Here's the chart a lot of us use.
To use the multimeter, take your battery out and look at the connector port (not the charging port.) You will see two holes. Put the negative probe in one and the positive probe in the other. If you guessed right, it will show the current voltage. If you guessed wrong, the needle will try to go backwards. You won't hurt the multimeter doing this so don't sweat it. Just change the probes, and remember which is which for the next time.
The voltage on the display is OK, not as precise as the multimeter but close. Mine has always been within .2 or .3 volt of what the multimeter shows. Close enough for engineering work. (Haha. My dad was an engineer and had a ton of engineer jokes.)
I don't worry too much about the 20-80 thing. I know it makes a difference, but if I only get 800 recharge cycles before the battery drops to 80% of nominal capacity, vs. say 1000 charge cycles, I've still gone many many thousands of miles, and can still go many more with the same battery. I just have to recharge after every 3rd ride instead of every 4th ride. Big deal. I have yet to see anyone on this forum say "my battery is unusable because I charged to 100% every time."
BTW, the stock Juiced charger only goes to 95% or thereabouts. Not as good as 80%, if you absolutely have to get every possible watt-hour out of it, but it will give some extra level of longevity over charging to 100%. I believe, from a comment Tora Harris made a few years ago, that this is intentional.
What makes more of a difference is going too low (I really do stay within that 20% margin), leaving the battery out in freezing weather, charging it when it's too cold (l let mine get up to room temp. first), letting it get too hot, and stuff like that.
I love the zippy feel of the bike when I've got a full charge, and you never get that if you always only charge to 80%. There's more to life than getting the maximum possible life out of your battery. Don't abuse it, enjoy the ride, and you'll never regret it.
