Basics of Flash Photography
Four Fundamentals we must know

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Powering the Speedlight Flash — AA Batteries

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.

BatteryRecycle current   Recycle time
Energizer Ultimate Lithium   5.3 amps5.0 seconds
Duracell Alkaline4.8 amps5.0 seconds
Eneloop NiMH6.7 amps2.9 seconds

Battery Type Choices

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.

Battery charge status meters:

The simple battery charge status meters in our AA battery equipment are just a voltmeter, just shows the voltage, being designed to indicate the dropping alkaline voltage curve. Because of the constant voltage curve, this meter cannot work right for NiMH. The graphs show why it falsely starts out showing only 2/3 charge for fully charged NiMH (about 1.3 volts), 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 as useful for NiMH cells. Longer flash recycle time informs us when the flash batteries are getting low, or we can recharge NiMH at any time.

The Lithium Ion batteries in cameras and laptops and cell phones contain sophisticated chips, called Coulomb counters, which measure all battery input and output current to determine accurate charge status, primarily for safety purposes. But AA batteries only use the voltmeter, which doesn't show much information for rechargables.

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.

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

Nikon Recycle times, after full power flash, with fresh batteries
page H-23
2.6 seconds1.8 seconds
page F-21 / H-21
4.5 seconds4.0 seconds2.3 seconds
SB-800 page 197.5 seconds6 seconds4.0 seconds
SB-700 page H-273.5 seconds2.5 seconds2.5 seconds
SB-600 page 194.0 seconds3.5 seconds2.5 seconds
SB-500 4.0 seconds3.5 seconds
SB-4004.2 seconds3.9 seconds2.5 seconds
SB-3004 seconds3.5 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 batteries
2000 mAh
2600 mAh
page H-23
150 shots190 shots
page F-21 / H-21
230 shots110 shots165 shots190 shots
SB-800 page 19170 shots130 shots150 shots
SB-700 page H-27330 shots160 shots230 shots260 shots
SB-600 page 19400 shots200 shots220 shots
SB-500 (2 AA cells) 100 shots 140 shots
SB-400 (2 AA cells)250 shots140 shots 210 shots
SB-300 (2 AAA cells)70 shots110 shots

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