As others already pointed out, it uses Internal Resistance test to determine charging current. And there in lies the problem. In my experience, C4-12’s internal resistance results are somewhat inconsistent , which sometimes leads to questionable charging current rates. But for the most part, it is programmed to err on the safe side. So if you let it choose charging rate automatically, you may end up with very long charging times now and then. But you can always manually set the charging rate if you need it charged faster.