Battery conundrum - dying or not ?

[Crafty_]

Wondered what people's take on this is:

10 year old car on original battery.

Last weekend noticed it turned over slowly, but did fire, Tuesday (after a -4 night) slow single turnover but managed to fire.
Seemed faster crank in the warmer afternoon for return jouney, covered maybe 20-25 miles. Was stopped and restarted on the return journey, no problem.

The car then wasn't used at all, until friday mid morning, very lazy crank speed on one try and then dead. Weather has been much warmer than Monday/Tuesday over the past few days.
Battery does show as discharged according to the charger. Left on charge for a few hours, starts no probllem. usual fast turnover, made a short trip (say 3-4 miles), car stopped, restarts fine, drive home again, stop, restart - all fine.

At this point 14.7v when engine running, 12.2v immediately after shut off last night.
This morning 15+ hours later its still at 12v, frost overnight, starts no problem with usual cranking speed.

I kind of expected it to lose voltage overnight.

Positive that no interior light etc left on at any time, usage (admittedly light) hasn't differed from normal. terminals tight.

I have a new battery to put on, but not sure if I'm wasting my time and money if I can't fault the old one at present.

I'd be happy to say that the cold last week / start of this killed the battery off - but it started (just) ok on Tuesday in the cold and died between then and Friday, when it was warmer - last few days have been 1 degree low to 8 degree high.So doesn't quite add up.
So slow crank last weekend - only just started Tuesday, dead on Friday suggests whatever is going on was happening las weekend and through this week, apparently hasn't happened overnight in to this morning.

The only thng I can think of is for some reason the car didn't shut of al power and caused a drain, but for some reason is now ook and hasn't done that ovenright...I don't know how I'd even prove that either.

So do I condemn the battery and change it or return the new one as not needed ?
Voltage is obviously a bit low but isn't noticeable in terms of crank speed / any alerts on board.

[catso]

If a 10 year old battery is struggling on a cold morning then it's days are numbered, I'd replace it before it fails you completely.

[g3org3y]

10 year old and registering 12V, I'd be changing TBH (especially given you've got a new battery already there).

You know if you don't, sod's law it'll fail at the most inconvenient time.

[ashenfie]

Typical dead battery. 14.7 when running suggests charging is fine, just ensure the voltage does not go over that with say 2000rpm. Best done before you put the new battery in.

Edited by ashenfie on Saturday 10th January 10:59

[Crafty_]

Thanks folks, I was just put off by it not dropping overnight, maybe a trait of these newer AGM batteries. I'll change it.

Edited by Crafty_ on Saturday 10th January 11:33

[Jakg]

If it really is 12v, then thats quite discharged. See https://sunonbattery.com/agm-battery-voltage-capac...

Crafty_ said:

10 year old car on original battery.

Change it.

[OldGermanHeaps]

When they wear their internal resistance goes up, so a high charging voltage isnt a good indicator of anything other than the alternator works. Needs a proper battery test, but at 10 years just change it, its exceeded its expected life and is guaranteed to fail when its least convenient

[paul_c123]

Voltage is (relatively) meaningless if you don't know also what the current draw is at the time. For example, are you measuring voltage just after opening the car and unlatching the bonnet? Some cars can draw 10-15A in waking up various modules and doing initialisation processes on them. Unless you actually measured it with wires connected before unlocking it; AND knew the quiescent current draw at the time too, you're guessing. And what are you measuring voltage with? Many cheaper multimeters drift on voltage measurement quite significantly when their internal battery gets weak. And many cheap ones are basically hopeless at accurate voltage measurement in the first place.

But its an educated guess.

Best approach is to use a proper battery tester, there are old-style (which just impose a large current draw and monitor voltage over time); and newer ones which use imposed ripple current to determine State of Health more accurately, eg Topdon AB101 and similar.

[Matt_E_Mulsion]

As above, a battery that has lasted 10 years has done exceptionally well!

Its a no brainer, its starting to show signs of failure so change it now before it let's you down.

Oh and cheap batteries are cheap for a reason, they are false economy so always fit a decent one.

[E-bmw]

Just for context a brand new fully charged 100% healthy battery will show 12.6/7 volts at rest, every 0.1 volt under that is a loss of around 15% charge as a rule of thumb.

As others have implied this is not 100% cast-in-stone but a good rule of thumb.

So, your 12.0 volts is around 15% healthy (give or take) so that is your answer.

Your charging voltage is fine, so the alternator is working.

Your experience of warmer temperatures "helping" is normal as cold is the enemy of battery efficiency.

Change it & you will be fine.

When you do change it (properly) check all connections are good.

[fooman]

My 2014 diesel Merc is still on its original battery, I do consider changing it but it starts no problem even on the recent cold mornings, even after sitting a while. Maybe I'll check voltages though.

[OldGermanHeaps]

Some cars alter their charging curve over time to suit worn batteries, and you should tell the car in the coding that the battery has been replaced to not cause problems with the new battery.
Dont keep using your dying battery, low voltage can cause all sorts of problems on complicated recent cars.

Edited by OldGermanHeaps on Saturday 10th January 15:50

[trickywoo]

Treat yourself to a new exide with the highest cca for the spec you need.

Your existing battery is giving you a warning, don’t ignore it.

While the cranking ability reduces with cold so does the discharge rate. If it lasts long enough come the summer it’s likely to become significantly discharged over a shorter period of time.