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.

Two Exceptions

There are two exceptions to this rule. Both of these are when the battery is at the extremes of capacity.

  1. If the battery voltage is VERY low (say below 11 volts) and there is little running from it – it is clearly very flat.
  2. 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.

Tags: , , , , , , , ,

 

Has the information on this site been useful?

If it has, please consider using the donate button at the bottom of our home page. All donations are used to maintain and improve the website and we are very grateful for any support.

 

Do you offer a product or service that relates to caravanning or motorhoming?
Would like to tell the thousands of people that visit Hobohome.com each week, about your product?

Every week Hobohome.com attracts thousands of visitors. Like you, they are interested in motorhoming and caravanning in Australia. Some are building their motorhome, others are travelling in their caravan. If you have a product or service that relates to motorhomes, campervans or caravans and would like to advertise on our site, please
click here for more information.
We are also looking for new products to review.

10 Responses to “Why Battery Voltage Does NOT Indicate Battery Charge State”

  1. Motorhome and Caravan Info Australia » Blog Archive » Motorhome and Caravan Battery Life Says:

    […] and Caravan Technical Articles « Motorhome vs Caravan for long term Australian travel Why Battery Voltage Does NOT Indicate Battery Charge State […]

  2. Rob Says:

    Nice analogy, the water/voltage thing is common but the comparison to the pump is good.

  3. Alby Kramer Says:

    An interesting article Gavin, very good analogy between water and electrics. How can we determine the state of the batteries on the run then? I have had it when the 12v fridge is running and the 12v freezer is running early in the morning before sunrise the voltage is being dragged down to ,say, 11.8 – 12v and then when the fridge and freezer cycle off the battery voltage slowly climbs back up to 12.4-12.5v. So when it shows 11.8v you assume the battery is running low and then it shoots back up to 12.5v and you assume, oh well, it is still around >50%. My solar regulator does not have all the whishbang battery state info like some. What can I add to the system to be able to get a better idea of what is left in the batteries?

  4. Hobo Says:

    The only real to look after your batteries is by using a PROPER battery monitor. Xantrex make a very good one. This device does NOT use battery voltage to track remaining capacity. For about $250 – $300 it provides all the info you need.

    G

  5. Darryl Says:

    Sorry off topic Question Gavin Tracy
    What insurance do you have for break downs on the road.Like who would tow such a large bus.Why i ask is we are heading off soon in our bus around Australia but cant find a company that would help in a event of a break down needing towing .I am going with my son so want some peace of mine if things go really wrong Thanks Darryl or is this why most travel with a car & caravan set up

  6. Hobo Says:

    Hi Darryl,

    Our bus is insured with the CMCA insurance scheme. This automatically provides basic breakdown and tow insurance.
    Cheers

  7. Roger Says:

    Hi Hobo,
    Slightly off-topic but…
    When a battery is fully charged and is then “used”, is the rate of discharge linear over its normal high-low range?
    Cheers,
    Roger

  8. Hobo Says:

    Hi Roger,
    If your question is “is the battery voltage change linear given a consent discharge” – the answer is no.

    Keep in mind that a battery is a device that stores energy using chemical reactions. The liquid electrolyte changes its specific gravity (during charge or discharge) it moves within the battery (due to heat or convection etc). It is the “strength” of the electrolyte in contact with the plates of the cell that determines the potential (or voltage) that appear across the terminals of the battery. Some times this is a gradual change – other times it is sudden. Gradual or sudden changes in battery voltage result. If you averaged out these spikes and dips, the voltage between (approx) 90% and 65% would be fairly linear given a constant rate of discharge. Outside of this range the curve gets increasingly non-linear.

    One big problem with using battery voltage to even estimate remaining capacity is that the rate of discharge is very rarely constant – the situation is much worse if a charge is applied (eg solar).

    I hope this answers your question – if I mis-understood the question, please set me straight.

  9. Roger Says:

    No, that answer was just fine thanks. I have a fair idea what I’m drawing and replacing and am loathe to fork to fit a shunt. The thing that is screwing me at the moment is crappy laptop batteries (replaced this week to fix the problem) that required us to plug in to an inverter (true s/w) which was converting 12>240 which was then being reduced 240>20 by the laptop chargers. Lots of spare heat is a by product of this inefficiency….
    I have also dicked the 3-way and replaced with a 12v compressor fridge – best thing ever!!! Cheers, Roger

  10. john belle Says:

    hi there im wanting to put a xantrax lite in our bus which is 24v our house batterys are 12v can you please tell me if it can read the bus battery and the house battery one being 24v and the other 12v thanks john

Leave a Reply

Anti Spam Captcha - Please complete this math problem before submitting *