But it occurs to me that to fully know the health of a cell it would have to be discharged to some appropriate reference level before you charged it back up. Georges' little charger will (apparently) tell you exactly how many mAh's went into the cell during charge, which is pretty sweet. But I don't think it does any discharging.
Does such a thing as a 'discharger' exist? Or, have any of you made something to do the task? Thanks!
I just ran across this thread.
For most of the cases where I'm keeping good track of battery capacity, it is when the batteries are used with an electric plane or multicopter that uses one or more ESC's that shut the throttle off (or lowers throttle in case of small Quads), when the voltage has dropped to a certain level.
But that does not do you any good.
However, I can recommend another great little inexpensive device.
A Lipo Battery tester. Do an Ebay search for these keywords:
rc lipo battery tester alarm
Looks like this:
Leftmost pin is negative/ground, 2nd pin is +V for Cell #1+, 3rd pin is +V for Cell #2+, etc.
It displays the total voltage, then the voltage for each cell, up to 8 cells. For what you are doing
.. you'd only use the left pin for ground and 2nd pin for the +V from the single LiPo battery. Of course you'll also need to wire up a connector adapter for the little "stick" Lipo's
Now, it is not a discharger, in the sense of the fancy big chargers. But, it can be set to sound off a very loud alarm when the voltage drops to a pre-set level, such as 3.3V. So, you could plug the battery into some load and just be sure you'll be around to hear it when it sounds the alarm (the alarm is so loud you might want to muffle it. Or turn the loudness into a feature by doing the test in the workshop if you might be able to hear it in other parts of the house). What you don't want to do is to get so far away that you never hear it go off, then when you come back, the LiPo is dead, and as you probably know when a Lipo goes below 2.7V it starts to chemically break down, under say a volt and it's ruined dead-dead. So, that is one thing that the big brother chargers can do, a discharge to certain level then STOP automatically, even beginning a recharge cycle.
Of course, the old school glowing red LED display will draw some current by itself. For more current you could use say a small old-school incandescent 4 to 4.5 volt flashlight type bulb, since it lights up. But given the LED display, you could use most anything else that would be suitable, perhaps use a 500 ohm (or 100 ohm?) potentiometer and dial it down to get the desired current level.
Anyway, I really like that little unit
. and have several. Right now, one is double sided servo-taped to a multicopter as an inflight battery monitor. So, I'll hear it beeping long before the voltage gets too low to cause a crash (can hear it easily from a hundred yards away, like I said it is loud!).
Here's a Youtube video of it in use with a 2 cell LiPo battery, many similar vids out there but this one is short and to the point.
UPDATE - see message #24 for a better video of the tester/alarm
[video=youtube;tupNpOKHxJw]https://www.youtube.com/watch?v=tupNpOKHxJw[/video]
A pic showing 6 images of it in use with a 3-cell Lipo, note the 4 wires starting at left
Now, having said all the above
the HiTec 4X Charger I recommended was for the convenience of charging a lot of those little 3.7 V batteries, up to 4 at a time.
If you do not need to charge several very much, and DO want to be able to test capacity by doing automatic discharging, then you could consider this IMAX B6 charger that is a lot cheaper than the HiTec 4X. There was a thread on this a week or two back where ironically the guy asking how to charge a small Lipo battery (70-100 mAh) decided not to use a LiPo after all. But this is a "big charger", it simply does not charge more than one battery. And you can use it for lots of other batteries, maybe not right now but probably eventually.
Anyway, Hobby King has it ($25), but so do others on eBay for cheaper (but longer wait times from China).
Edit - I see Bob Krech also recommended it. He linked to the version that can run off of 120 VAC (as well as 12 VDC), which is definitely useful if you do not have a 12 power supply to run off of at home. Although until I got a "real" 12V power supply, I used to run field 12 V powered RC battery chargers by using a 12V battery with a 12V charger hooked up to keep the source 12V battery charged up. Worked OK for charging small batteries, since those plus their charger did not require more current in total than the charger (plugged into 120 VAC) was producing to keep the 12V battery topped off.
https://www.hobbyking.com/hobbyking...5A_Charger_Discharger_1_6_Cells_GENUINE_.html