Battery power

[nelmo]

I don't get electricity much - a multimeter across my battery terminal showed 12.5v but there wasn't enough power to even get the starter motor turning. A few minutes on a charger and it just about turned over a couple of times. 

Can anyone explain this to me? I've just been using the multimeter and assumed my battery was OK but it seems not - how do you test if a battery is any good? 

Edited November 29, 2020 by nelmo

[dandan62]

I'm sure somebody much more expert than me will be along soon but a fully charged battery should read a bit more than 12.5 v I think. I know that if it reads 12v it's at the end of its life probably. Personally I would make sure both your terminals are clean and tight. Check the earth connections are clean and tight on chassis and engine. Then give the battery a full charge and see how you go. Good luck. 

[nelmo]

Left it charging overnight, showing 12.7v but still not enough to start the car. Jump started the car and went for a short blast - will leave it on the charger now and see if the voltage increases or if my battery is just knackered. 

[tractor]

if you can do it try and get a multimeter reading whilst turning it over 

[alantoon]

It's the season for battery failures, i have just had to replace mine

 

[Andi]

After charging it will show high figures. But leave it over night then check.

Like Alan said. Tis the season for knackered batteries.

[geordie40]

Halfords and such places will do free battery checks, as mentioned before a fully charged battery should read about 12.6-12.8V but even then it can be the battery is just knackered something to do with the lead plates and acid loose their electrical properties

[alanrichey]

I’ve also noted that when I was a young lad (many years ago ) car batteries used to gracefully degrade and give you plenty of warning of impending failure. These days they seem to fail catastrophically overnight with little or no warning.  I’ve also just had to replace mine as 24hrs after a full charge it wouldn’t start the car.

[sylvia.fry2]

Try a heavy boost charger if possible --- it will (should ) dislodge the sulphate on the plates & allow (hopefully ) a higher amount of charge & then able to start the car.

But it's probably a warning that a new battery is required, even if the above gives an engine start. 

[Sparepart]

I'm no expert, however, a brand new fully charged car battery has thick electrodes and strong sulphuric acid. The energy is in the acid. As current is drawn the atoms on the surface of the electrodes migrate into the acid. The acid gets "weaker" and becomes more and more a salt. When the battery is charged the current returned causes the electrode atoms in the salt to return to the electrode surface. These two processes take their turns as the battery is discharged and charged. In a perfect world this would go on for ever. In reality, slowly, less and less of the electrode surface is returned because the atoms dont bind well with the surface and fall to the bottom of the cell and so loose electrical contact other atoms also tend to contaminate the surface. Also the structural integrity of the electrode is compromised and it becomes more fragile and brittle. The electrode is organised as a series of thinnish parallell plates to maximise surface area to be able to generate large currents on discharge. Unfortunately the currents also unleash magnetic forces between the plates, so they have a tendency to twist. Each cell creates a little over 2 volts of potential, there are 6 cells, each cell has two multiplate electrodes. The battery manufacturer wants to make as much profit as possible, so the construction of the cells and the thickness and purity of the plates and acid are all at a minimum to satisfy the spec and "lifetime" guarantee. Put this all together, and you can see that Voltage by itself is no guarantee of the state of the battery. Yes if one cell should fail, the voltage will drop immediately but its the voltage under load that is more important. A geriatric battery can fail suddenly because a trauma has caused a plate to fall to pieces or a plate to go short circuit to the opposite plate or if the high current magnetic effect causes the same things. An old battery starting a big engine on a cold morning is as deadly as CORVID19 to it. Charging does not help because the electrode surface area is too small to generate enough current.  Still after all this babble, how can you tell if it's a good battery ?, the best way is by owning it from new and noticing how it performs, as it ages you will definitly see that it is more and more reluctant to get out of bed in the morning, treat it with respect, don't bump it or drop it, if you have to charge it, try and do it slowly and gently with care and respect. Then as it gets into old age be prepared to say goodbye and change it before the inevitable heart attack occurs.

[nelmo]

Jees, if that's not being an expert, I'd be scared to meet one 

Thanks for that - makes a lot of sense, in particular the bit about voltage alone not being much use, which matches with my current experience.

I will check the voltage when cranking - that seems to be recommended elsewhere as a good test as well.

Edited November 30, 2020 by nelmo

[nelmo]

So when cranking, the 12.7v drops to 10.5v and the starter motor just about turns but not enough to start the engine. Online wisdom suggests anything below 9-9.5v when cranking is dead battery time, so not quite there but as its not starting my car, might as well be

Off to Halfords we go... 

[magh]

Generally I’ve found that to be a bad cell. But yup new battery time. 

[Miked5]

A battery can read 12v but have no cold cranking amps which is the power/load it needs to turn the starter. I've seen this before. Sounds like new battery time

[theduck]

10.5v might be low enough that the ecu powers down.