Flash power can mean the batteries of course. There are battery choices for speedlights, affecting recycle speed and capacity (number of shots). Recycle is the time 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 maximum power. Published values from the Nikon flash manuals are shown for comparison.
|Nikon Recycle times, after full power flash, with fresh AA batteries|
page F-21 / H-21
|Lithium||4.5 seconds||7.5 seconds||3.5 seconds||4.0 seconds||4.2 seconds|
|Alkaline||4.0 seconds||6 seconds||2.5 seconds||3.5 seconds||3.9 seconds|
|NiMH||2.3 seconds||4.0 seconds||2.5 seconds||2.5 seconds||2.5 seconds|
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.
|Nikon Minimum Number Of Flashes, full power flash, with fresh AA batteries|
page F-21 / H-21
|Lithium||230 shots||170 shots||330 shots||400 shots||250 shots|
|Alkaline||110 shots||130 shots||160 shots||200 shots||140 shots|
|NiMH 2000 mah||165 shots||150 shots||230 shots||220 shots|
|NiMH 2600 mah||190 shots||260 shots||210 shots|
However, there are TWO types of NiMH rechargeable batteries today:Regular NiMH cells - These self discharge sitting on the shelf, 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 today, or at least this week.
Newer type NiMH - which may be called PreCharged or Low Self Discharge, but they definitely should say something about a long storage time. Sanyo Eneloops are the classic leader, but there are several good brands now, licensed from Sanyo - these are the way to go today. One difference is that these 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 so, the Eneloops will win. Because, the really big deal is that these hold their charge a very long time. Sanyo Eneloops say 75% capacity will remain after three years (unused), 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 month or two, this choice is necessary. This long storage time is a huge plus if you only take a few flash pictures a week, these are still good to go after months. The packaging says you can use these right out of the package without charging first, because they don't lose their charge on the shelf. However, it won't hurt to charge them first, as the factory does not fully charge them. 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 much. Using Eneloops, my SB-800 recycle time is reduced slightly, to 2.9 seconds.
Wikipedia says "eneloop is a brand developed by Sanyo encompassing rechargeable batteries and associated devices, introduced to the market in November 2005. The batteries are low self-discharge NiMH, which lose their charge slower than previous NiMH batteries; 10% in the first year compared to about 20% on the first day and 1-4% on every successive day. eneloop batteries are sold pre-charged and can be used immediately, while regular NiMH batteries lose their charge so fast that this isn't feasible." This is a pretty big deal.
UPDATE: There is a new improved AA Lithium battery now, called Energizer Ultimate, which limits the current less, and better, and faster. Those I saw recycled at same speed as alkalines in flashes now. If using a lithium AA, flashes will want the new one called Ultimate.
The previous data sheet (battery named "Advanced") previously said their current is limited to 2 amps for pulse loads, to prevent hazard. The current new L91 data sheet (battery named "Ultimate") now says 5 amps maximum peak, with less internal resistance, and seems as fast as Alkaline now, and should 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".
FWIW, here are approximate numbers I checked in a SB-800, 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 not claiming any precision measuring the current amps for pulses. It is not a constant value, it is a decreasing pulse (continuing weakly a few more seconds after the Ready LED (allowing a couple more seconds can't hurt). But this chart is suggestive of the scale of things. The difference between today and the SB-800 manual in 2003 is the 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|
We might imagine that Alkaline batteries could have some advantage, being 1.5 volts instead of 1.2 volts for NiMH. However, this is not the actual story, the opposite actually. The 1.5 volts is very fleeting, not practical reality, and the other downsides are so great anyway. The batteries simply charge the flash capacitor during the short recycle period, to around 300 volts, and then the capacitor is what powers the flash tube. And over most of the batteries life cycle, the NiMH battery actually has higher voltage than alkalines. The graphs below are from the specification data sheets of two AA Energizer batteries, first Alkaline E91, then rechargeable NiMH NH15-2300.
Energizer E91 Alkaline
Note the Alkaline battery is hardly ever at 1.5V, and is higher voltage than the NiMH only during the first fraction of its life. The voltage drops steadily until it is gone (many devices quit at about 1.1 volts). This is why recycle becomes very slow as the battery runs down. We likely discard them before it reaches bottom.
The Alkaline curve shows constant power 250 mW, and the NiMH curve shows constant current 230 mA, for both around 10 hours life. But if we assume NiMH is constant 1.25V, then the power is 1.25V x 230mA = 287 mW. Very similar situations in these two charts, but is a tiny load, a couple of magnitudes less than typical flash use.
Energizer NH15-2300 rechargeable NiMH
Note the NiMH battery holds at a more constant voltage, and is above 1.2V almost all of its life. NiMH gives a generally flat voltage curve, until it is gone.
The simple battery charge status meters in our equipment just show the voltage, being designed to indicate the dropping alkaline voltage curve. This meter cannot work right for NiMH. The graphs show why it falsely starts out showing only 1/2 charge for fully charged NiMH, but it will stay there all the time, until the battery is dead. My notion is this is why flashes don't have these battery meters, the meter is not useful for NiMH cells. Longer flash recyle time informs us when the flash batteries are getting low.
Internal resistance: Alkaline: 150 to 300 milliohms (recycles slow)
Capacity: Depends on discharge rate. The first data sheet bar chart shows about 2800 mah capacity at 25ma discharge, but only about 1300 mah capacity at 500 ma rate.
Internal resistance: NiMH: 30 to 40 milliohms (recycles fast). This is what the flash recycle needs, does not limit the current.
Capacity: NiMH mah rating is specified for a discharge of ANSI 0.2C rate (approx 460 ma for this 2300 mah capacity). The Capacity of a battery (example 2300 mah) is called "C", and a charge or discharge rate 0.1C means 0.1 x rated 2300 = 230 ma rate (for this 2300 mah cell). 0.5C is a good charge rate for NiMH, which is 1 amp for 2000 mah Eneloops, and 0.5C means that a fully discharged cell will charge in two hours (0.5C is good for best detection of full charge state).
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 these data charts show (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.
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 charged. The better chargers have circuits to monitor battery voltage and state, and shut off when the battery is actually full. This is not 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 Eneloop data sheet). It's not real simple, relatively difficult, and best performance requires some smart. Even if a better design, some of these cheaper ones monitor and control a pair of two batteries combined (without seeing individual cell detail). The cheapest charger is really NOT what you want today, at least not for maximum battery performance.
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. They all come out evenly and fully charged. Otherwise, with most other chargers, 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.
The cost savings of recharging, instead of buying replacements every time, will repay the cost of the charger, and meanwhile, your flash can enjoy the better NiMH recycle speed and capacity. But do get a decent charger because the cheapest one is not a bargain. The flash performance is better with rechargables, and you can have fresh batteries anytime you want.
The Maha C401FS is excellent, and does all we need it to do. This one can even operate on 12 volts from the car lighter socket while you drive. On mine, the power cube AC is 120 volts only (not 120 or 240V, however there are international versions available). Has the four LEDs for individual status, and charges each cell individually, providing whatever that one cell 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. (0.5C means two hours for a dead battery, but one hour is my more usual case).
If you want fancy, the Maha C9000 with the LCD continually shows time, voltage and current, and total mah capacity recharged (for each individual cell). It also has 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 (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).