Hi Dave and you have hit on one of the most confusing areas of voltage measurement to monitor a battery's available capacity ( State of Charge or SoC ).
There are two type of voltage monitoring used with Lead Acid Batteries. OPEN CIRCUIT Battery Voltage and LOADED Battery voltage.
PLEASE NOTE, WET CELL ( also known as FLOODED ), AGM and GEL batteries are ALL LEAD ACID type batteries. The Term LEAD ACID battery is often mistakenly used to refer to just WET CELL batteries, but all the above are actually LEAD ACID batteries.
Using battery voltage to gain an indication of the amount of battery capacity that is available is not an accurate indication of the true SoC of a battery but it is still a very reliable indication, if you know the drawbacks of using voltage measurements to keep track of your batteries capacity while the batteries are in use, such as when on holidays.
OPEN CIRCUIT voltage readings
OPEN CIRCUIT voltage readings, while the most commonly used, are almost totally irrelevant when it comes to monitoring lead acid batteries in RV use.
The first and primary requirement before you can take an OPEN CIRCUIT voltage reading is that the battery must be in a No-Charger/No-Load state for at least 6 hours, but to get an accurate reading, the battery must be in a No-Charger/No-Load state for at least 24 hours, before an OPEN CIRCUIT voltage reading becomes useful.
No-Charger/No-Load state means the battery must not have a charger connected or any load connected to the battery.
It is this single primary requirement that makes the use of OPEN CIRCUIT voltage readings unrealistic for monitoring batteries in RV situations.
The reason that a battery must ?REST? for at least 24 hours is because the chemical reaction of the battery's electrolyte does not cease for up to 24 hours after the battery was last charged, or supplied energy to power a load.
While this chemical reaction is taking place, OPEN CIRCUIT voltage readings will not give an accurate indication of the true SoC of a battery.
Even when this 24 hour ?REST? period is carried out, there is still other factors that need to be addressed, before the readings are of True value.
OPEN CIRCUIT voltage are battery SPECIFIC, which means you need the correct OPEN CIRCUIT voltage chart for a given battery.
For example, most WET CELL batteries have a fully charged OPEN CIRCUIT voltage reading of around 12.7v to 12.75v, many AGMs will have a reading of 12.8v to 12.95v and then you get batteries like Optima, where all their Yellowtop and ?MOST? of their Bluetop battery have a fully charged OPEN CIRCUIT voltage reading of 13.1v
So you get an idea of how inaccurate these reading can be if you do not have the correct OPEN CIRCUIT voltage chart for the specific battery you are testing.
Another major potential drawback with using OPEN CIRCUIT voltage readings is one touched on by Ron.
You need to know that a battery is in good condition before you know that the OPEN CIRCUIT voltage readings is accurate.
This is pretty hard to do, even as Ron explained, even when the batteries are being professionally monitored.
You can take an OPEN CIRCUIT voltage reading and it indicates the battery is fully charged and then you connect up a load and within minutes, your battery is flat.
LOADED Battery voltage Readings.
Taking LOADED Battery voltage Readings with a faulty battery can also produce an inaccurate indication that the battery is fully charged when it is actually dead flat.
The difference is to get a correct reading, the voltage must be stable, and because you have a load connected, you will more than likely see the voltage dropping as you try to take the reading.
So while a LOADED Battery Voltage Reading is also susceptible to giving an indication of a battery being fully charged when it is actually faulty, you are far more likely to find a faulty battery when using the LOADED Battery voltage Reading.
Here is the LOADED Battery Voltage Chart I have been using for the last 25+ years and it is pretty accurate for what it is intended for.
NOTE, the is for measuring the SoC of a battery, using the battery's terminal voltage. This is not going to give you an SoC of say 84.6%, it will return a reading somewhere in the 80 to 90% range, which is quite satisfactory for when you are off camping and want to know whether your battery will be good for another few days, or will see you through the night, or is in need of topping up there and then.
The load you have connected to the battery while testing it, should not be greater than 5% of the total battery capacity otherwise the error margin makes to reading less reliable.
For instance, if you have a 100Ah battery, the load should not be greater than 5 amps.
https://www.aulro.com/afvb/images/im...016/06/640.jpg
In the last few months I can across this chart on the LIFELINE Battery Web Site.
This gives an excellent comparison of LOADED Voltage Readings, with different current loads.
https://www.aulro.com/afvb/images/im...014/12/243.jpg
