereas the others don't have any energy stored between 4.2V and 3.6V. You can also see this effect at the end of the discharge curve where this added capacity at the beginning of the discharge curve shows up in pushing the end of the discharge curve further out.



These curves show the results

for the 4.2V lithium ion chemistry for comparison

