Originally Posted by
Manfred M
That being said, the real limitation on small flash is the power supply; i.e. the AA batteries you put into the flash. Flash power is related to three factors; the voltage, the current draw and the duration of the flash . If you look at studio flash units, these are generally rated on the power that they draw and these are rated in W-s.
AA batteries nominally have a voltage of 1.5V and, for example, the popular rechargeable Panasonic Eneloop batteries can deliver 2000 mAH. If you look at the Godox flash I mentioned in #2, it's battery delivers 11.1V and 4500 mAH (effectively over four times the output of a Speedlight. The Elinchrom ELB 500 TTL that Bill mentions in #4 delivers even more power.
If you add an external battery pack, these do not increase the voltage, but rather the current that can be delivered, so cycle time will decrease, but the power output will not. The speedlights are designed around the power that a AA can deliver, so the capacitor used to fire the flash tube is limited by this design factor. Adding more batteries in series is not going to overcome the charge that the capacitor can hold; all you are doing by applying an over-voltage is to charge it up more quickly; add too much additional voltage and you risk damaging the flash circuitry, without any output power gain.