I've never tried any of these. I've read reviews that suggest that HF sells cells that were swept off the floor at the place that dug rejects out of someone else's dumpster.
I do have some experience with drills bought from HF. Those batteries were uniformly horrible. And I bought some HF closeout tool batteries with the intention of using the cells elsewhere. No go...all crap...six cells for 99-cents and it still was a bad deal. I was never able to quick charge them and slow charging only worked a few times.
There exist real rechargeable C cells...
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is the first google hit. Looks like just over $4 each. Have no opinion on that brand. Twice the cost of HF. But more than 3x the RATED capacity, and I'd bet $$$ that they're actually usable in a real application.
Most end-user grade C-size rechargeable batteries are a smaller, often much smaller cell embedded inside. You can tell by the weight. At 1500 mAh, if you don't need the form factor, you're probably better off with a quality AA cell.
Thermal lag. It takes a while for the battery to become warm to the touch. If the -deltaV sensor says it's done at a conservative point, you'll never feel the heat (which is a good thing). With a fast charge, you could easily overcharge before the internal heating has time to make it to the surface. We're also not talking about much heat. 0.5C charge into 1.35V is only about 1.4 watts. Heating shouldn't be a problem with a slow charge, which might simply radiate the heat before it gets excessively too hot. Extra credit for manufacturers that include small fans in their chargers which insures that the temperature sensors don't work.
However, I've never seen a cold and overcharged battery. With reasonably fast charging, the batteries I've played with all get warm or hot as they approach full charge. The battery manufacturers are making sure that the battery gets warm by playing with the ma-hr capacity number. For example, a 2300 ma-hr NiMH battery, may achieve the rated capacity at 0.1C discharge. However at 1C discharge, it's only good for 2000 ma-hr. Less for higher discharge rates. Old batteries also lose surface area, and therefore lose capacity. Trying to charge a moving target like with a fixed timer charger, is just not going to work. It will overcharge.
Now, if you really want entertainment value, try putting a sealed zip-lock plastic bag over the charger and give it a nice hefty overcharge. The bag will inflate slightly with even a modest overcharge. Where does the gas come from? It's from blowing the vent seal on the battery and boiling off electrolyte. Note the pressure curve: The safety vent opens at about 130 psi and stays open forever. The gas is mostly hydrogen, so you probably make a small bang. You can sometimes see condensed electrolyte on the bag surface. The potassium hydroxide can be detected with ph test strips. Do that a few times and there will be little electrolyte left in the battery. I "rejuvenated" at few NiCd cells by filing off a corner of the cell, and using a hypodermic to inject KOH back into the cell. I'm not certain how well it worked, because there may have been other damage inside, but it worked well enough to run an HT for a few months.
I guess that's one advantage to 0.1C charging. It will never get hot enough to vent.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Not necessarily difficult, but a world away from Ni-xx. The primary ingredients for Li-Ion charging are a current-limited constant voltage source that has good voltage control. Near enough is not near good enough. We have used bench PSU's on occasion as part of out testing.
While John is reading up on the material you linked, he could also check out the available documentation from some charger-controller chip makers. Back half a lifetime ago I designed a commercial charger using the MAX1737, and the data sheet included a lot of good info for (Li-Ion) beginners on care and feeding requirements,
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