I didn't mean to imply otherwise
@LARGEONE. I only meant as a general rule not to trust voltage on lithium the same way as you can with lead-acid.
The chart you show is right on but a graph to visualize how narrow the voltage difference between 100%, 80%, 20% and 0% is between the two types. You want to stay between 20% and 80% on lithium for long life and safety but the high/low change is on the order of 0.25V across this working region while on a lead deep cycle you'd have 2V of room to voltage change to work with over that range.
The point I failed to make is when you have this narrow margin of error your meter quality comes into play as to how much you're sure you're staying between the ditches.
For example a 3-3/4 digit DMM might specify a 0.1mV to 100mV resolution depending on range and 0.5% + 4 counts accuracy.
This means if it has a 40V range it can show 04.00V to 39.99V, thus the resolution is 10mV per step. When displaying 13.10V the 0.5% accuracy will means the measurement could be as much as 0.0655V in error plus the 0.01V resolution plus 4 counts of the least significant digit.
So that means when the voltage is 13.10V the DMM could show as low as 12.99V up to 13.22V and be within it's specifications. It's probably better than this (likely close to 13.03V to 13.18V) but you don't know. In any case this amount of uncertainty is only enough to give you an estimate within about +/- 25% as to state of charge across the plateau.
Using 13V as the re-charge point is safe in the sense you'll never risk dropping off the cliff below 20% but you might not end up using a lot of capacity if it's really closer to the 45% state of charge when you think it's at your 20% floor.
On the charging side this theoretical DMM is good enough to make sure you don't go too high (this being above 14.6V) as long as you stop at around 14.45V.
On lead-acid using a DMM to judge state of charge based on DMM voltage you're able to estimate within a couple of percent because the SOC-vs-voltage response is so much larger and linear.
There's not much you can do about meter accuracy, even calibrated it is what it is based on what you're willing to spend. So if you want to really know it's considerably cheaper to get an accurate enough (because 1% is fine due to the larger changes in magnitude and time averaging) current meter than it is a better voltage meter in this situation. To further complicate the question a cheap DMM is often still quite a bit better than your typical panel mounted volt meter, which may only be full unit accuracy (+/- 2% is not unusual), so showing 13.1V could very plausibly be as much as 0.25V in error and that's enough you could be at 20% and think you're at 80% or vice versa. To some extent you can calibrate by comparing to a known good measurement but the offset may not be consistent, such as the error may drift with temperature.
It's totally a question of what you need, though. Warm beer in a fridge on a weekend trip is not a problem vs living full time off grid losing a fridge of food that could be a real problem vs running medical equipment where it's critical.
I know this isn't perfect, but for how many charge cycles this will see, I think I will be OK.
