mAh, Eneloops and battery packs.

Started Oct 31, 2013 | Discussions thread
WFulton Senior Member • Posts: 2,697
Re: mAh, Eneloops and battery packs.

PP2K wrote:

This may be a silly question, but it's something that's been bothering me for a long time. Mind you, my knowledge of watts, amps and volts, is miniscule. For me, it's always been enough just to know how many pops a given battery can give me.

So, I do some of my stuff with speedlights and been looking at these external battery packs. Let's say, I put 8 AA Eneloops inside my speedlight, 1900 mAh each, and they give me, say, 250-300 full power pops. Then there's this external 2000 mAh pack, that gives 320 pops. Then there's another 4500 mAh pack that says 1800 pops.

Ahem... 8 x 1900 is 15 200. And why does the second pack advertised, give so much more pops than the first one? What's the math here, between mAh's and volts and all that shizzle?

We don't have any clue what flash and what external power pack we are discussing... It may have more battery cells in it?

Four AA NiMH cells ought to give 150 to 250 full power flashes, but of course it will depend on how much power the flash can use. With the same batteries, a small flash can do more of its full power flashes than a bigger flash will do. The purpose of the larger flash is to use more power in each full power flash.

Putting four cells in series to be 6 volts does multiply the voltage, and does leave the mah capacity unchanged, but it also delivers 4x more power (than one cell). Power is voltage x current.  Capacity is current x time.

The typical flash uses four AA cells, but they have an internal power converter to raise this to be 300+ volts to charge the flash capacitor, which drives the flash tube. The recycle time is the time to recharge that HV capacitor. High current ability of the NiMH is a big plus there.

The 1900 mah is a total capacity, milliamp hours, not a rate. The bigger speedlight flash will draw 6000 or 7000 ma  for the few seconds that it is recharging the capacitor. Then it eases off to vastly lower current, to only keep it charged, and to power the other electronic chips.

The external power packs are also power converters, outputting the 300+ volts direct to the capacitor through a special port for the purpose.   Around 325 volts is common.

The overall number of maximum flashes depends on the power of the flash, and also depends on how much capacity it can get from the number of cells attached.  More cells is more power.

Only if it is the same flash and same conditions and same number of cells, then the mah number can be compared more meaningfully.  But it is a capacity, not a rate.

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