Still trying to figure out why my battery is discharging. Dealer said they could not find anything wrong, no codes. They tested the alternator, but said it still could be the alternator clutch which is managed by the ECM and BMS.

​​​Unplugged the ctek and the voltage read 13.2 at the ctek connector before starting. Started car and remeasured voltage at ctek connector to be 12.2. Went for a drive turned car off, double locked doors and massured 11.3 at ctek connector. I believe this is telling me the alternator is failing to charge the battery. The BMS should not let the battery to go below 12.2 volts.

My car calls for a flood battery and I have a new AGM. I'm hoping the AGM battery is not interfering with the BMS. My battery is good. After a full charge it reads 13.4 then settles at 12.6 after a few days. I ordered new Denso alternator today. Hoping this this fix the problem. When I had the car out for a drive the voltage on the Ado kept going down from 12.2 to 11.2.
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So when they checked the alternator what voltage did the dealer say it was?

 

They never gave me a voltage number, but said it was generating sufficient amps.

 

Hopefully the new alternator will cure the issue.

 

That's a very strange metric for a battery charging system. An alternator could be generating 1000A but if it's only producing it at 12V then it is never going to charge the battery.

Richard

 

Start car, measure voltage. That's what the alternator voltage is...

 

except, if the battery is old and doesn't carry as much reserve then the measured voltage will be lower then that of a newer fully charged battery. While driving my old battery is only showing a constant 13.9v but when the battery was is better shape it would be @ 14.2.

wj

 

The difficult thing with 5l xk models is the BMS. The alternators are expensive because they communicate with the ECU and BMS to control the charge.

 

Today I installed a new Denso alternator hoping my ADU would show higher voltage. The install went easier than I anticipated. However, my voltage readings haven't changed, so far. My AGM battery was fully charged. Resting the ADU shows 12.2. Running from 10.3 to 12.4. Running at idle with full electrical on 11.3.
These readings seem very low to me.

 

How did you come to the conclusion that your battery was fully charged? Did it read at least 12.6 with a multimeter? If not, it wasn't fully charged. With your vehicle running, your voltage should never be as low as you are getting.

 

Yes. After fully charging using a multimeter it shows around 12.8. I thought my regulator in the alternator was bad. I have no codes and had Jaguar test everything. I'm wondering if it is the AGM battery or BMS.
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what does the multimeter read at the battery with the car running? seems to me and granted, I haven't read the entire thread in a long while but are you just trusting that the ADU is correct? Have you tried something like Torque Pro? That will give you a readout while driving, but maybe first explain the voltage at the battery vs ctek leads, ADU, etc.

 

Yes, I confirmed the readings from the ADU are the same as torque pro. After fully charging the battery and waiting 48 hours the multi meter at the battery is 12.6 to 12.8. At rest with car running the reading is 12.2 to 12.4. After going for a drive the readings keep going down. I have battery health indicator attached to the battery where I connect the battery maintainer. The light is green after charging the battery and stays green until I take the car for a drive. After a drive it display amber, indicating to connect the battery maintainer to charge the battery. If I done connect the battery maintainer for a week and drive the car with all trips over 20 minutes to 30 minutes the battery reading keeps going down measuring it at the charging terminals. The battery is a new AGM battery and now the alternator is new also. Don't know what else to do. Replace BMS and next battery will be a flooded non-AGM battery.

 

"At rest with car running the reading is 12.2 to 12.4" - I assume you mean at the battery posts? I would inspect (R&R) all of the ground points before replacing the BMS.

 

Just looking in the electrical and service manwells:

On in service vehicles fitted with a Battery Monitoring System (BMS), the BMS module must be reset following the
installation of a new battery. The BMS module reset procedure must be performed using an approved diagnostic
system.

If you have got the manual there is a diagnostic procedure on page 2137:

Download manual from stickies section

 

BMS was reset.

 

I see you reset it with the Autel. I wonder how effective that was against using the SDD?

This guy has a vid on youtbe - read his comments, not quite sure what he's saying - I didn't watch the vid.

 

Yes, thanks for reading the thread. I did reset it with the Autel and continued to get low voltage readings. After replacing the AGM again with an AFM I took the car to the dealer in the fall and they reset the BMS. They inspected the car for any voltage drain and found nothing. They said the alternator was producing sufficient amps, but could not eliminate it as the problem completely. Consequently, I bought a replacement alternator and installed it. I have no codes and everything works as it should expect over time the battery is drained with voltage readings decreasing. The original battery was a flooded battery. When I drive the car it is normally for 20 to 30 minute trips each way, no short trips.

I understand when the BMS is reset the car drains the battery to below 12.2 and only then rechanges the battery. However, my battery is discharged below 12.2 and keeps going lower. Maybe, the car is not liking the AGM battery. I ignored the low voltage with the first AGM battery I installed and never put the car on the maintainer for a month because I was driving it. No unusual codes. However, the car early turned over to start after a few weeks and decided to put it on the maintainer to find out the battery became bad. I think the first AGM battery became bad from not being maintained at a sufficient voltage my the car.

 

I can confirm there’s an issue here. I’ve got BMS on the 2015 and with a new AGM fitted recently, as has always been, with the engine running it always reads 13.9-14.2. With the engine switched off, voltage holds in the high 12s. I think I'm on battery #5 or 6 now, they last about 2 years, even with the top of the line H8 from Interstate. When they replaced this last one, their Midtronics tool had the battery right at the edge of green and claimed it had plenty of life but with my voltage readings constantly dropping into the 11s I insisted on a new one. And I was right, new battery performs flawlessly unlike its tired predecessor.

So I would first replace with a new battery, fully charged and see if there’s any change. If the result’s the same then consider BMS. Main dealer replaces mine twice early on in warranty.

 

How did you get your voltage readings, at the battery, torque pro, ADU etc. This is my second new AGM battery

​​​​​​I understand the ICU and BMS receive data from the battery and the ICU trumps the BMS if there is a problem.