While a number of manufacturers have stated that the minimum charge rate be greater than 0.5C to generate a strong enough signal for chargers to detect and stop charge, there was a point raised that it should be a "fixed" number or fixed current level...especially for smaller capacities, see below:
If you look at the numbers, the AAA cell terminates well and is charging at 0.4/2000 or 0.2C rate. The AAA cells is charging at 175/800 or 0.218C or slightly higher but does not terminate.
I have another charger that shows exactly the same pattern with vibrant AAA eneloop batteries where the rate is 0.14C (175mA) and that batteries do not terminate, yet, the AA batteries at the same 0.14C rate do terminate ???
When I alter the AAA charge to about 280mA (increase of 170 mA), the termination is very reliable.
Based upon these empirical tests on a few chargers, I would suggest that a minimum current of 280mA be used for both AA and AAA batteries if smart termination is to be used. Out of curiosity, I want how low it can go ?
Last, as the charge current drops, I believe the termination algorithm has be that much better (than at higher currents). The units that terminate with the lowest currents should be the most tolerant of less than vibrant batteries.
One thing I'll note about the CEF21 is that while it appears to terminate the charge very reliably on AA cells at 400 mA, it frequently misses termination on AAA cells at 175 mA -- even on good cells like eneloops.
This is a shame, but it leads me to think that the C-rate recommendations for charging need to be modified with the battery size. Smaller cells like AAA need to be charged at a relatively higher rate, and larger cells like D need to be charged at a lower rate. For instance I don't think you should charge a 10 Ah D cell at a 1C rate of 10 A...
I have another charger that shows exactly the same pattern with vibrant AAA eneloop batteries where the rate is 0.14C (175mA) and that batteries do not terminate, yet, the AA batteries at the same 0.14C rate do terminate ???
When I alter the AAA charge to about 280mA (increase of 170 mA), the termination is very reliable.
Based upon these empirical tests on a few chargers, I would suggest that a minimum current of 280mA be used for both AA and AAA batteries if smart termination is to be used. Out of curiosity, I want how low it can go ?
Owners of programmable charge rate charges (eg. Maha C9000 or La Crosse BC-900) could easily verify the claim and perhaps find a lower limit by using the this current with a 10 minute temination test. A lot of La Crosse owners already know that default 200mA does not work for a lot users. Early Maha C9000 units were set to a 1000 mA default.
All things being equal, the smaller AAA battery should be able to terminate more reliably at a lower current level but I believe it does scale (with capacity) from the AA number.
Last, as the charge current drops, I believe the termination algorithm has be that much better (than at higher currents). The units that terminate with the lowest currents should be the most tolerant of less than vibrant batteries.