Battery charge quickie

[jakesmith]

Trying to diagnose a battery drain issue, battery was new in December, Bosch S5 AGM so decent. It has gone flat a few times due to a so-far undiagnosed fault

Last time car was in the garage they traced it to my Dashcam, put it on its own circuit (apparently it was causing the cars canbus to wake due to the fuse socket that I had put the piggy back on)

Anyway the garage rewired all that and tested it by putting the meter between the battery and the car and the residual current usage was super low like 5ma or something so seemed fixed

But a week later after standing, it still had lost a load of charge and I needed a booster to start it

I’m wondering if the fact that before it went into the garage, it did go flat a few times, has damaged the battery and it now won’t hold a charge

Thought I’d test it by buying a trickle charger, charging it up, then testing later

I’ve read about surface charge overstating the battery’s charge. Does that apply after trickle charging or just after driving, ie if I trickle charge the battery to 100%, can I test it with the multimeter straight away for voltage and again in 12/24 hours? Or do I need to wait 12 hours to take my initial reading?
Or should it be over 12.7v after a 12 hour gap after trickle charging anyway?

[GreenV8S]

jakesmith said:

I’m wondering if the fact that before it went into the garage, it did go flat a few times, has damaged the battery and it now won’t hold a charge

Almost certainly it has. Car batteries are designed to provide high current for short periods. They really don't like being deeply discharged. Even just once or twice can significantly reduce the battery's capacity. So this definitely won't have done the battery any favours and may be the only problem you have left. But before you replace the battery, measure (or get somebody to measure) the car's residual current drain to see whether there is still a problem. It would be a shame to splash out on a new battery and then knacker that by running it flat again.

[paintman]

After fully charging off the car leave the battery disconnected & standing overnight.
As good a guide as any:
https://haynes.com/en-us/tips-tutorials/how-test-c...

Don't forget to make a note of your radio security code before disconnecting the battery & you may need to reset windows etc.
If in doubt as to your ability probably best to leave it to the garage or get a good indy auto electrician to have a look in case the garage are missing anything.

Edited by paintman on Monday 25th February 20:37

[jakesmith]

paintman said:

After fully charging off the car leave the battery disconnected & standing overnight.
As good a guide as any:
https://haynes.com/en-us/tips-tutorials/how-test-c...

Don't forget to make a note of your radio security code before disconnecting the battery & you may need to reset windows etc.
If in doubt as to your ability probably best to leave it to the garage or get a good indy auto electrician to have a look in case the garage are missing anything.

Edited by paintman on Monday 25th February 20:37

Thanks, if I charge it off the car with a trickle charger (just bought a Chelsea mxs 5) presumably that will tell me when it is fully charged and I take my second reading 12 hours after that? Can I read it with a multimeter straight after trickle charging or does the batter get ‘surface charge’ even after trickle charging?

[Mignon]

You can remove surface charge by turning on the headlights for 3 minutes.

[jakesmith]

Mignon said:

You can remove surface charge by turning on the headlights for 3 minutes.

Cool ok thanks

[paintman]

jakesmith said:

Thanks, if I charge it off the car with a trickle charger (just bought a Chelsea mxs 5) presumably that will tell me when it is fully charged and I take my second reading 12 hours after that? Can I read it with a multimeter straight after trickle charging or does the batter get ‘surface charge’ even after trickle charging?

No idea how your charger works, probably best to read its instructions.
From the link: "If you get less than 12.6 volts, disconnect the battery and fully charge it with a battery charger. Then test after an overnight rest."
I sometimes wonder if people read links.
Or do as Mignon has advised.

[Penelope Stopit]

A battery can't be tested with a Multimeter, a battery needs load testing with a tool made for the job
Charge the battery one more time, leave it standing for several hours to cool down and then have it tested by a specialist

Edited by Penelope Stopit on Tuesday 26th February 16:59

[MatrixXXx]

You can test A battery with a multimeter, it will give you the general state of charge ie 12.6+ charged, 12V flat.

take your battery to a garage and get them to carry out a discharge test, they apply a high load for a few seconds and see how quickly the battery volatge drops. flatening a lead acid battery will shorten its life but not damage it , lithium ion is different, flattening or overchaging then will cause damage, ie on hybrids.

if the battery is faulty then it should be still under waranty?

sounds more like your car is not switching off completly and draining the battery, the dealer will be able to plug into your car and see what is active when the ignition is off.

