NiMH Chargers, Old and New(er)

Written by  on January 5, 2015

Many  flavors of Lithium batteries are popular now, Lithium-ion (Li-ion), and  Lithium polymer (LiPo, or Li-poly).  They do hold more charge (specific energy) than Nickel Metal Hydride (NiMH) ones, some almost twice as much, for the same weight. They can be recharged more times, and maybe even charge faster. You will find Lithium batteries in every cell phone and quadcopter.

But NiMH AA batteries are each sporting as much as 2500mAh at 1.2V (nominal). It does take 3-4 of them to get to the LiPo cell voltage of 3.6V. But the NiMH batteries hold that 1.2V level through just about the whole discharge cycle, which the Lithium ones do not. Most importantly, Lithium batteries are now viewed as a serious fire hazard, because they can get mighty hot, if not charged correctly. So hot, that some incidents were reported, of laptops catching fire. Now each Lithium battery cell and each device using them must have warning labels. There are rules about having them on planes or in postal parcels, as if they were some kind of a high explosive bio-weapon, which they are not. But, NiMH are considered safe, by default.

NiMH batteries are not as environmentally suspect, as the NiCd were, because of serious toxicity of Cadmium. And modern battery designs have no ‘history’. Remember, how once in a while we had to discharge NiCd batteries all the way in order to restore them, to make sure they did not ‘remember’ a smaller amount of charge? No more, with NiMH. One does have to charge NiMH with some care, so as not to overcharge them, as they can heat up a bunch, and also not to trickle charge them forever.

I found in my garage a 15-year-old Kodak NiHM charger, and also bought a Lenmar 8 Port Battery Charger over a year ago. Brands had shifted and diminished, but let’s find out what changed technologically in fifteen years. You will see that there are systematic differences, but also surprising similarities. There are four screws holding the Kodak unit together, tiny hex screws, and I was too lazy to look for a set of hex wrenches. A couple of small flat screwdrivers worked.

Lenmar went even further. There are six safety screws on the back, of a slightly unusual variety, with two slots on the sides. Not something common, like the star with a pin sticking out in the middle, which I do have bits for. Well, two different sizes of scissors did the job in the end.

The Kodak plugs straight into the wall outlet, but Lenmar has a walwart that brings out 12VDC. The beauty of such a setup is that you can have a walwart and a plug specific to every country and set of regulations, and the main circuit is the same, and only has low voltage. So everyone does this now.

But why would Lenmar use the funky screws, as inside there is no high voltage whatsoever. And these screws must have cost at least a dime a pound more than the common variety. Would those extremely pragmatic folks in Shenzhen ever do this on their own, or was this the one design contribution by their US counterparts?

The mechanicals of battery contacts and the utilitarian color and layout of the PCB top feel very similar between the two units. There is a bit of a difference in how actual electronics are accomplished. Besides the whole 110V AC issue that the Kodak has to contend with, they used a load of discrete components and a couple of ICs, TI LM324 quad Op Amps, which are a dime apiece these days.

Lenmar sports two Michrochip TC4427 MOSFETS, $0.90 each, though they must be getting them a whole lot cheaper. Also, one can see an Alpha & Omega Semi AO4606 30V complementary MOSFET, maybe $0.20.

So, now you know. The more things change, the more they stay the same.