After eight years of the same battery in my car I decided to proactively replace the old battery with a new one - a diehard platinum AGM unit of the correct size that was fully charged before installation. I have never had a battery drain issue with the car after OBD port access - red triangle always turns off after about 15 minutes. I always put the car on a CTEK if it hasn't been driven for more than 2 weeks.

I recently started using the Torque app to monitor PIDs and one is the battery voltage. The thing I noticed is the battery voltage after an hour drive is usually approximately 12.5 to 12.7 volts with the engine off and the car asleep (red triangle off). When the car is started and idling the voltage is between 13.0 and 14.6 volts. When I am driving (highway cruising) the voltage is usually between 12.1 and 12.6 volts except when I'm off the gas petal or I'm accelerating and the engine rpm is above 3500 to 4000 rpm.. Then the voltage is 13.6 to 14.5 volts. Is this normal? I thought the charging system should always stay above 13.0 to 13.6 volts. If not what could the issue be? The behavior I am seeing it doesn't seem like I have a belt slipping or that the alternator is dead/bad. BMS or current sensor on the negative battery terminal?

The car is throwing no codes and there is no idiot light light on the dash. I also installed a separate battery voltage monitor (cigarette lighter style) and it is always in close agreement with the PID.

 

14.5V is normal for AGM charging. At least I hope it is as mine has done it since 2018 to my knowledge and the battery is still fine. As you have a CTEK have a look in the handbook that came with it - it should show the various voltages it applies at each stage of charging.

 

I also have a plug in voltmeter and see similar behavior.
It is perfectly normal. Do not worry.
I will leave it to others to explain the reasons.

 

I don't understand the voltage being so low when cruising; can't see why the voltage should be less than when you are ticking over. 12.1 seems too low. Maybe there's a reason with these new style batteries....

 

I declined offering an explanation in my earlier post because I am not an expert, but my understanding is as follows.
I do not think the alternator charges continuously at the highest current (read battery voltage).
The charging current is modulated depending on the engine speed and state of charge of the battery.
The voltage reading reflects the net flow into and out of the battery.
When the vehicle is cruising at decent speed and has a fully charged battery, the alternator does not need to keep charging the battery at full tilt.
When you are stopped at a junction and the AC is on there is current deficit that requires more input.
Also, FWIW, even though I drive regularly, whenever I put the CTek on (before leaving for a few weeks) it takes several hours to get to the green state of charge.

 

It still seems wrong that the battery, not the rate of charging which goes down according to battery state, should drop to 12.1 - 12.6
the alternator should surely keep the battery at about 13.2 or so, and never below 12.7?

 

If you're measuring it through the 12V socket I'm not convinced that that is the true voltage at the battery. Maybe some regulation/protection circuitry getting in the way? My socket often shows less than 12V when parked and off.

 

I think as far as the OP is concerned, there is no need to worry.
Presumably there has not been a battery alert and the car starts and runs OK.
You can "drive" yourself nuts trying to understand the minutia!!

 

Update

Went for two more 45 minute drives (round trip). At the start the battery voltage was 12.4V with the car asleep (measured at the booster terminals under the hood with Fluke). When car was started the voltage went up to 14.5V and stayed there. At the end of the first 45 min drive forgot to look at battery voltage with car off and asleep. For the second 45 minute leg the battery voltage started at 14.5V with the car running and slowly fell to 14.1V. At the end of this drive the battery voltage was 13.1V with the car asleep .

Further research on a UK Jaguar forum (in the XJ section I think) there was an explanation that this behavior was the BMS recalibrating itself to the new battery. Apparently the BMS cycles the battery through two discharges from full to 70% and then goes back to a normal charging regimen. Maybe that is what was going on.

Stay tuned.

 

As stated in Post 5, these days rate of charging is based on electrical draw. That is, after the car is started, the car's electronics will provide full output (~14V DC ) for a while, but once the battery is charged to a sufficient level, it will be allowed to 'run off the battery' (12V DC) until needed.

If you are worried about alternator output, monitor voltage with the lights on, and the interior fan on HIGH. That should make the alternator be voltage seen as 14V.


Many newer cars have systems like this. Saves gasoline, as does electric power steering assist. Why load the engine down driving a PAS pump or alternator all the time?

 

Yeah, more the power for the supercharger!

 

If you are really interested in keeping tabs on your battery. Just install an Ancel BM300. this simple devise installs in minutes and data logs your voltage constantly. I use these on my EV's to keep tabs on the 12v battery.

 

You need them on EVs, they chew through 12V batteries in a couple of years! Those batteries are not fit for purpose wrt EV charging technology.

 

That is not accurate. My I-pace has had the original 12v battery since new in 2019. Older Tesla's had issues due to poor BMS programming, but that is a thing of the past. My Hummer EV charges the 12v battery as soon as it goes below 12.6v.

 

You'd be in a minority on the UK i-pace forum, then! Loads of them have failing 12V batteries after 3 years - quite a few people on there recommend changing as a matter of course after that time to avoid future issues.