11.9v indicates the battery is completely discharged. Anything under 12.2v means the battery is in a state of discharge, and a healthy battery should show 12.6v in a static state. Yours needs to be replaced.

We also might refer to a battery as being knackered, or whatever. But in scientific terms like anything else there is one of many things that might be wrong with a battery, and all will produce different symptoms. a battery tester cannot understand the term 'knackered', but they will carry out a particular test depending on the type of tester.

I was taught a lot about batteries and battery faults when I was an AA Patrol, especially considering battery related problems are one of the biggest reasons people use breakdown services.

Firstly the engine starting or otherwise is not conclusive proof of battery condition. There's a lot more to it than that. But a very good reliable test is as follows.

With a cold engine, and first thing in the morning measure the battery voltage. If it reads 12.6v there's a good chance it's healthy. When the engine is running the battery voltage will be around 14.4v. This builds up a surface charge on the battery which must be removed before testing. You will not get an accurate diagnosis if it isn't. If the engine has run switch on the headlamps for about 20 seconds to take the surface charge off, then carry out the following test.

You will need to prevent the engine from starting by pulling the ECM fuse or something like that. Then connect a voltmeter across the battery and crank the engine for a minimum of 10 seconds. Observe the battery voltage whilst cranking and a petrol engine must maintain at least 10v for that cranking period. A diesel must maintain 9.5v for that time. When you stop cranking the voltage should return to around 12.6v after a short while. Crucially during cranking the voltage change between the start and the end of cranking must not be less than 0.3v, eg if it starts at 10.5v it must not fall lower than 10.2v while your cranking for 10 seconds. If it does the battery needs replacing.

If it doesn't it's the best indication you can get that the battery is in good condition.

Thanks for the advice all - sounds like it's well past time to get a new battery, I'll start with that.

You might be wasting your money

Measuring the voltage of battery under no load is not the whole story. The important thing about batteries is not the voltage but the internal resistance. Now if you have an ohmmeter don't try to measure this. If you try then you are putting a full short across the battery which is very dangerous.

A good battery will have a very low internal resistance so whatever load is applied across it the voltage across the load will be much greater then the voltage across the internal resistance of the battery. In a car the starter motor is by far the biggest load you can apply. So when starting the car almost 12V will be applied to the starter with a good battery. with a bad battery this will be much lower. Generally if battery voltage drops below 10V when cranking then battery either needs to be charged or replaced if it won't take charge. So you could try once you get car started by either jump starting or charging the battery with a battery charger for at least 8 hours. Measure the voltage when actually trying to start the car

If you don't fancy this then you could get a battery tester. This measures the internal resistance of the battery by placing it under a known load and the meter is a voltmeter with red, yellow or green calibration marks to show the condition of the battery. Just seen a battery and alternator tester on Google search for £15. Might be worth buying, cheaper then a new battery. Last time I had battery issues local auto factor tested it for free, I think most places should do this.