Hi,
The Eneloops come precharged and have a very low self discharge rate compared to regular rechargeable batteries but it's always wise to top them off for heavy duty work like flashes. When batteries deplete so will their voltage from 1.45V on full to a 0.8V when empty, so high power applications benefit from full batteries
There are many good chargers out there but need a little more info to provide a solid advice like the use of li-ion 18650 / 16340 batteries, the need for charging in a car or by using solar panels or power banks, etc. How many batteries do you want to charge at once, 2, 4, 8?