Flash power can mean the battery source rather than intensity level. There are battery choices for speedlights, affecting recycle speed and capacity (number of shots). Recycle time is the few seconds that we have to wait for the flash to become Ready for the next shot. Recycle time is virtually immediate after low power flashes, but we have to wait a few seconds for a maximum power flash. In practice, we often are using a lower power level, and then many more shots are common. What requires greater flash power is greater subject distance (especially including bounce), greater f/stop number, or lower ISO. A good round number starting point for speedlight bounce flash (on camera when standing under ten foot white ceiling) is ISO 400, f/5, however flash model capability varies, and ceiling heights vary (eight foot ceilings need less exposure, twelve foot may need a little more exposure).

FWIW, here are approximate numbers I checked in a Nikon SB-800 speedlight, using a Fluke DVM to measure recycle battery current, and an iPhone video camera for the recycle time to the Ready LED (average of three tries, full power). I'm certainly not pretending any precision measuring the current amps for pulses, and any fast pulse is surely higher, but the meter still indicates a lot of current. It is not a constant value, it is a decreasing pulse (and allowing another second or two after the Ready LED can't hurt). But this chart is suggestive of the scale of things. The improvement in recycle times between today and the SB-800 manual in 2003 is the new Eneloops and Ultimates.

Battery	Recycle current	Recycle time
Energizer Ultimate Lithium	5.3 amps	5.0 seconds
Duracell Alkaline	4.8 amps	5.0 seconds
Eneloop NiMH	6.7 amps	2.9 seconds

Battery Type Choices

• Zinc carbon AA batteries (heavy duty) are the less expensive regular old type, which are the worst choice for a flash. They simply cannot supply the high current required by flash (slow to recycle), and in such use, they will not supply very many flashes before they go dead. Maybe OK for flashlights or mp3 players, but I would say "unacceptable" in speedlight flashes.
• Alkaline AA batteries are available everywhere. Flashes need high battery current, and while alkalines cannot match the rechargeables, the alkalines do greatly better in speedlight flashes than ordinary zinc carbon cells. At minimum, you want alkaline batteries in the flash. Alkaline recycle speed is slower than NiMH rechargeables. but faster than Lithiums (previously true, but new lithiums are more equal today). Current alkaline battery shelf life is said to be 10 years, so these can be good as backup spares in the camera bag, for just in case. Alkalines are NOT rechargeable.
• NiMH rechargeable AA batteries. Nickel Metal Hydride. Fastest recycle, because of lower internal resistance. These can be recharged hundreds of times, basically for free, however a good charger is necessary. NiMH rechargeables are extremely popular with active speedlight users.

  However, there are TWO types of NiMH rechargeable batteries today:

  Regular NiMH cells - These work great, and are a top-choice, except they self discharge when just sitting around, and will be near dead in a month or two, used or not. So these really must be recharged soon before any serious use. These regular type would be worst choice in something like a clock, or in the camera bag as backup spares (pointless where long life is needed), but they do work great and fast in flashes, if recharged recently, like at least this week.

  Newer type NiMH - which are labeled Pre-Charged or Low Self Discharge, or maybe Ready To Use. This means they hold their charge for a very long time. Ready to Use means they won't self discharge and run down sitting on the store shelf in a few months awaiting purchase. IMO, these are best choice for a flash. The packaging says you can use these right out of the package without charging first, because they don't lose their charge before you can use them. However, it won't hurt to charge them first, as the factory does not fully charge them, only to maybe about 70%.

  Eneloop%20batteries (originally Sanyo, now Panasonic and Fujitsu). Sanyo was the inventor and classic leader of this newer low self-discharge type, but there are several good brands now, licensed from Eneloop. These are the way to go today for flash (flash is a high current device, which needs more than alkalines). Another difference is that low self discharge types have slightly less capacity than the regular type (2000 mAh vs perhaps 2700 mAh), so the regular NiMH might be bigger for a big shoot tomorrow, but after a week or two, the Eneloops will win. Because (other than being NiMH), the really big deal is that these hold their charge a very long time (due to less self discharge). Eneloops say 70% capacity will remain after years of storage, so these could be backup batteries too (still probably always good to rotate your backup batteries now and then). If you want them to still be charged after a few months, this low self discharge choice is necessary. This is a huge plus if you only take a few flash pictures each week, and want them to still be good to go after months. NiMH batteries do not suffer the same "memory effect" as did NiCd, and while "recharge early and often" is a good thing for NiMH, Eneloops hold their charge well, and so don't really need it as often.

