Speedlite & Battery Pack

The INTERNAL batteries perform a number of functions on the speedlight:

• Power the zoom head.
  
• Power the LCD panel.
  
• Power the control buttons including on/off.
  
• Power the trigger circuit.
  
• Charge the capacitors to ready the unit to actually flash.
  

Typically, with a 4xAA flash unit, the whole of the circuitry is designed to work at 4.8v to 6v power supply. The EXTERNAL power port, on the other hand, is designed to work at something in the region of 300-360v (depends on the flash unit make) and it is that increase in voltage which gives the faster recycling. In very simplistic terms, the external power port bypasses the upscaling part of the flash unit circuitry and applies a high voltage directly to JUST the capacitors. You still need the <=6v power source to make all the other functions work - including that on/off switch! Using C.Fn on certain flash units, you can configure to draw recycling power from ONLY the external power pack rather than both but you do still need the internal AAs for the control functions.

As David says, it's perfectly possible to 'hack' a the battery compartment via the dummy battery / external daisy-chain of cells route and get the increase in number of flashes. BUT, that daisy-chain must be <=6v as it is feeding into a 6v capacity circuit. If you simply grabbed one of the many power packs designed for application to the EXTERNAL power port, cut off the plug and substituted your dummy batteries feeding into the INTERNAL circuitry, your flash will go BANG. It will only do it once. It will then be a †† deceased †† flashgun/speedlight. So that's a US$500 flash wrecked to try to save US$5(?) on AAs - not sound economics!

Bear in mind that the hack option is literally a HACK as you either have to butcher the battery compartment cover or leave it open to get the cable in from your daisy-chain of cells totalling <6v. Inelegant and not really a viable option, appealing though it may at first seem.

If I remember correctly, Metz units used to (dunno if this still applies) work without internal batteries when used with Metz's own P45/50/70 power packs. There may be others which can be 'adjusted' to work this way - but it seems an awful lot of fuss simply to save the cost of those 4 internal AAs. With a perfectly standard external power pack plugged into the external port (giving you enhanced recycle times AND greater number of flashes because of the increase in capacity (mAh) of a greater number of external cells), the drain on those 4 internal cells in pretty small so the expense of replacing them is only going to be occasional.

The whole 'hack' idea seems to me to be a lot of messing about / ruining any 2nd hand value of the unit / compromise for no real benefit. Like others here, I would strongly recommend either the Pixel TD3xx (Elv's review on FlashHavoc.com) pack with it's dual-bank design (pretty fail-safe, unlike the single-bank units such as the CP-E4 et al) if you want to stay with NiMH cells throughout. Alternatively, the terrific Godox PB960/Cheetahstand L4500 (same product, different name on the side) lithium pack. The Pixel is cheaper, the Godox/Cheetah is rather more bombproof for those who delight in abusing equipment.

You're very welcome. Take a look too at the Eneloop XX (XX = the Adult version :oops

• 2500mAh instead of 2000mAh, so 25% more storage capacity.
  
• LSD design again but not quite as long lived as the vanilla Eneloop.
  
• The main compromise lies in the life expectancy - 500 cycles claimed instead of the 1800 cycles claimed of the Mk 3 Eneloop now available.
  

The biggest problem I 've found with Eneloops, of either kind is that they do tend to spoil you a little for any other make/type. I occasionally get different brands (eg "FREE" ** with another charger) and they have never failed to disappoint by comparison. Hence I have quite a few non-Eneloops which are just parked on my desk doing nothing - then when I do need them for something, they've self-discharged! Dunno why I don't just throw them at someone I don't like - let them suffer instead.

** I've usually found that these 'free' accumulators are worth just marginally less than I paid for them. Which was nothing, so .....