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In that case the approxmiate time will be
T = 2.4 AH / 0.85 A
T = 0.82 H
T = 0.82 * 60 Minutes
For perfect charging you should use chargers which works on positive or negative delta peaks of battery voltage and turn tehm off when battery is fully charged. One such charger IC is MAX712 from MAXIM (www.maxim-ic.com)