  Eneloops were previously branded as Sanyo Eneloop, until Panasonic bought control of Sanyo, and replaced Sanyo names with Panasonic. Panasonic Eneloops are now made in China instead of Japan. The Panasonic Eneloop info site. The internet story is that Eneloops were made in Japan by a company Sanyo Twicell, owned by Sanyo then. But governmental terms said the patents had to remain in Japan, which excluded Twicell from the Panasonic sale, and Twicell was instead sold to Fujitsu (in the name of FDK). Here is a record of that transaction. The FDK company history does also say they purchased Sanyo Energy Twicell in 2012 (timed with the Panasonic deal). And says that Fujitsu had been a 43.3% owner of FDK since 1965.

  The Fujitsu low discharge batteries are made in Japan, and have a very good reputation. Here is from the FDK website (but Fujitsu and Made in Japan are all over the page.) Panasonic bought the Sanyo name Eneloop, but Fujitsu bought the Eneloop factory and patent (called FDK now), so along with other companies, Panasonic must now license rights to make the new low discharge NiMH batteries from FDK. The internet notions now are that the Fujitsu low discharge batteries are actually the original Eneloops now? I think they are both good, but I might favor Made in Japan.

  You can find discussions online. Wikipedia says "Sanyo was acquired by Panasonic in 2009. As part of that deal, the Japanese Eneloop factories were sold off to Fujitsu, who since then produces 2nd-generation Eneloops under its brand. Panasonic Eneloops, starting with the 3rd generation, are made in China. As of November 2015, Eneloop Pro remains made in Japan."

  My Eneloops, 16 of them all marked "Made in 2006" (12 years) still recharge to test with discharge capacity of ± 1% of spec, between 1980 and 2017 mAh (using a Maha C9000 charger following IEC test procedure). You might see other numbers discussed, but realize that the mAh capacity numbers absolutely depend on the discharge rate, and using the exact IEC procedure is extremely important to agree with the exact advertised numbers.

  The only reason I can think of to not consider this new type of low discharge NiMH cells is if you want a larger capacity for one long shoot. There is a Eneloop Pro variety now, with a larger 2500 mAh capacity (25% more), but for double price. But carrying a spare set of batteries works too, and to me, two sets of regular Eneloops with double capacity for double price seems a better buy.

• Lithium AA batteries Lithium/Iron Disulfide (Li/FeS₂). These 1.5 volts and are NOT the same as the Lithium ion batteries used in cameras. These work well, and with some advantages, except they are not rechargeable, and will recycle more slowly than any of the others. Energizer Ultimate Lithium AA Batteries (8-Pack) can provide more flashes than the others. Better life than alkalines at high current loads (flash units), and much better performance than any in extreme cold temperatures (to -40°C per Energizer data sheet — this is what you want in sub-freezing temperatures). Even the weight is lighter. Shelf life is 20 years. A bit more pricey, but rechargeable is better in a few ways except for less recycle time and less heating concerns (and of course replacement cost). Recycle time is important for speedlights, but these lithium cells contain protection to limit the maximum current for safety reasons (heat).

  Note that this above is now a new improved AA Lithium battery, which limits the current less, better and faster recycle. Those I saw recycled faster, at least at same speed as alkalines in flashes now. If using a lithium AA, flashes will want the new one called Ultimate. But these cells start out at 1.6 volts. Be very aware of heating conditions (fast shooting speed makes it worse). In fact, Canon put out an advisory stating: Do Not Use AA Lithium Batteries in a Flash.

  If shooting rapid flash shots, the lithiums can get especially hot, especially fast, and could damage the flash, and may just quit (thermal fuse). A single flash only every now and then is one thing, but any rapid shooting is surely better off with NiMH (but rapid is always a concern to a flash). The lithium flash recycle times are longer too, which is a biggie when waiting for the next shot. Eneloop NiMH rechargeables remain my own choice for my speedlight use.

  The previous data sheet (older battery named "Advanced") previously said they limit current to 2 amps for pulse loads, to prevent hazard. The current new L91 data sheet (battery named "Ultimate") now says only limited to 2.5 amps continuous, or 4 amps maximum peak (2 seconds On, 8 seconds Off), with less internal resistance, and seems as fast as Alkaline now, and will last longer than alkaline in flashes. The lithium high internal resistance was artificially added as a safety factor. See their Application manual page 19-20 for reference of this safety protection (It says L91, but both were L91, each in their time. This document dated 2001 is the older version). Flash wants the new version called "Ultimate".

  These AA size lithium are NOT rechargeable. Lithium rechargeables are not AA size, because lithium rechargeables are about 3.3 volts instead of 1.5 volts for AA (lithium ion for cameras and laptops are a different chemistry. 3.6 volts per cell). The big point is, lithium rechargables are NOT 1.5 volts. Lithium AA cells are instead Li/FeS2 chemistry.

