...For a typical Lead Acid Battery, 12.05V is around 50% discharged and generally the lowest you would want a battery to go to. As a rule of thumb, on any installs I do or kit I supply, I tell folk to avoid ever being below 12V
There are quite a few charts around, but this is a pretty typical one and a decent example to reference:
Note: the table above is for a battery at rest i.e. Not being used in the last 30 minutes or so and not having been charged for an hour or so.
If you take a note when say the solar panel is providing a charge, you can think the battery is at a much higher charge than it is (and conversely if under a load it can look as much more discharged than it really may be)....
I put the above message up a couple of days ago. I thought I would add a couple of charts to see how well those charts relate to the 'real world' and if they are useful....
This chart is a graph of my own battery bank over a period of 11 days. I have taken the start from when I just removed the mains battery charger and the batteries were reported as 100% SOC (so Full) and stops when I decided to plug in the mains charger again and the batteries had got down to 55%
Not tried this before, but I can make my own version of that table and see how it matches up. So here goes:
| Battery SOC | My Battery Voltage | Table Voltage |
| 100% | 13.10V | 13.00V |
| 90% | 12.72V | 12.75V |
| 80% | 12.59V | 12.50V |
| 70% | 12.37V | 12.30V |
| 60% | 12.16V | 12.15V |
| 55%/50% | 12.11V (55%) | 12.05V (50%) |
Now the original table reading is based on a battery that has been allowed to rest before taking the readings. My system is live, so the voltage readings will fluctate somewhat as loading comes on (the 'ragged' voltage line in the chart is the result of the fridge coming on around 20 times or so a day and when on, causing the voltage to dip) and charging happening from the solar panels (the little bumps around the middle of each day).
But you see while the voltage bounces up maybe 0.25V during that charging time, the SOC only increases by 1-2%, and not the 15% or so the voltage table would suggest (which is why it is so important when reading a voltage on a meter to know if charging is occuring).
Effect of loading on the voltage?
Well my batteries in the graph above are always under load - I am pulling a pretty constant 1A day and night due to the various kit I have always running (Victron GX, Router, MiFi, BMV and illiumination on the bunch of USB sockets dotted around the van) so the voltage will always be very slightly down compared to a resting voltage.
I have added back the current in the drill-downs below so you can see the loading and how that when the current draw increases (fridge goes on), the voltage dips (by around 0.1V for the extra 4.5A in fact).
This first chart shows the voltage at the regular 1A draw when the battery is at 62.4%
This next chart actually captures the in-rush of the fridge compressor (it happens every time, but the sampling doesn't always grab it)
When the demand shoots up from the normal 1.0A to 41.15A (a fair old current draw), the voltage dips from 12.20V to 12.05V - a 0.15A drop.
Again, if by happenchance you looked at a voltmeter at that precise time and then used the table to check the SOC, you would say your batteries were down to 50% whereas in reality they are at 62.4% (as the SOC chart shows).
So yes, a Voltmeter and a Voltage vs SOC chart is a useful tool when used correctly and will give you a pretty good idea of your batteries. It can be upto 10 points out if the batteries are under a fairly higher load or charge (most people would not draw more than 15A or charge greater than 15A at any time) but still gives a decent indication.
An SOC Meter (Victron BMV, NASA Marine BM, plus numerous others) are a lot better however as they will give you a much closer indication of the batteries State of Charge and can be read at anytime without considering loads and charging.
