Car battery health testing - voltmeter & voltage drop
Discussion
Hi,
I am trying to test the health of my car battery as I think it's worn out, the specs are as below. The battery obviously shows approx 12V when turning the key to the ON position but goes as low as 8V when cranked and starts the car, albeit sluggishly. If the car is not started for ~2 weeks the battery and goes from 12V to as low as 6V when engine cranked and will not start the car. Halfords say my battery health is at 29%.
Does anyone know what the **voltage drop** should be on the voltmeter when **starting** the car and how that **corresponds to** the **battery health**?? I've read various numbers that the voltage should not drop below 10V if battery is new, but for used batteries others say 9V, some say 8V and I've even read as low as 7.2V is OK to start the car!
I'm wondering if I could use this voltage drop vs battery health as a test when buying a second hand battery as I plan to sell the car in the next 12 months. I don't own the fancy battery health checker that Halfords have but have a voltmeter and so thought this may be a way to check for good battery when buying.
Car = 2007 Auris 1.6L Petrol
Battery:
Voltage 12 Volts
Capacity (C20) 55 Ah
CCA (EN) 500 A
I am trying to test the health of my car battery as I think it's worn out, the specs are as below. The battery obviously shows approx 12V when turning the key to the ON position but goes as low as 8V when cranked and starts the car, albeit sluggishly. If the car is not started for ~2 weeks the battery and goes from 12V to as low as 6V when engine cranked and will not start the car. Halfords say my battery health is at 29%.
Does anyone know what the **voltage drop** should be on the voltmeter when **starting** the car and how that **corresponds to** the **battery health**?? I've read various numbers that the voltage should not drop below 10V if battery is new, but for used batteries others say 9V, some say 8V and I've even read as low as 7.2V is OK to start the car!
I'm wondering if I could use this voltage drop vs battery health as a test when buying a second hand battery as I plan to sell the car in the next 12 months. I don't own the fancy battery health checker that Halfords have but have a voltmeter and so thought this may be a way to check for good battery when buying.
Car = 2007 Auris 1.6L Petrol
Battery:
Voltage 12 Volts
Capacity (C20) 55 Ah
CCA (EN) 500 A
Edited by PistonAFC on Tuesday 9th September 19:17
Edited by PistonAFC on Tuesday 9th September 19:18
General rules of thumb.
A fully charged healthy battery should read 12.6/12.7 volts.
Every 0.1 volt under that represents a loss of charge of around 15% so 12 volts is basically around the 30/40% mark, which tallies in line with what Halfrauds have told you.
That is an APPROXIMATE rule of thumb.
When starting the volt drop on the battery will be affected by many things including starter health, wiring, temperature etc, so any drop in voltage can be very difficult to use to predict battery health, but as another rule of thumb the more it drops below around 10 volts the worse the battery health is as a good one shouldn't really drop below 10 volts.
A fully charged healthy battery should read 12.6/12.7 volts.
Every 0.1 volt under that represents a loss of charge of around 15% so 12 volts is basically around the 30/40% mark, which tallies in line with what Halfrauds have told you.
That is an APPROXIMATE rule of thumb.
When starting the volt drop on the battery will be affected by many things including starter health, wiring, temperature etc, so any drop in voltage can be very difficult to use to predict battery health, but as another rule of thumb the more it drops below around 10 volts the worse the battery health is as a good one shouldn't really drop below 10 volts.
paul_c123 said:
Battery health in a vague way, or actual "State of Health" which is a quantitative measurement of actual capacity vs stated capacity? There's a number of tools to measure SoH accurately on a battery, for example Topdon Artibattery 101 is about £30 and does everything you'd need.
I'm assuming something like this is what Halfords use to measure battery health (I guess they'd call it state of health) and device like the Topdon is not really worth investing in as a home user with one car, as I rarely buy car batteries. I was just wondering if anyone knew of a cheap and cheerful way of determining state of battery health using my FLUKE multimeter, and thought starting the car and determining the lowest the voltage drops to would give some indication of that.
E-bmw said:
General rules of thumb.
A fully charged healthy battery should read 12.6/12.7 volts.
Every 0.1 volt under that represents a loss of charge of around 15% so 12 volts is basically around the 30/40% mark, which tallies in line with what Halfrauds have told you.