• There is an exception: There are now rechargeable AA size lithium cells called LiFePo4 cells, approximately only 600 mAh (there are 400 mAh versions that are Not AA size). These cells are 3.2 volts (2x too high). These might be used in flashes with “dummy fake battery setup shells” (Amazon), one cell of each type is a set. For a four AA flash, you would use two each (two 3.2 volt batteries and two of the dummy fake shells) to complete the 6 volt circuit. For a two AA flash, then one of each. It should work, but it does seem the wrong tool. I can’t say I see any advantage for a flash, because of the very low mAh capacity, the requirement for a special charger, and there would always be the risk of mistakenly putting them where a 3.2 volt cell should not go.
• NiCd rechargeables - Nickel–Cadmium. This older technology still works, also with a fast recycle, and they were great in their day, better than alkaline in flashes. But we don't see many of them any more, because they only have a fraction of the capacity of NiMH (NiCd has perhaps no more than 1/3 capacity of NiMH), and they don't hold a charge very long at all. NiCd AA cells are only available now in Chinese brands, but not in any major brand now. NiCd is still used in some cordless tools (but which are shifting to lithium ion now), but NiCd is no longer a good choice for flash today, and NiMH is their modern AA and AAA replacement. A NiMH charger will be fine for NiCd, but an old NiCd charger is not right for NiMH today (it needs to be replaced now).

When your batteries (any type) start recycling slowly in the flash, it is time to refresh them — recharge if applicable, or replace non-rechargeables.
However, NiMH can be recharged anytime, early and often is good for NiMH, no problem at all.

Battery Facts

We might imagine that Alkaline batteries could have some advantage, being 1.5 volts instead of 1.25 volts for NiMH. However, this is not the actual story, the full opposite is more true. The alkaline 1.5 volts is very fleeting, not practical reality, because the voltage immediately drops when used. (Many small electronic devices will quit near 1.1 volts, but a flash can struggle to a bit lower.) The other downsides are so great anyway (recycle time, and not rechargeable). But NiMH holds its voltage constant and high. Over most of the batteries life cycle, the NiMH battery actually has higher voltage than alkaline, and higher current and capacity ratings too, faster and longer life, and rechargeable. The flash batteries feed a power converter which charges the flash capacitor to above 300 volts during the short recycle period. Then the capacitor powers the flash tube, but then it has to be recycled (recharged) to be Ready for the next flash shot. This recycle becomes slow on low batteries, but if it is able to still be accomplished, the final capacitor voltage always reaches the same point.

The graph below is Panasonics voltage comparison (Under a continuous discharge at 500 mA, so the graph represents 2 hours and 4 hours). It conceptually represents all NiMH, not just Eneloop. This graph is idealized with the small 1/2 amp load, whereas a flash recycle draws several times more current (maybe 6 or 8 amps for a couple seconds during recycle) than this light load shown. At the flashes recycle load, both curves fall faster and steeper, so neither are this pretty... but the alkaline battery certainly has no voltage advantage.

This next one is from the Eneloop data sheet). This link is the 4th Generation Eneloop (after Panasonic). It does show a 6000 mA curve, not quite as flat, but that higher current is greatly harder on alkaline life than NiMH. The alkaline AA curve at same 6 amps would pretty much be a straight vertical line down. The alkaline data sheet curves do not even imagine 6 amps (6000 mA). Rechargeables are much better and longer life for heavy loads, like flash and cameras.

This constant-voltage property (and the longer life, and the faster recycle, than alkaline) will be very useful to your flash. Specifically, the power is good until gone. For AA batteries, NiMH is the modern solution, and Eneloop is the latest and best of those. The Eneloop advantage is that they do not self discharge (run down) in storage. The only downside of NiMH is that you have to buy a battery charger (Don't use an old NiCD charger for NiMH, and don't skimp, the cheapest charger is the worst bargain, insufficient performance). But the NiMH advantages are 1) you have the performance of constant voltage NiMH batteries, which 2) also have a higher capacity (charge lasts longer), and 3) recycle is faster, and which 4) you don't have to keep buying more of them (pays for the charger), and 5) you can recharge to peak performance at any time (to recharge early and often is no problem for NiHM). The big advantage of Eneloop (the newest low discharge type of NiMH) is if you only take flash pictures now and then, Eneloops should still be good to go in a few weeks or months, where regular NiMH may be self-discharged then. There are also several other brands of low self discharge rechargeable NiMH now, licensed from Eneloop.

The NiMH battery is a really great thing, except for rapid self discharge. The Eneloops solve that now. Note that the speedlight is exceptionally hard use, with battery current MANY TIMES higher than most charts show (just for a couple of seconds after each flash, while it is recycling). This high current is why the batteries get hot, but speedlight recycle speed needs what NiMH can do. So, it would seem that any but casual use surely wants NiMH batteries in the flash. And if you want them to hold a charge a few weeks or more, get Eneloop class NiMH batteries. And get a good charger too, which will properly take care of them.

