My new favorite AA Charger: The Maha/PowerEx C-204W
Reviewer: Dave Etchells
(Review posted: September 20, 2004)
 |
* | Charges AA or AAA cells. |
| * | Safely detects Alkaline cells, won't charge. | |
| * | "Smart" Charger, won't over-charge, unusual ability to "resurrect" dead cells. | |
| * | Built-in battery conditioner to keep cells fresh. | |
| * | Universal, international internal power supply. |
Introduction
Out of all the various electronic devices on the market, digicams continue to be one of the harshest tests of battery capacity, making rechargeable battery technology a critical issue for digicam owners. While we're seeing more cameras using LiIon batteries these days, many models continue to use the more affordable and versatile AA cells.
As I discovered some time back, optimum battery life is not only a matter of buying the highest-capacity NiMH cells you can find (see my Battery Shootout page), it's also important to get a good-quality charger. Some chargers overcharge and can damage batteries, others drastically under-charge, making your high-capacity NiMH cells little better than garden-variety cheapies, so your choice of charger is critical to getting the best use from your batteries.
Three years ago (in September 2001), I identified the Maha Powerex C-204F as the AA-cell charger I most preferred for routine use. The C-204F was fast, didn't overcharge cells, had discharge-conditioning circuitry, and seemed to strike a good balance between fast charging and gentle handling of the batteries.
As you'd expect though, over the course of three years, even something as seemingly ordinary as battery charging technology has evolved. In part based on our my test results (see the aforementioned Battery Shootout page), Maha has developed a charger that does an even better job of "topping off" batteries than the C-204F did, and added a host of other advancements as well. The result is the new Maha Powerex C-204W, the subject of this review. Thanks to the advancements that Maha has made, the C-204W has finally taken over the top spot as the charger I most prefer for routine use.
In the Box
One of the first things you'll notice when you unbox the C-204W is what isn't there: There's no sign of the traditional "wall wart" AC power adapter that many chargers use as their power source. The C-204W has a true international power supply built into it, so all you need is a power cable with the right shape plug on the end to interface with your local power system. You can plug the C-204W into any power source, from 100-240 volts, and 50-60 Hz.
Of course, there are tradeoffs with anything, and the C-204W is no exception. While the universal power supply will let it operate virtually anywhere in the world that has AC power, it's no longer an option to just plug it into a car cigarette lighter for charging on the go. - If you need to charge on the road, you'll either need to get a power inverter to give you AC power in the car, or just stick with the original C-204F, which comes with a cigarette lighter adapter right in the box.
NiMH Only
Another tradeoff made with the C-204W is that it now supports only NiMH batteries, dropping the support for NiCd cells that the 204F offered. In reality though, this isn't much of a limitation, as NiCd cells have all but vanished from the marketplace, thanks to the generally superior characteristics of NiMH batteries for digicam use.
More importantly, the C-204W has a pretty sophisticated algorithm for detecting Alkaline batteries, which are unsafe to recharge. (Besides their chemistry being ill-suited to recharging, Alkaline batteries lack the sort of protective venting found in NiMH cells, designed to prevent an explosion in the event of overcharging. Alkaline batteries literally can explode if recharged by a NiMH charger without the sort of detection/protection circuitry built into the C-204W.)
Charging
The C-204W is a "smart" charger, which means it applies a high rate of charge to the batteries when first inserted, then cuts back to a "trickle charge" once peak voltage is reached. Like the 204F, the 204W again has two independent charging circuits, so it can charge batteries either two or four at a time. This is an important feature these days, as many cameras operate on just two AA batteries, and it's important to charge batteries in the same groups that they are used in, to keep from over- or undercharging cells with different usage patterns.
When fast-charging, the 204W delivers current to the batteries in two-amp (2000 mA) pulses for AA cells, 700 mA pulses for AAA ones, alternating pulses between the two pairs of batteries. With a set of four typical ~2000 mAh AA cells, this will result in a charge time of about two hours, but if only two cells are charged at once, the charging rate will be twice as fast.
While it has very a sophisticated end-of-charge detection system, the high current delivered by the 204W does mean that you shouldn't use its one-hour quick charge mode (with only two batteries in the charger) on lower-capacity cells with less than 2000 mAh capacity. If you need to charge lower-capacity cells, make sure you charge them four at a time in the C-204W.
