What's a good battery level monitor?

It is not just theory. For many of us who don't plug in, you can get several partial charge cycles in a row (several days of non optimal solar for example) and as a result your SOC can be significantly off. Without plug ins, we also generally don't sit at float for hours upon hours to get your monitor back to 100%.

Yes you can make it work without these conveniences, but you have to remember when the last time you were on float for multiple hours, or try and check the battery voltage when you know things are at rest.

My point was that the good battery monitors deal with this, and you can be fairly sure when you glance at the display that the number is pretty close to reality. Cheaper monitors are a compromise.

I use lithium batteries, so the voltage doesn't tell me much about the SOC. However I have a good battery monitor that is correctly programmed for my batteries, and I can trust it.

SJ-Chris wrote:
Regarding the drifts and possible inaccuracies due to a variety of behavioristics of charging and/or the batteries themselves....

I can see how IN THEORY you could have an issue if every time you charged your batteries you immediately STOPPED charging once your monitor said you are at 100%. If you were in fact not 100% full (...let's say the true SOC is only 95% even though your monitor incorrectly says 100%) and then you used your battery down to 50% as stated on your monitor, you would in actuality only be at 45% SOC. Then if you charged back up to what you thought was 100% and immediately stopped charging once your battery monitor says you are at 100% but in fact if it once again fell short you might only be at 90% true SOC. If you repeated this many times in a row, then I can see you'd have a problem. (drift)

Perhaps a good rule of thumb would be to make it a habit to continue to charge your batteries for a little while beyond the time when your monitor says you just reached 100%. Or, if your monitor shows amps going into your battery while charging, you can get a sense that way as to how much "charging" is still happening once your monitor reaches 100%.

This "drift" problem gets fixed/reset anytime you do have a situation where your charger stays on/active for a long enough time after your battery monitor says you are at 100%. For example, if you do find yourself plugged in at a campsite and your monitor reaches 100% (even if your true SOC is less then 100% because it has been drifting) while you are sleeping and then continues getting you to true 100% until you unplug. Or, if you are driving from one destination to another and your alternator is charging the battery...if your monitor gets to 100% (when your batteries are actually at less than 100%) the batteries will continue to get closer to 100% as long as you are driving. This "erases" any of the drift delta that might have built up.

Also: Most battery monitors tell you the Voltage it sees on the batteries. From most 12v SOC charts you can see that 12.06v resting voltage corresponds to 50% SOC. So if you ever see your battery resting voltage at 12.06v REGARDLESS OF WHAT YOUR BATTERY MONITOR SAYS percentage-wise, you should charge your batteries. Likewise, when your batteries are truly fully charged to 100% they will likely read ~12.7v at rest. If your monitor says it is 100% full but your batteries are only at 12.4v or 12.5v then you should continue charging until you get all the way up to 12.6-12.7v at rest. If your batteries never get there, then there is likely a problem with your monitor voltage reading (you can test by putting a voltmeter directly on the batteries) or a problem with your batteries.

-Chris