The reason there’s not a simple relationship between battery voltage and state of charge is that your solar system is constantly doing different stuff – electricity is coming and going all the time.  However, there is a simple relationship between the two if the battery is disconnected from all chargers & loads and has been sitting idle for an hour or so, but you don’t want to do that every time you want to check the system.

It’s a good place to start, though. This is called the “open-circuit” or “resting” voltage of the battery.  Resting 100% fully charged 12V batteries are around 12.8-12.9V, and discharged ones are at 12.0V, so 12.4V on a resting battery means that it’s about 50% charged.

Battery Voltage Overview:

In general, chargers raise the actual voltage on the battery above its resting voltage, and loads lower the actual voltage below its resting voltage. Crank up your charger and your voltage could go up to 14.0 – 14.5V. Turn on a big load like the microwave and voltage dives down into the 11.5-11.8V range, even on fully charged batteries.  Again, in general, the bigger the charging current or load, the further above or below your resting voltage your system will be.   So, looking at the voltage doesn’t give you an accurate reading on how charged your battery is – you must adjust for loads and charging sources.

Charge Voltage: When a battery is charged the plates will polarize and develop a resistance to the charge (surface charge). This resistance will add to the battery voltage and therefore using this voltage reading will not reflect the true state of charge. All the so-called “surface charge” will be removed when the battery is being discharged. In general, the battery voltage will recover or increase when the load is removed. This is especially true if the load is very high.

An Easy Way to Determine the SOC In Your Battery

The foolproof way to accurately determine the SOC is to use a hydrometer – a simple device that measures the density of your battery electrolyte. Fully charged batteries have lots of sulfuric acid in the electrolyte, depleted ones don’t – the acid changes into lead sulfate on the battery plates as the battery discharges. Since sulfuric acid is heavier than water, fully charged batteries have electrolyte specific gravity in the 1.255-1.275 range, 50% depleted batteries read 1.175-1.195, and dead batteries read 1.095-1.115.  A hydrometer can be purchased at any hardware store and they are inexpensive. In addition to the specific gravity numbers, they’ll have color-coded ranges that say good battery, so-so battery, or dead battery.

Now, to use the hydrometer you have to pull out your battery tray, open one of the caps, and suck up some electrolyte into the hydrometer, so you won’t be doing this often, or on the road. If you have AGM batteries, you can’t get to the electrolyte at all because there are no caps. However, you can use the hydrometer to calibrate your voltage display under known load conditions, and you’ll know what voltage reading goes with which state of charge.

What Is Known Load Conditions?

Let’s say that we have a site where the TV is always on, and we usually have a fan going, and one or two lights are turned on. I charge up my batteries, verify that they’re charged by making sure the hydrometer reading is in the 1.26 neighbourhood, and look at my voltage display when these loads are running (which is the operating word). 12.5V is on the voltmeter which indicates that the battery is fully charged. This is the Voltage-under-load and not the resting voltage.

Now I leave the stuff on for a few hours, go back with the hydrometer, and see that specific gravity is down to 1.18 – half discharged. My voltmeter says 11.7.  Now I know that, under this load of TV plus fan and lights, this is the point that I need to recharge my batteries – don’t run your batteries below 50% unless you’re in an emergency situation. Now I can put the hydrometer away – I have a voltage number, 11.7, that corresponds to 50% discharged and time to recharge the battery.  Easy.

People have different numbers or different types of batteries and different typical loads.  So, 11.7 volts on your system may mean more or less than 50% discharged, but the principle is the same – there’s a particular voltage that corresponds to different states of charge for your batteries under load. Some battery manufacturers state the voltage figure for 50% discharge as 11.58 volts, so that’s probably using a larger load than mine. So to arrive at the 50% voltage for your system, test your own system and use a load that’s known.  With this load on your batteries, there’s a foolproof battery voltage that corresponds to 50% discharge. Stay above that voltage, and you and your batteries will live long, healthy lives.

How to find out the 50% voltage for sealed batteries?

Get a clamp-on ammeter, figure out the amperage going out of your batteries under load and time it. Say you have 4 batteries with 90Ah @ C20 – that’s 360Ah. In this case, we get 18A for 20 hours before it’s dead. Charge them all the way, turn loads on until you’re drawing 18A (about 200-250 watts) and time it – 10 hours is half discharged. Your voltage under load after 10 hours will be your 50% discharge level. This will only work with good, new batteries at the 20 hour rate.

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