That is an APPROXIMATE rule of thumb.
When starting the volt drop on the battery will be affected by many things including starter health, wiring, temperature etc, so any drop in voltage can be very difficult to use to predict battery health, but as another rule of thumb the more it drops below around 10 volts the worse the battery health is as a good one shouldn't really drop below 10 volts.
^^ Thanks, that's useful to know.A fully charged healthy battery should read 12.6/12.7 volts.
Every 0.1 volt under that represents a loss of charge of around 15% so 12 volts is basically around the 30/40% mark, which tallies in line with what Halfrauds have told you.
That is an APPROXIMATE rule of thumb.
When starting the volt drop on the battery will be affected by many things including starter health, wiring, temperature etc, so any drop in voltage can be very difficult to use to predict battery health, but as another rule of thumb the more it drops below around 10 volts the worse the battery health is as a good one shouldn't really drop below 10 volts.
Maybe the 7.2V I read comes from this and is the minimum voltage need to start the car:-
CCA stands for cold cranking amps and is the maximum current a battery can sustain continuously for 30 seconds while maintaining 1.2V/cell (7.2V for a 12 volt/6cell battery) while the battery is at 0 degrees F (-18 degrees C).
Also some useful information in this post : https://www.liveabout.com/vehicle-battery-and-load...
PistonAFC said:
I'm assuming something like this is what Halfords use to measure battery health (I guess they'd call it state of health) and device like the Topdon is not really worth investing in as a home user with one car, as I rarely buy car batteries.
I was just wondering if anyone knew of a cheap and cheerful way of determining state of battery health using my FLUKE multimeter, and thought starting the car and determining the lowest the voltage drops to would give some indication of that.
The only way to really test a battery is by putting a known load on the battery, a cars starter motor isnt a known load, the tester that halfords used probably was a load tester.I was just wondering if anyone knew of a cheap and cheerful way of determining state of battery health using my FLUKE multimeter, and thought starting the car and determining the lowest the voltage drops to would give some indication of that.
If in doubt , i would change it with winter on its way, batteries arent that expensive. And there is never a good time for your car to not start.
The rule of thumb based on voltage another poster said is a fair indication.
PistonAFC said:
Hi,
I am trying to test the health of my car battery as I think it's worn out, the specs are as below. The battery obviously shows approx 12V when turning the key to the ON position but goes as low as 8V when cranked and starts the car, albeit sluggishly. If the car is not started for ~2 weeks the battery and goes from 12V to as low as 6V when engine cranked and will not start the car. Halfords say my battery health is at 29%.
Does anyone know what the **voltage drop** should be on the voltmeter when **starting** the car and how that **corresponds to** the **battery health**?? I've read various numbers that the voltage should not drop below 10V if battery is new, but for used batteries others say 9V, some say 8V and I've even read as low as 7.2V is OK to start the car!
I'm wondering if I could use this voltage drop vs battery health as a test when buying a second hand battery as I plan to sell the car in the next 12 months. I don't own the fancy battery health checker that Halfords have but have a voltmeter and so thought this may be a way to check for good battery when buying.
Car = 2007 Auris 1.6L Petrol
Battery:
Voltage 12 Volts
Capacity (C20) 55 Ah
CCA (EN) 500 A
see this. https://www.youtube.com/watch?v=tsJUuLu1cw0I am trying to test the health of my car battery as I think it's worn out, the specs are as below. The battery obviously shows approx 12V when turning the key to the ON position but goes as low as 8V when cranked and starts the car, albeit sluggishly. If the car is not started for ~2 weeks the battery and goes from 12V to as low as 6V when engine cranked and will not start the car. Halfords say my battery health is at 29%.
Does anyone know what the **voltage drop** should be on the voltmeter when **starting** the car and how that **corresponds to** the **battery health**?? I've read various numbers that the voltage should not drop below 10V if battery is new, but for used batteries others say 9V, some say 8V and I've even read as low as 7.2V is OK to start the car!
I'm wondering if I could use this voltage drop vs battery health as a test when buying a second hand battery as I plan to sell the car in the next 12 months. I don't own the fancy battery health checker that Halfords have but have a voltmeter and so thought this may be a way to check for good battery when buying.
Car = 2007 Auris 1.6L Petrol
Battery:
Voltage 12 Volts
Capacity (C20) 55 Ah
CCA (EN) 500 A
Edited by PistonAFC on Tuesday 9th September 19:17
Edited by PistonAFC on Tuesday 9th September 19:18
PistonAFC said:
I was just wondering if anyone knew of a cheap and cheerful way of determining state of battery health using my FLUKE multimeter, and thought starting the car and determining the lowest the voltage drops to would give some indication of that.
That is what I gave you in my reply.SOH you cannot get without a dedicated battery tester, hence why I gave you the closest you will get with a Fluke multimeter.
Bit of a thread bump instead of starting a new one, this is the situation I'm in with 2 cars:
Mazda 6 2010
Yuasa HSB053 , 3 years old 580CCA
Voltage this morning after 24 hours of "rest" : 12.04V
Voltage when running (Alternator voltage) : 14.4
However, it's not lazy when starting, far from it in fact and certainly doesn't give any sign of it being drained.
Next, is an Impreza WRX 04
Bosch S4 024 6 years old 540CCA
Voltage this morning after 12 hours: 12.45V
Voltage when running :14.5
However, the Bosch just about managed 4 cranks before it sounded as if it was ready to throw in the towel.
Both starters and alternators on the cars are originals, but touch wood, never given any issues.
I can't work out why the Yuasa, is showing less voltage yet has plenty of power in it. whereas the bosch is the other way around. Would 40 CCA difference be the reason?.
Last question, if it looks like the Bosch needs to be replaced, would the Yuasa be the better option (for the sake of £22 difference on Tayna?)
Mazda 6 2010
Yuasa HSB053 , 3 years old 580CCA
Voltage this morning after 24 hours of "rest" : 12.04V
Voltage when running (Alternator voltage) : 14.4
However, it's not lazy when starting, far from it in fact and certainly doesn't give any sign of it being drained.
Next, is an Impreza WRX 04
Bosch S4 024 6 years old 540CCA
Voltage this morning after 12 hours: 12.45V
Voltage when running :14.5
However, the Bosch just about managed 4 cranks before it sounded as if it was ready to throw in the towel.
Both starters and alternators on the cars are originals, but touch wood, never given any issues.
I can't work out why the Yuasa, is showing less voltage yet has plenty of power in it. whereas the bosch is the other way around. Would 40 CCA difference be the reason?.
Last question, if it looks like the Bosch needs to be replaced, would the Yuasa be the better option (for the sake of £22 difference on Tayna?)
Where are these cars parked? It's cold this morning. The temperature difference between an attached garage and parked on the drive will be absolutely massive.
Battery voltages are generally assessed at something around room temperature. Battery efficiency/voltage reduces with temperature.
Battery voltages are generally assessed at something around room temperature. Battery efficiency/voltage reduces with temperature.
As above temperature is a massive issue to battery efficiency.
As is lights/fan/stereo being on at the time, as they are drawing current they drop the battery voltage quite a lot.
To do a direct comparison you need to make sure you are comparing apples & apples.
ETA.
That is before you get to possible wiring/connections/starter not being perfect.
As is lights/fan/stereo being on at the time, as they are drawing current they drop the battery voltage quite a lot.
To do a direct comparison you need to make sure you are comparing apples & apples.
ETA.
That is before you get to possible wiring/connections/starter not being perfect.
Ahh...bugger....
It was 1 degree C this morning when I checked them both, both outside overnight. I'd not taken the ambient temp into consideration, so that's a factor. I checked with everything off on the car, auxiliary wise.
I'll run the check again tomorrow afternoon, hopefully it's a bit warmer.
I have searched online, Tayna via Ebay with a voucher are 15% off at the moment, which is better than the website with no current offers.
Cheers Fellas
It was 1 degree C this morning when I checked them both, both outside overnight. I'd not taken the ambient temp into consideration, so that's a factor. I checked with everything off on the car, auxiliary wise.
I'll run the check again tomorrow afternoon, hopefully it's a bit warmer.
I have searched online, Tayna via Ebay with a voucher are 15% off at the moment, which is better than the website with no current offers.
Cheers Fellas
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