Hi Eevo, before going into charging details I think it might be useful to explain what SoC means.
SoC, stands for State of Charge, but there is also the reverse abbreviation and this is DoD. DoD stands for depth of discharge.
SoC indicates the amount of usable battery capacity reminding in a given battery, whereas DoD indicates the amount of battery capacity already discharge from that same battery.
For simplicity, I will only use SoC in my articles as a means of giving measurements relating to battery capacity. I will post up a charter a little further down, showing what the SoC measurement stands for in a battery but there's a lot more to it than just straight out voltage equals existing capacity.
Ah or ampere hours marked on a battery, roughly indicate the amount of hours you can draw a one amp load from a battery.
In theory, if you have a 100Ah battery and you apply a one amp load to that battery, say a 12W globe ( which will have roughly a 1 amp draw ), then that globe should glow for 100 hours.
In reality, the amount of usable ampere hours is controlled by a number of factors. Factors like the age of the battery, how well the battery has been maintained and so on.
Now to confuse the issue.
The SoC measurement of a battery is an indication of the State of Charge of the battery’s capacity that can be charged, and I will try to explain.
As above, age and condition of a battery will have a bearing on the actual amount of a battery’s original capacity that is still usable. When you measure the voltage to determine the SoC of the battery, you will only be measuring the SoC of this usable capacity.
For example, if you have a 100Ah battery that is partially sulphated ( I will cover the sulphation in more detail in the next article ) and you actually only have say 80Ah of usable capacity. If the battery is fully charged, and you get a 12.7v voltage reading, which indicates an SoC of 100%, in this case it does not mean you have 100% of the original 100Ah fully charged, you only have 100% of the 80Ah now available that is full charged.
The problem is that from the SoC measurement, you have no idea that you only have 80Ah available, and in this case, your 12w globe will only run for 80 hours.
You will need something that is capable of measuring current in and out as well as the voltage, and be able to analyse this data, to be able to get an accurate capacity indication. I am not going to cover that here.
So a simple voltage reading is not a reliable indication of the true available capacity of a battery, but in most RV applications, it will give you the info you need.
SPECIAL NOTE. There are two different types of voltage measurement used to determine the SoC of a battery. This article is based on using the voltage measured from a battery that is in use. There is also OPEN CIRCUIT VOLTAGE READINGS, where the battery must site in a No-Charger/No-Load state for at least 24 hours, before the voltage reading is taken.
OPEN CIRCUIT VOLTAGE READINGS are far more prone to erroneous results and are pretty useless while on holidays, when you need to have the battery sit around used for 24 hours, just to see how available capacity there is.
The chart below is comparatively accurate when measuring any form of lead acid battery. OPEN CIRCUIT VOLTAGE READINGS are battery specific, which means you have to have the correct OPEN CIRCUIT VOLTAGE Chart for the specific make and type of battery you are trying to monitor.
1 SoC Table.jpg
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