[jakesmith]

Thanks the problem is after it was fixed, the garage did a consumption test when locked and the car was well within acceptable power draw

They identified and fixed the cause of the excess drain via a parasitic test, and retested it after and said it was all sorted

A week later the battery was flattish again requiring boost to start the car

So I am wondering if, as the garage already did the draw test that the battery itself is to blame for going flat?

Either that or the garage did the draw test and then another problem developed or the original one hadn’t been correctly identified

It is a very well known and regarded garage so I have faith that they know what they’re doing

Edited by jakesmith on Wednesday 27th February 15:04

[Penelope Stopit]

I'm confused

Have the battery load tested

[Penelope Stopit]

MatrixXXx said:

You can test A battery with a multimeter, it will give you the general state of charge ie 12.6+ charged, 12V flat

No you can't test a battery with a multimeter

You can measure its voltage

[pcn1]

Had a similar problem years ago with a newish (about 6 months old) Bosch S4 battery going dead after a few days. Turned out to be the electric ariel was constantly pulling current. This happened many times untill I figured it out.
Anyway unplugged the ariel but battery would still go flat after week. Thought battery must be knackered as well.
Returned to online battery supplier (at my cost....) , who charged and tested it with their gear. They said its fine mate, nothing wrong with the battery. So I asked the guy, just keep it 1 week and test it again will you ? If still OK send it back to me.
He called a week later and said its dead, dont know whats wrong with it. Sent me a warrenty replacement

The moral of the story in my huble opinion.... Batterys are still a unpredictable even to the experts. If In any doubt, I just buy a new one rather than be stranded by the roadside. Also I buy from local Euro car parts now, just as cheap and easier to return if a problem.

[jakesmith]

It’s about 2 hours minimum to change the battery on an R8, the wiper arms have to come off along with 4 large trim pieces and then the trunk. Doing it myself took 4 hours as I was being so careful

I am keen to diagnose it first

[MatrixXXx]

Penelope Stopit said:

No you can't test a battery with a multimeter

You can measure its voltage

you can see the state of charge, the EMF of a battery is its voltage ,12 V and its is flat, 12.6 and above it is charged, that is if the battery internal resistance is in range.
if the battery is faulty the resistance test will show it.

when charging a battery it is all done on voltage, I am currently working on battery testing on a defence project for over 2 years.

[jakesmith]

MatrixXXx said:

you can see the state of charge, the EMF of a battery is its voltage ,12 V and its is flat, 12.6 and above it is charged, that is if the battery internal resistance is in range.
if the battery is faulty the resistance test will show it.

when charging a battery it is all done on voltage, I am currently working on battery testing on a defence project for over 2 years.

Seems legit!

How do I test battery resistance? Can I do that with my trusty multimeter? I mean I know it can measure resistance but what should I be doing / looking for

When the RAC guy was trying to jump start me he said that it seemed like there was high resistance somewhere, maybe the battery or starter?

[Penelope Stopit]

MatrixXXx said:

Penelope Stopit said:

No you can't test a battery with a multimeter

You can measure its voltage

you can see the state of charge, the EMF of a battery is its voltage ,12 V and its is flat, 12.6 and above it is charged, that is if the battery internal resistance is in range.
if the battery is faulty the resistance test will show it.

when charging a battery it is all done on voltage, I am currently working on battery testing on a defence project for over 2 years.

I haven't once mentioned that the voltage can't be measured
A battery can't be tested with a multimeter
A battery can be tested by applying a load and measuring its voltage

[jakesmith]

If the battery is discharging itself when off the car, can a multimeter test that? By taking a volts reading after charging and removing surface charge, waiting 12 hours and then reading volts again?

[MatrixXXx]

Penelope Stopit said:

I haven't once mentioned that the voltage can't be measured
A battery can't be tested with a multimeter
A battery can be tested by applying a load and measuring its voltage

Put it another way, you cant test a battery without a voltmeter (which is the point i was trying to make)

[Penelope Stopit]

jakesmith said:

If the battery is discharging itself when off the car, can a multimeter test that? By taking a volts reading after charging and removing surface charge, waiting 12 hours and then reading volts again?

Yes you can, at least it will give you something to go on
Follow the information posted above by MatrixXXx