Puzzled by the way my battery behaves
Discussion
I have a Renault Captur 1.5 diesel. It is fitted with an AGM battery as it has stop/start.
My problem is that after a run when I test the battery the voltage it is around 12.8/12.9 however the following day it has dropped to 12.0. Normally I would think that the battery was on its last legs but I have recently left it parked for 12 days without using it and the battery is still on 12.0. it does not seem too degrade any further after the first few hours. I would have expected it to be down to the high tens or so and strugle to turn the engine over. Has anybody got any thoughts on this? it always bursts into life at first touch of the button.
My problem is that after a run when I test the battery the voltage it is around 12.8/12.9 however the following day it has dropped to 12.0. Normally I would think that the battery was on its last legs but I have recently left it parked for 12 days without using it and the battery is still on 12.0. it does not seem too degrade any further after the first few hours. I would have expected it to be down to the high tens or so and strugle to turn the engine over. Has anybody got any thoughts on this? it always bursts into life at first touch of the button.
loggo said:
I have an intermittent fault with my heater fan which forums ascribe to low battery condition hence my interest in the battery
Maybe start with that next time. The battery voltage may be a symptom of the problem, but if you've got a healthy working car and just see a meter reading you don't understand, most likely you're worrying over nothing.12.8V is about right for a fully charged healthy lead acid battery at rest.
12.0V is about right for a battery that is roughly half discharged.
It's unusual for a battery voltage to drop that far overnight.
Perhaps you have a high current drain that stops after a few hours, such as a vac pump, cooling fan or similar.
Perhaps your battery is really unhealthy and not actually taking a full charge. It is not hard to start a healthy engine n warm weather and even a weak battery won't necessarily show any problems until you get to cold weather or an engine that is reluctant to start.
Perhaps your meter is inaccurate.
Perhaps you aren't measuring the voltage directly across the battery.
A battery that is in the high tens isn't 'struggling to turn the engine over', it is below the fully discharged voltage and probably damaged.
Thank you GreenV8S. A very comprehensive response.
The reason I focused on the battery issue is because questions like this often fly off at something of a tangent.
"12.8V is about right for a fully charged healthy lead acid battery at rest"
I am up to speed on led acid batteries but this little tinker is an agm which is something of a closed book to me.
"12.0V is about right for a battery that is roughly half discharged"
As you say however There is no obvious reason why the battery should be half charged
"It's unusual for a battery voltage to drop that far overnight"
Indeed
Perhaps you have a high current drain that stops after a few hours, such as a vac pump, cooling fan or similar.
Checked and iliminated
"Perhaps your battery is really unhealthy and not actually taking a full charge. It is not hard to start a healthy engine n warm weather and even a weak battery won't necessarily show any problems until you get to cold weather or an engine that is reluctant to start"
Very true and something I shall be keeping an eye on in the next few weeks. The only real possibility but I am perplexed by the rapid dive to 12.0 with no further drop
"Perhaps your meter is inaccurate"
I get a 12.7/12.8 reading from my other 2 cars and my boat so I think the meter is accurate
"Perhaps you aren't measuring the voltage directly across the battery"
As above I seem to be getting a an expected reading from my other tests so I guess I have the hang of reading the battery correctly
"A battery that is in the high tens isn't 'struggling to turn the engine over', it is below the fully discharged voltage and probably damaged"
Just as you say however not relevant to my question as I was merely stating what I would have expected not what the actuality is.
The reason I focused on the battery issue is because questions like this often fly off at something of a tangent.
"12.8V is about right for a fully charged healthy lead acid battery at rest"
I am up to speed on led acid batteries but this little tinker is an agm which is something of a closed book to me.
"12.0V is about right for a battery that is roughly half discharged"
As you say however There is no obvious reason why the battery should be half charged
"It's unusual for a battery voltage to drop that far overnight"
Indeed
Perhaps you have a high current drain that stops after a few hours, such as a vac pump, cooling fan or similar.
Checked and iliminated
"Perhaps your battery is really unhealthy and not actually taking a full charge. It is not hard to start a healthy engine n warm weather and even a weak battery won't necessarily show any problems until you get to cold weather or an engine that is reluctant to start"
Very true and something I shall be keeping an eye on in the next few weeks. The only real possibility but I am perplexed by the rapid dive to 12.0 with no further drop
"Perhaps your meter is inaccurate"
I get a 12.7/12.8 reading from my other 2 cars and my boat so I think the meter is accurate
"Perhaps you aren't measuring the voltage directly across the battery"
As above I seem to be getting a an expected reading from my other tests so I guess I have the hang of reading the battery correctly
"A battery that is in the high tens isn't 'struggling to turn the engine over', it is below the fully discharged voltage and probably damaged"
Just as you say however not relevant to my question as I was merely stating what I would have expected not what the actuality is.
Have it tested on a proper battery tester that measures cca, capacity and internal resistance. Voltage measurement only tells a small part of the battery health story.
In my experience my 2 stop start vehicles the batteries were noticably tired and needed replaced by 4 years old where i have had 6 to 10 year old batteries work just fine in non stop start vehicles.
Bit of a pain as stop start batteries are more expensive.
In my experience my 2 stop start vehicles the batteries were noticably tired and needed replaced by 4 years old where i have had 6 to 10 year old batteries work just fine in non stop start vehicles.
Bit of a pain as stop start batteries are more expensive.
loggo said:
I am up to speed on led acid batteries but this little tinker is an agm which is something of a closed book to me.
They are chemically the same as any other lead acid automative battery and have the same charge/voltage characteristics. The glass mat construction just means the electrodes are supported and isolated better.SlimJim16v said:
I found this when I was looking for info on AGM vs normal batteries. The minimum OK voltage surprised me for the AGM.
That surprises me too. I can't refer to a technical spec, but I've seen numerous guides relating to AGM batteries giving voltages which are similar so wet lead acid batteries. Possible that data is relating to a deep discharge storage battery? They do have different voltage characteristics but aren't usually used on vehicles.Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff