I now use rechargeable NiMH batteries for almost everything. It is quite likely (and a great thing too) that I will probably never buy another non-rechargeable battery. I use them to power my collection of flashlights, headlamps, and photographic gear.
The claim is that after you use and recharge a NiMH battery 10 times, it has paid for itself. You also have to consider the cost of a charger.
I should also point out, for the purpose of bowing to strictly correct language use, that in most cases what I call a battery in the following is in reality a cell. A true battery is an aggregate of several cells. Now that we are clear on this point, we will forget we ever mentioned it, and call an AA cell a battery from now on.
The whole thing started when I got a Canon 580EX speedlight. I began to sweat and worry about all the AA batteries I was going to be using. So, I dove in and got myself some NiMH rechargeables (and a charger).
It turns out this is a very smart thing (in the flash that is) for a reason aside from expense. Plain old alkaline batteries have significant internal resistance. This means that in a high current application (such as a speedlight), much of their capacity gets wasted just heating up the battery itself. Rechargeables (including NiMH batteries) have much lower internal resistance, and it is estimated that for this reason alone, you will get around 4 times as many flashes with the rechargeables. In other words, 200 flashes rather than 50 before the batteries need to be recharged (or tossed out).
At first, I just chose the highest maH rated batteries, but I have since decided that for the way I use batteries, low self discharge NiMH batteries are far superior, in spite of their lower capacity. The reason for this is that conventional NiMH batteries self discharge rather severely. Most drop 5-10 percent of their charge the first day, then stabilize to a 0.5 to 1.0 percent daily self discharge rate. An experiment was done over a 6 month period that verified a 0.7 percent daily self discharge, and after 6 months, 37 percent of the original charge remained in a set of 2500 maH Energizer cells. The rate is temperature dependent (and as you might expect increases at higher temperatures). At 70 degrees F, a conventional NiMH battery looses 40 percent of its charge in a month. The low self discharge cells retain 75-80 percent of their charge after a year. (Sanyo claims a 15 percent self discharge in a year, i.e. 85 percent retention for their eneloop batteries.)
This means that if you don't use (and recharge) your batteries frequently, you are likely to pick up your gear and find yourself with a bunch of almost discharged batteries.
It has been claimed that after a month in storage, the eneloop 2000 maH AA batteries will hold about the same charge as 2700 maH conventional AA rechargeables, After that, the eneloop batteries will have them beat significantly.
I have chosen and like the "eneloop" batteries by Sanyo. There are many other brands and makers including Hybrio and others. Any battery labeled as "pre-charged" is probably a low self discharge type, but check the specifications.
Here is a good link with reviews, and even plans to build a battery charger.
As an interesting aside, certain headlamps were (perhaps are) not recommended to be used with NiMH rechargeables. The reason is that in the extremely unlikely situation where something shorted out, the NiMH are capable of delivering enough current to melt wires, potentially dropping hot melted stuff onto a persons eyes and face. Alkaline batteries, because of their internal resistance inherently limit the current they can deliver and would not cause such a hazard. This is why you are advised not to short out NiMH and Ni-Cd batteries. The can (and will) deliver enough current to start fires, overheat the batteries, and produce mayhem.
The charger I have had the longest is the MAHA C401FS. This has individual charging circuits for each battery, with a single status LED for each battery. It offers a choice of two charging speeds, selected by a slide switch. Fast charge is 1000 mA (500 mA for AAA), which will charge a modern high capacity (2700 maH) NiMH battery in about 2 hours, look out! Slow charge is 300 mA (maybe less for AAA?), which will charge a modern battery in something like 8 hours (some specs say 5 hours, but they must be talking about older 1600 maH batteries). Once charging is complete, batteries are trickle charged at 50 mA.
My first unit burned up when in fast charge mode, destroying a set of 4 Sony batteries. This is clearly not a good thing. To their credit, Thomas Distributing was quick to send me a replacement unit without hassle, which is still working fine, but I avoid the fast charge mode now. Interestingly, some time after this experience, I read a review of this unit on the Candle Power Forums that said they found the fast charge on this unit scary!. The review said that it made batteries so hot you could not hold them and had them rushing to remove batteries as temperatures soared. They also noted that each charging slot behaved differently when in fast charge mode. I may open up my unit and permanently disable fast charge mode. Given that it is selected by a slide switch that could be accidentally slid, this seems prudent. Buy the La Crosse BC-700 instead.
I also recommend that if you buy or have this unit, you immediately remove the clear plastic cover and throw it away. It serves no purpose and will only trap heat while the batteries are charging, which is not what you want at all.
After misplacing my C401FS, I ordered a La Crosse Technologies BC-700 charger. I have just received it, and like it a lot, maybe better than the MAHA. Like the C401FS it offers 4 individual charging circuits, but with LCD monitors telling you various values and parameters, which is cool. You can see charging current, battery voltage, elapsed charging time, and amount of charge accumulated. The last value can be deceiving, as it is not the amount of total charge in the battery, but the amount of charge that has been added to the battery during the current charging session. It also has clever modes to recondition batteries and three charging levels. It defaults to slow charging (at a kinder and gentler 200 mA), which will require 13.5 hours to charge a 2700 maH battery. Also available are 500 and 700 mA charging rates, which must be selected via button pushes when charging begins. The 500 mA rate will take about 5 hours, and the 700 mA rate about 3.5 hours to charge a 2700 maH battery. It then trickle charges at an unspecified rate. The wall wart feeds it with 3 volts (or so its label says). One misgiving is that it does not have a 12 volt car adapter like the MAHA C401FS. Something could certainly be rigged up, but the simple and easy approach is to use a small inverter in my vehicle.
There are lots of other chargers. The LP 4000 from Ripvan (where LP stands for "lightning pack") is cheap and liked by people who have screwed around with lots of different chargers. It does charge in groups of two though, which I think sucks.
I just (9-2009) bought a 12 volt NiCd battery from Battery Space. The price was right, and I'll let you know how it is when it arrives. (It arrived in perfect shape, and so far exceeds my expectations!) Most people, speaking imprecisely would call it a battery "pack". Any composite of cells is technically speaking a battery. A single Ni-Cd (or NiMH) cell produces 1.2 volts. It takes 10 such cells to produce a 12 volt battery.
He notes that a single cell requires 1.5 to 1.6 volts to charge and will generate from 1.2 to 1.25 volts when charged (my pack of 10 now gives me 12.4 volts, so he seems to be right about the high end). My 10 cell pack can yield 2100 milliamps. He says:
Trickle charge is 3.3 to 5 percent of 2100, i.e 70 to 105 milliamps.
Slow charge is 10 to 20 percent of 2100, i.e 210 to 420 milliamps. (8-15 hours)
Fast charge is 50 to 100 percent of 2100, i.e 1050 to 2100 milliamps. (1.5 hours)
Expect about a 66 percent charging efficiency. Don't worry about a memory effect, this requires many precisely repeated partial discharge cycles, and is unlikely to be produced by normal use. What you should worry about instead is properly charging your batteries.
What you do not want to do is to try to fully discharge a battery. What will happen is that some cell will go dead first and then be charged in reverse by the other cells delivering power to the load, and this is quite bad.
Once a battery is fully charged, it will begin to generate gas and build pressure (and heat).
One of the circuits above uses a comparator to stop charging a two cell (2.4 volt) NiCd battery when the voltage reaches 2.81 volts. This is a simple approach and certainly better than endlessly charging. What manufacturers recommend is either using a thermistor or other temperature sensor to detect a temperature rise in order to terminate the charge, or to monitor voltage to watch for the "negative delta V" effect. What happens is that the battery voltage peaks and then drops 20 millivolts per cell when full charge is reached.
National Semiconductor once made an LM3647 universal battery charger chip. The data sheet might well be instructive, as most national semiconductor sheets are! Also look at the MAX712/713 chips (the 712 charges NiCd, the 713 charges NiMH).
Tom's Digital Photography Info / tom@mmto.org