Mobile battery test
So, I've ended up with four batteries for my Gnex, but I only have one back-up charger. The most sensible thing it seems would be to take my two best batteries, and keep one in the device and one in the charger at any given time, but it's hard to know which batteries are better, I don't even know which two are the newest at this point.
What's the best way to tell which of my batteries is objectively the best of the group? I know there are lots of apps for this, I can see them in the store, but do they actually do anything? Has anyone used one that they recommend?
I don't know anything about anything, but would measuring them all with a multimeter when they are all at full charge be a good indicator?
No. I have not used one of these apps, but what they should do is activate your phone's "burn battery" mode which drains the battery at maximum power dissipation and measures the total watt-hour energy to depletion. If the app is doing it right, it will require you to be plugged into a charger to do the test. Laptop battery calibration works the same way.
Does it have to drain the whole battery? Is that the only way to test it? I feel like I could do that myself by leaving Netflix turned on or something. I feel like there should be a better way to test.
It sounds like you don't care what the numerical value is; in that case you can just run Netflix or w/e and measure time to depletion on each battery.
Also, worth noting that you should NEVER attempt to charge or discharge a lithium battery unless it's connected to a battery management circuit. Lithium battery chemistry is not particularly stable and you can damage the battery or start a fire.
This plot is the reason why a voltmeter is a bad choice for trying to determine how much energy is left in a lithium battery:
The "100%" mark for older batteries is somewhat less than that for fresh batteries. Lithium batteries lose ~10% of total capacity per year since manufacture; this is accelerated by keeping it at high charge states and frequent cycling.