If the 21700 is really 70 mm long, yes. My C4 can handle cells up to 70.5 mm long. My C4-12 however can take cells up to 73mm long.
(Edited 2/22/18, took actual measurements by stacking magnets on batteries rather than using a ruler, C4 lost 0.5mm length capacity)
Being able to adjust the new 2nd generation C4 can be a double-edged sword.
My old 1st gen C4 always charges all the batteries in a “set” at the same rate, so they all finish at pretty much the same time. With only two charge rate choices (350mA or 800ma), it doesn’t have any trouble picking the appropriate one from that short list.
However, since my new 2nd gen C4s can pick charge rates from 100 to 800mA (apparently in 50mA increments) its chosen rates may differ by 100mA or more among the four batteries in a “perfectly” matched/equally discharged set.
For example, earlier today I charged four Eneloop Xs and the automatically selected rates varied from 600 to 750 mA, so there probably would have been more than the usually expected difference in finish times. I forced all of them to a reasonable and leisurely 700 mA, and don’t really consider this worth worrying about, just an observation.
Slow charging shouldn’t be a problem with Li-Ions, BUT charging AA NiMH batteries at much less than ~300 mA can make the Delta V termination point awfully hard for some chargers to detect, which can result in cooked batteries. I haven’t tested it on this one and don’t plan to, because I’m not HKJ! This charger looks like it will allow some inappropriate rates to be forced, one just has to know enough not to go there.
I am having a similar issue with a C2-6000 charging new protected 18650 ga&bs. Picks a very low rate for some reason and does not boost it much .
Any plans for a coupon for the C4-12 charger Leo?