Many thanks Puggit Auld Jock, a fantastic explanation and information.
I will monitor the voltage and if I change the battery will report back here with how it went, as it may help somebody else.
Does anyone know the best way to check a battery out of the car?
I realized I said above that battery deterioration can be slowed by good practice and maintenance but then said nothing about what that might mean in practice. With access to individual cells on modern batteries being very difficult you can't easily check electrolyte levels - although often the case is semi transparent so you can see the fluid level even if there's not much you can then do about it if it's low. I understand sealed modern batteries have a slightly different composition to their electrolyte to reduce gassing so they shouldn't loose much during a normal lifetime and you shouldn't expect to need to top them up anyway.
Maintenance? One of the most neglected components on many cars is the battery. How often do you open someone's bonnet and the battery top is absolutely filthy? A dirty battery top allows moisture to accumulate which creates a path between the battery terminals and allows the battery to, very slowly, discharge. So keeping the top of your battery clean is a must. As is looking for corrosion on the terminals - often a greenish white crusty deposit. If you see this you really need to remove the terminal and clean up both the battery post and terminal. Tools like this work pretty well and are not expensive:
https://www.ebay.co.uk/itm/20375931...CBBxOElelYgKzJvN39tJZSrmTvTz8RFxoCHwwQAvD_BwE
Here's a video showing one in use:
Having done that and got a nice shiny metal to metal contact I like to neutralize any existing "gunge" by sprinkling a little baking powder over the terminal and then cleaning it off with an old toothbrush dipped in lemon juice. This completely gets rid of any lingering corrosive. Now reassemble the terminal and give it a light coating of terminal protector - which can be bought in spray cans or manually applied grease - which will stop further problems. (Simple petroleum jelly - Vaseline - works very well too). If the car is quite new and/or has no signs of corrosion on the terminals, it's worth applying some terminal protector anyway.
If you're going to disconnect a battery terminal there are two big problems you can quite easily avoid if you know about them. First, and probably most important, always disconnect the negative terminal first and reconnect it last. This is because if you tackle the positive terminal with the negative still connected to ground then if you inadvertently touch a metal part of the car with the tool whilst undoing the positive terminal then the tool will complete the circuit and cause a massive short. The tool might get so hot it burns your hand, the sparks could ignite battery gasses and blow it up! but even more expensively, you might cause damage to electronic components. So negative off first and on last. You also need to be aware that disconnecting batteries on modern cars is fraught with problems because some of the electronics need to be permanently powered on. So simply disconnecting a battery terminal can cause stuff like window winders, central locking and many other things, to stop working as they should until reprogrammed. This can be very easily avoided by connecting a "saver" to the diagnostic port:
Amazon product ASIN B098SWMF5H You can get ones that connect to the cigar lighter socket but on many cars you need to have the ignition turned on to complete the circuit and it's too easy to forget to do this so you can connect it to the battery terminals (or a slave battery) plug it in and forget to turn the ign on. Whereupon everything "dies" as you disconnect the vehicle battery. Much more fullproof with the Diagnostic port connector:
If you want to explore battery maintenance in more depth try you tube and google, there's lots of advice. but if you keep your terminals in good condition and the battery clean you're more than half way there.
Then there's "good practice". In simple terms this concerns keeping the battery well charged. Modern charging systems are pretty good at charging the battery but if you're only doing short journeys all the time it's not giving it a fair chance. A good run out from time to time will ensure the battery gets a good charge and also is very good for the engine, especially it's oil, which tends to collect contaminants like unburnt fuel and condensation. If the engine is allowed to get nicely up to temperature and that includes the oil which on an "average" engine will take about 3 times longer to get up to temperature than the coolant does! - many people don't know that - then stuff like condensation and unburnt fuel will be vapourized and pass out of the engine through the breather system. This is one of the reasons why you shouldn't ignore oil changes even if you do a very low annual mileage.
It's a good idea to check standing battery voltage from time to time because a fully charged battery will sulphate much more slowly than a well charged one. One of the worst things you can do to a lead acid battery is not use it. They like to be "worked". So, if we're going to check it with a voltmeter what are we looking for? Well, first thing is, don't be "frightened" to use a voltmeter. A voltmeter has almost infinite resistance so it passes no discernable current and can't damage anything, unlike an ammeter with which you can do no end of wicked things! So you're going to apply the positive (red) probe of the meter to the positive (red +) battery terminal and the negative (black) probe to the negative (black -) terminal of the battery. Don't worry if you do connect them wrong way round because most digital meters have protection - most will just show the reading with - in front if you do this. So what are you looking for. Well, if the engine is running the alternator will be charging the battery and you'll likely see somewhere around 13.5 to 14.5 volts. This may seem strange though because this is a 12 volt battery isn't it? Well yes, but actually a 12 volt lead acid battery actually makes about 2.1 to 2.2 volts per cell and they have 6 cells, so, in theory a fully charged 12 volt car battery should show about 13 volts. Nothing in this world is perfect though so, in real life, what we get is around 12.5 to 12.8 volts BUT you need to let the battery rest for at least an overnight after the engine has run for the battery to equalize it's charge. (The surface of the plates react with the acid so can assume a higher state of charge then deeper into the plate - called surface charge - and you need to let this surface charge disipate into the depth of the plate to get a true reading). So, if you've been running the car then check it first thing next day for a meaningful reading. Of course modern cars have permanently live systems which pull current all the time - one reason why sometimes your car won't start when you come back from your annual holiday - so if you leave it for say, a week without starting before checking it don't expect a meaningful reading. As a general guide an open circuit voltage (that is a battery not connected to anything) will read about 12.6 volts after resting, around 12 volts when half charged and 11 volts when three quarters discharged.
The trouble arises when you try ascertain a battery's condition using simply a voltmeter to take these readings. A battery with quite severely sulphated plates can often be charged up well and when checked with just a voltmeter - remember the voltmeter doesn't actually consume any electricity - will display good figure (see above) but when you try to use that battery on a vehicle it won't turn the engine over. This is because with large areas of the surface of the plates sulphated what you have in effect is a very small battery because much of the plate surface area is incapable of reacting and therefore ineffective.
So now to your last question - How to check the battery when out of the car? You can start off by putting a voltmeter on it and seeing if you're getting at least somewhere over 12 volts. This tells you whether the battery is accepting a charge. If the figure is low you'll need to charge it and check it again before knowing if it's worth going any further. If you think the battery may be Ok then you need to find out what condition the plates are in and whether they are reacting with the electrolyte satisfactorily. You can do this the age old "primitive" way of whacking a large load on it. Simplest way to do this is to put it in a vehicle, pull the fuel pump fuse so the engine can't start (this may set a trouble code) and try cranking the engine. If it cranks well the battery is probably good (although there can be some problems this won't detect - like internal plate shorts for instance which can cause a slow discharge) You can simulate doing this cranking test with an old style load tester:
https://www.machinemart.co.uk/p/cla...qPn9ZT9sK6-VjL8nh7RoC6bIQAvD_BwE&gclsrc=aw.ds There's you tube videos if you want to see one in action. It was an early version of this with which I learned to test batteries and it works quite well if you know what you're looking for the gauge to show you. Now a days there are sophisticated electronic testers which, I think, pass a low current through the battery and measure internal cell resistance. The one I fancy is the TOPDON BT200 - Is it you PB who recently bought one? These seem to be safe to use even when the battery is still connected to the car whilst I'd maybe think twice before using the old heavy load tester type what with all the electronics just waiting for an excuse to cost me a fortune! Trouble is I so far haven't managed to make a good enough case to Mrs J for funds to be allocated!
