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Old 06-17-2019, 12:40 AM   #11
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Join Date: Jul 2018
Location: Minnesota
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Voltage is a very poor way to evaluate state of charge for lithium batteries since the voltage almost flat lines after an initial drop and then drops rapidly when the battery reaches about 20% remaining charge. You can see that in the chart posted above.

Moreover, voltage is hard to use and not very accurate for any battery that is in actual use. To get an accurate measure of voltage requires the battery to be rested for a day and even then the voltage measured may not accurately reflect its actual state of charge depending on the conditions. So its not clear getting a more accurate reading of voltage will give you any more accurate a measure of state of charge. The vagueness of "idiot lights" may just accurately reflect the uncertainty around any measure.

The solar charger adds to this confusion for our Roadtrek. Whenever we are in the sun, the voltage reading reflects the voltage the solar panel is putting into the battery, not what is available. Roadtrek labels this idiot light level "charging", but it is the same as when the battery is fully charged.

So two things may be going on:

1) You are reading the voltage during the day while the battery is being charged by your solar panel.

2) Your "idiot lights" are set to reflect 100% charged while voltage is anywhere in the flat part of lithium batteries' voltage curve.

There is an alternative way to monitor lithium battery state of charge that involves tracking actual usage. I am not sure how practical that is for an RV.
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