Flat battery…Faulty or not?

So it’s dark and cold outside, you jump into your car, turn the key, and nothing… First thought, my battery has died, followed by a call to a roadside assistance company, and they confirm your battery is faulty.

But how do you know they are right? How do you know that your battery has really failed, and is there anything that you can do to check?  The answer is yes.

The first thing we advise you do is to check the voltage of the battery, or ask the roadside assistance man to advice you of this. If the voltage is less than 12.7v, then your battery is not fully charged, and if it is not fully charged, then it should not be tested. If the voltage is around 12.1v, then it is 50% charged, and any voltage lower than 10.5v is fully discharged.

The voltage level of the battery could give you the first indication of what fault you may have. If you see a voltage of around 10.6v, and the battery will not charge above this, then it is possible that your battery has “dropped” a cell.If the vehicle has recently been used, and the battery performed fine, but you now have a voltage in single figures, then it is possible that something has caused your battery to discharge.If your battery is showing around 12.7v or above, this would indicate your battery is fully charged, and the issue could lie elsewhere.

Okay, now we know a little more about battery voltages, it time to delve in to the mysterious world of battery testing.   I say mysterious, as very few places test batteries in the same way, and there is so many battery testers out there, from digital to hydrometers. The results of can be interpreted quite differently.

We use a testing method approved by the manufacturers we deal with, and these are as follows

  1. Voltage Check. We make sure that the battery is able to charge fully, this as mentioned previously is 12.7v or above, and this reading is taken after putting a small load across the battery to equalise the battery cells.
  2. CCA Test. This is done with a Midtronics, and will give a numeric CCA ratting, and not a percentage.
  3. Load Test. This test is sometimes known as a drop test, and is done by putting a resister across the terminals of the battery to test the batteries ability to hold load. A healthy battery will hold a load of above 9.6v for up to 30 seconds.
  4. All of the above tests must be done with the battery fully charged.

If the battery fails any of the above tests, then it is replaced if under warranty.

There is one final scenario that the above tests are not able to catch, and that is a battery with a high rate of self-discharge.

A healthy battery will be able to hold its fully charged voltage, when disconnected from a car or charger for a long period of time, if the battery has a high rate of self discharge, they can appear healthy on the above tests, but when left for a number of days disconnected, their voltage will keep dropping below 12.7v. However it is possible that a parasitic drain on the vehicle is responsible for this and not the battery, and there is a very simple test you can do at home to find out.

The first thing you need to do is make sure you battery is fully charged, 1 hour after the vehicle has been used, then disconnect the battery, leave for the period of time the battery would normally fail within, and then check the voltage again. If the battery is at fault, then you will see a voltage lower than the 12.7v+ that you began with. If the battery is not at fault, the voltage will still be nice and high, 12.7v+.

This test can also be repeated with the battery hooked up to the car, and again left unused for the same amount of time. If the battery has now lost voltage, then you can presume that the car is at fault. If the battery has still not lost any charge, then this would indicate either an issue with the car’s charging system, or possibly something on the car not being switched off or closed down properly (boot light is a classic).

It is also possible to check the health of a battery with a hydrometer, but as most car batteries are now the fully sealed, maintenance free type, this is not really possible to do. I will try an cover this later on in the year however, for all of you that use the classic hard rubber batteries, and open cell traction batteries like The Trojan T-105s.

Hopefully this will help you if your battery does show signs of failing, and has not been to boring (I do like my technical information 🙂 ), you will now probably know more than the people arriving to test your batteries!!

Dan Jones Written by:


  1. Ilkin Jamalli
    February 19, 2020

    My CCA dropped from 570CCA after full day of charging to 535CCA, although it meant to be 720CCA. So I fully charged it and voltage went up from 11.xV to 12:56V, disconnected it from the car and left disconnected for 3 days. Voltage dropped to 12.3V and lost 35CCA. Battery tester was showing healthy after the charge but 3 days later it said Charge it. The battery is 1 year old, Do you think it is healthy?

  2. Tim green.an
    July 9, 2021

    My voltage shows 12.7 when I switch on the ignition without engine, and 14.2 when the engine is running. Im assuming my battery is healthy

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