Why Battery Voltage Does NOT Indicate Battery Charge State
The most common phrase I repeat when talking to motorhome and caravan owners about their electrical system is “Battery voltage is NOT a good indicator of the battery charge state”. The idea that there is a direct correlation between the voltage across the terminals of the battery and the amount of energy stored inside is so engrained that often I have to provide examples to show why this is false.
Using my favourite water metaphor let me explain exactly why your voltmeter is almost useless when it comes to deciding how full your batteries are.
Consider this diagram. The top portion depicts a pump connected via a large pipe to a water storage tank. There is a pressure gauge attached to the pipe near the tank.
So long as that pump is not turned on, the pressure gauge will tell us roughly how much water is in the tank (based on the weight of the water above the gauge). Now consider what will happen if we turn the pump on and start pushing water into the tank at a great rate. Now the pressure gauge will no longer show us how much water is in the tank, but how much pressure is coming from the pump. The more the pump pushes, the higher the needle on the pressure gauge will go.
Now – imagine that we can reverse the pump and suck water OUT of the tank at a great rate. What will the pressure gauge do? Of course it will show less pressure, as the sucking action of the pump will lower the pressure in the pipe.
Let us agree that the pressure gauge will tell us almost nothing about the water level in the tank so long as the pump is turned on (pumping in either direction).
This example is functionally equivalent to the lower part of the diagram. A voltmeter measures electrical pressure. A solar panel (or any other form of charging) pushes electrons into the battery for storage. Anything running from the battery is sucking electrons out of the battery.
When the solar panel is in the sun, the voltmeter will measure the voltage (pressure) coming from the panel. When the sun goes down and you have lights using power from the battery, the voltmeter will measure the “suction force” as your lights pull electrons from the battery. Larger appliances create more suck on the battery as they pull electrons out of the battery at a greater rate. Clearly in this case the voltmeter is reading the suction and NOT the pressure left in the battery.
The obvious solution to this problem is to disconnect the battery and then take a reading with the voltmeter – this has got to work … right?
Well yes you can get a reasonable idea of the state of charge if you disconnect the battery bank FOR 12 – 24 HOURS! Unfortunately, it takes time for a battery to settle down after being charged or discharged. If you want to use a voltmeter to measure how much charge is left in your battery, you will need to disconnect it completely for at least 12 hours.
There are two exceptions to this rule. Both of these are when the battery is at the extremes of capacity.
- If the battery voltage is VERY low (say below 11 volts) and there is little running from it – it is clearly very flat.
- If the battery voltage is VERY high (say 14.8 volts) and there is not much charging it – it is clearly very full.
Using a voltmeter to assess the charge remaining in your battery (outside of these two extremes) is like trying to tell how much fuel is left in your car using the temperature gauge.
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