Battery discharge question (iMAX b6 Mini)

My normal intellectual state of mind is around 50% confusion and 50% “Aha!” but the last couple of days this has moved towards 100% WTF!? I thought I was smart enough to figure this one out myself and I might have but I would still like some input from you guys. So what’s the big deal then? Well my confusion is based on how my iMAX b6 mini charger behaves when discharging batteries (in this case 18650’s).

If I use the NiMH discharge program and set the discharge current to 1 A and the cut off voltage to 3.0 V it will discharge the battery with 1 amp until it reaches 3.0 volt and then call it a day and display XXX mAh of available capacity. Now if I use the supplied ChargeMaster software and use the Li-Ion discharge program (with the same settings as the above mentioned NiMH program) it will discharge with 1 amp until the battery reaches 3.01 volts and then continue to discharge but while lowering the current all the way down to 0.XX amps and then stop. This method will of course yield a much higher capacity and on top of that the discharge takes forever - or, well maybe not forever but a fair bit longer. Pictures below (volt and current in that order).

My question is simply, what kind of discharge strategy is the best? My guess is that there is no good answer to this and that it depends on what type of real life load one use to drain the battery.

Your thoughts?

EDIT: i forgot to attach some pictures of the ChargeMaster discharge…


I find that odd, can anyone answer question above? I similarly tried discharging, but LiIon cells. I find about 10-20% difference when discharging in ‘normal’ discharge moge - instead of going constant current till voltage limit hit, it goes constant current till 3.0V, and then it gradually lowers the current while maintaining 3.0V . Is this all right? I mean thats usually nowhere near what would happen in a real life application, when either protection circuit would kick in, or the voltage would be sucked in below safe levels.

At vanheim, You can think of a real battery as an ideal battery and a resistor in series. When you discharge with 1A to 3V you're measuring 3V not of the ideal battery (Open circuit voltage) but of the ideal battery minus the voltage drop (I*R) across the internal resistor.

So V=Voc-IR=3.0V

but if you back off on I then V comes up above 3.0V. The real open circuit voltage hasn't hit 3.0V yet.

You're right, there's no right answer to your question. First what is your purpose for discharging? If it's just to measure capacity, then there is no one right way to measure capacity. If you need to power a 1A load and no less and need to do so above 3.0V then the first method tells you you capacity for that. If you need to squeeze every drop of power out no matter how slowly, while staying above 3.0V, the the second method tells you that.

Real life use? If your flashlight turns off because it hits low voltage protection on turbo mode, but you're in the dark and being chased by zombies, you can turn it back on on low mode and still scare the zombies off for awhile. What's not "real-life" about that? But if you're bicycling at high speed, moonlight mode won't really do you any good and you'd be more interested in something like the first method maybe.

Of course HKJ has great data for almost every battery you should care about already, with capacities and curves for a dozen or so different current levels to let you know how the battery performs, so there's no point remeasuring all that really. If you're just monitoring relative health of your batteries over time, then probably a moderately high-current discharge is good for that. Just do it the same all the time and see how your battery degrades. Or better yet, don't discharge it at all, just keep notice of how long it lasts you in real use and throw it out when it doesn't make you happy anymore.

Doing a Constant current followed by a constant voltage discharge is how the data sheets are rated.

So you want the one that drops current until the end, it will match the data sheet better. Although the data sheet generally discharges down to 2.5v.

It is the charging cycle but reversed.

Hmm, looks like you are correct, not sure why I thought I had seen that.

Maybe not, but I think they're rated at pretty low discharge current aren't they? like 0.1C or something usually, which works out not so different from that.