Battery Chargers

Maha Powerex chargers are a fine brand — most of their models charge each battery individually (with individual cell Status reporting, a separate status LED or LCD for each battery slot). Which is not absolutely required — cheaper chargers work too, in their way — but an old NiCd charger is a problem for NiMH cells. Individual cell charging is a big plus, not just because we can charge only one battery, but because each battery is optimally charged individually — it gets whatever it needs itself, independent of the state of another battery.

The cheap chargers are of a few types. A few just always charge at a low slow rate and never shut off. Some shut off after a constant time period, regardless of the battery size or previous charge state. Other faster ones shut off when the batteries get too hot, which implies they are charged and have had enough.

The good chargers have computer circuits to monitor battery voltage and state, and shut off when the battery is actually full. This is not at just some simple maximum voltage like car batteries, instead the curve has a little hump after which the voltage falls slightly (called Delta V), and this is much less pronounced in NiMH than in NiCd, and also temperature dependent (see 4th generation Eneloop data sheet). Charging is not real simple, and good performance requires some smart in the charger.

Some chargers only monitor and control a pair of two batteries combined together (without seeing individual cell detail). These cannot charge just one battery, it has to be two or four. But instead, the better chargers monitor each battery individually (multiple charging circuits with individual status LEDs for each cell slot). Each cell is properly and fully charged, per its own needs, regardless of the others. Regardless of their prior state, they all come out evenly and fully charged. Otherwise (without individual monitoring), both while charging and in final usage, mixing fairly fresh cells with run down cells is really bad news. Because, the flash operates the four cells in series. If the cells were not all charged properly (evenly), full discharge causes really bad problems, when one weak cell reaches bottom, but the other stronger cells keep pumping current through it anyway. Being forced negative can ruin the one battery, and of course overall performance is very low. We really want batteries more matched, and individual charge circuits provide that.
So IMO, the minimum requirement is one individual status LED for each battery slot, indicating individual chargers for each slot.

The cost savings of recharging will repay the cost of the charger, and meanwhile, your flash can enjoy the better NiMH recycle speed and capacity. The flash performance is better with rechargables, and you can have fresh batteries anytime you want. But do get a decent charger because the cheapest one is no bargain.

I use and like these two Maha Powerex chargers. Both of them charge and monitor the batteries individually (four individual charging circuits, and so can charge only one battery at a time), so each battery always gets its full complete individual attention. Many less expensive chargers can only charge a couple of batteries in series at once, unaware of any individual cell differences or needs or status. IMO, the most important feature to seek is separate charging circuits with separate charge status indication for each individual cell. If a charger were to simply report "Bad cell", but does not know which battery it applies to, that's not the best help.

The Powerex MH-C204GT Smart Charger is excellent and simple to use, and does all we need it to do. This one can even operate on 12 volts from the car lighter socket while you drive. It has the four LEDs for individual status, and charges each cell individually, providing each cell with whatever it needs. It has a switch for fast and slow charge. Fast is 1 amp for AA, which is the ideal 0.5C rate for AA Eneloops. The 0.5C means two hours recharge time for a totally dead Eneloop battery (2000 mAh capacity / 0.5C = 1000 mA rate, and 2000 mAh capacity / 1000 mA rate is two hours), but one hour is my more usual case, when not fully discharged. I tend to recharge early and often, it's so easy.

The Powerex MH-C9000PRO charger is fancier, with the LCD that continually shows elapsed time, voltage and current, and total mAh capacity recharged (for each individual cell). The rate is programmable and it offers a few modes (Charge, Initialize, Discharge, Analyze, Cycle - see the C9000 User Manual). 100-240VAC operation. Its default AA charge rate (insert batteries and walk away) is 1 amp (which is the ideal 0.5C rate for Eneloops), but individual slots can be set from 200 mA to 2 amps. The LCD continually cycles to show each value for each cell, one at a time. These values remain visible at completion, the final mAh value shows how much capacity had to be replaced (previous state of discharge). Or it will discharge the cells reporting the total mAh capacity at whatever discharge rate you select. The EIC Laboratories specification for published mAh specs is 0.2C discharge rate, which for Eneloops, is 400 mA, and about 5 hours.

Powerex also makes a similar design Powerex MH-C801D 8-cell charger (with 8 chargers) which is popular, but I have not used it. It looks very nice, but I bought mine earlier (before 8 cell models were available), so I just used two chargers anytime haste was needed.

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Published expected values from the Nikon flash manuals are shown for comparison. The chart is for Maximum full power flash level.

As comparison examples of capacity, the Nikon manuals give these numbers for the Minimum Number of flashes, for a full power shot every 30 seconds (every 120 seconds for lithiums), from freshly charged or new batteries.