Nikon SB-500 AF Speedlight - Manual (Page 30)

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Operation
Basic Operations
Replacing/recharging batteries
Refer to the following table to determine when to replace batteries
with fresh ones or recharge batteries according to how long the flash-
ready indicator takes to light up after turning the SB-500 on or flash
firing.
1.5 V LR6 (AA-size) alkaline battery 20 sec. or more
1.2 V HR6 (AA-size) rechargeable Ni-MH
battery
15 sec. or more
Low battery power indication
When battery power is low, the
flash-ready indicator flashes repeatedly
twice per sec. for approximately
40 sec. Replace or recharge batteries.