Any commercial Li-ion charger capable of charging the industry standard 18650 cells can be used.
BioSemi uses this charger: Nitecore, Ci2

With the Nitecore Ci2 connected to a low power USB charger (for example an USB charger of 5W), then the charging time will be about 4 hours.
With the Nitecore Ci2 connected to a high power USB charger (for example an USB charger of 18W), then the charging time will be about 1,5 hours., then