Lithium AA batteries These work well, and with some advantages, except they are not rechargeable, and will recycle more slowly than any of the others. Energizer Lithium AA batteries can provide more flashes than the others, if given enough time. Better life than alkalines at high current loads (flash units), and much better than any in extreme cold temperatures (this is definitely what you want in sub-freezing temperatures). Even the weight is lighter. Shelf life is 15 years. A bit more pricey, but better in most ways except recycle time (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. This means that flash recycle times are longer, which is a biggie when waiting for the next shot (Eneloop NiMH rechargeables remain my own choice for my flash use). AA size lithium is NOT rechargeable. Lithium rechargeables do not exist in 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).
UPDATE: There is a new improved AA Lithium battery now, called Energizer 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".
NiCd rechargeables - this older technology still works of course, also with a fast recycle, and they were great in their day, but we don't see them any more, because they only have a fraction of the capacity of NiMH (perhaps no more than 1/3 capacity of NiMH). Not a good choice today, and NiMH is their modern 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).
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 |
Advanced AA Battery Facts
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.
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Energizer E91 Alkaline
from Energizer AA E91 data sheet
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
from Energizer AA NiMH NH15 data sheet
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. |
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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.
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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.
Battery Chargers
Maha Powerex chargers
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).
Copyright © 2012-2013 by Wayne Fulton - All rights are reserved.
