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. . .unless there has been a change, the 'free' charger that comes in the box with eneloops is a 'dummy' chargers, constant current with no peak detection or temperature shutoff. They just keep charging the batteries and never shut off or go into trickle charge mode.
Access, it appears that Sanyo has made a few different models of chargers, and some are "smarter" than others. I have two models: NC-MQN06W (two of these), and NC-MQN06U (four of these). They around $13 each, if you buy them alone without any batteries. They are reasonably "smart" chargers; when you first insert depleted AA batteries, they charge at about 300mA (or 150mA, for AAA). During this time, an LED light blinks green. When the batteries reach a target voltage, the current drops to a trickle charge and the LED changes to continuous green. In one independent test posted online, the tester measured 286 mA average during the charging phase, and 33 mA average during the trickle charge phase.
Some of this is discussed in Sanyo's data sheet for the NC-MQN09U. There is additional independent test data on similar models in the thread I referenced in the prior paragraph.
From my use, I can attest that the amount of time it takes for the LEDs to change from blinking to solid (indicative of the charger changing from charging current to trickle current) correlates well to the level of charge left in the batteries when I put them into the charger. This change occurs rather quickly for barely-used batteries that I'm topping off, and much more slowly for deeply discharged batteries. I can also attest that fully-charged batteries left in the charger (while it is powered on, with the LED glowing solid green) for a couple of months never feel warmer than room temperature to the touch; there has to be some heat energy being released, but it must not be all that much.
While looking this up, I read data sheets for several other Sanyo Eneloop chargers. Some appear to behave like mine, while others operate on a timer--they supply a charging current for a set amount of time, then switch to a trickle current. The LED light behavior also differs from mine. These chargers do seem less smart than the ones I use--but I wouldn't want to assert, without test data, that this materially affects Eneloop battery life or performance. And I see that Sanyo also offers "fast" chargers for Eneloops. This sounds even more sub-optimal to me, but I have not looked for data to confirm or dismiss that sense.
Many of my Eneloops have been recharged dozens of times--but this is nowhere near the 1,500 cycles claimed possible by Sanyo. Will my cheap chargers give me a few hundred cycles shorter life than an expensive charger would? Only a lengthy test would tell. Even if this turned out to be the case, I'm not sure that the additional cost of a fancy charger would make economic sense, given that Eneloops are very, very cheap on a per-recharge basis (about 1/10 of a penny in battery depreciation), so it would take many added uses to recoup the cost of the fancy charger. If you are looking to spend an extra $50 on an expensive charger vs. a cheap one, you'd need to get around 50,000 additional charging cycles out of your Eneloops to break even. How many of us are going to do that?
Meanwhile, with six cheap chargers, I can recharge 24 batteries at one time. This keeps up with my rate of use most of the time, and lets me recharge in a few goes after a trip. For the same money, I could have bought one really nice charger with four-battery capacity--and be forced to feed my discharged Eneloops slowly through the queue.