Special Stuff: "Topping Off" Batteries
In my own testing of batteries and chargers, I found that fast chargers invariably leave the batteries in a less than "full" condition when the quick-charging cycle is over. Some chargers (like the earlier 204F) drop to a trickle charge mode after the fast charging is complete, which does help to "top off" the batteries if you leave them in the charger overnight. But even the 204F couldn't seem to get the last 2-5% of capacity into a battery: For that, I always had to resort to overnight trickle-charging in a rack that I built, that gave the batteries a straight DC input. (Actually, rectified, but unfiltered AC.)
Noting my results, Maha invested a fair amount of effort in revising their charging algorithm, with the aim of developing a charging profile that would deliver true 100% charge capacity in a single unit, without the need for external trickle charging.
What they came up with is a three-stage charging process, that's implemented in the 204W for the first time. While Maha is deliberately a little sketchy about the exact details, they did tell me that the 204W switches from a fast charge to an intermediate charge level after the charger initially detects that the battery is "full" (the point at which the 204F and most other rapid chargers would either stop entirely, or switch to trickle-charge mode). After a while in this "topping off" charge mode, the charger will finally drop down to trickle-charge mode, which in the case of the 204W means applying brief pulses of current for a few tenths of a second every 7 seconds or so. (They found that this pulsed trickle mode did the best job of maintaining battery charge, with the least impact on battery life and chemistry.)
The bottom line of this is that the C-204W does the best job of "topping off" batteries of any charger I've seen to date. When the LEDs initially turn green at the end of the fast-charging cycle, batteries will typically be at about 92-95% of maximum capacity. (As good or better than most other fast chargers.) Given just a little longer though, the 204W brings batteries very close to their absolute maximum capacities: Charging Maha's own 2300 mAh Powerex batteries, I found that the 204W would bring them to 99% of maximum capacity within about 4 hours. Leaving the batteries in overnight didn't seem to add any more capacity, but the gentle trickle cycle means you should be able to leave good-quality NiMH cells in the 204W indefinitely, to keep them fresh. (Pure DC still seems to be required to eke out the last 1% of capacity, but that's such a small increment that it's both academic and within the range of variations I see between successive charge/discharge trials anyway.)
Special Stuff: "Reviving" Old Batteries
NiMH batteries don't like to be left laying in a drawer for months, as they lose a few percent of their charge every day. If they're left unused for as little as a month or two, cells can actually go entirely dead, the result of "micro shorts" that can develop internally.
Faced with really old, "dead" NiMH batteries, most chargers figuratively throw up their hands, and refuse to charge them. (Pumping current into a dead short could damage the charger, if kept up long enough.) It turns out though, that the micro shorts that can develop in unused batteries can often be cleared fairly easily, simply by applying a modest charging current for a minute or so.
Recognizing this, Maha developed a more sophisticated dead-battery detection algorithm for the C-204W. Rather than deciding whether batteries are shorted as soon as they're inserted, the C-204W applies a moderate charging current for 80 seconds before checking their voltage. In many cases, this "conditioning" pulse is enough to clear the micro-shorts and allow the battery to be charged normally afterward. Once it's successfully revived a set of "dead" batteries, the C-204W applies a special, more gentle charging cycle to more fully restore them. (If you do use the 204W to revive an old set of batteries, leave them in the charger for 24 hours or so to gain maximum benefit.
This "revival" capability of the C-204W is a real plus: Saving just one or two sets of old, "dead" batteries could pay the entire cost of the charger.
Conditioning
While it's less of an issue with NiMH cells than the older NiCds, it's a good idea to fully discharge NiMH batteries every now and then, before recharging them again. While experts argue about the need for this, I've definitely encountered sets of batteries in my own use that regained significant capacity after being "conditioned" by fully discharging them.
Like the C-204F before it, the C-204W has discharge-conditioning circuitry that drains the batteries fully to about 1.0 volts/cell before recharging. Just plug the batteries into the charger, push the little gray button between the LEDs at the bottom, and walk away. The LEDs will blink yellow to indicate that the discharge conditioning cycle is underway, switch to red when charging begins, and light green when finished.
This doesn't need to be done very often, but is a good idea after about
every 10-20 normal discharge/charge cycles. The discharge rate in conditioning
mode is pretty low, so allow for an overnight discharge/charge cycle
when conditioning batteries in the 204W.