This is an update to this thread I started some time ago:

https://www.jaguarforums.com/forum/xk-xkr-x150-33/voltage-gauge-rate-239220/

I installed an Antigravity Battery Tracker on my battery which records and tracks charging behavior and other interesting battery phenomenon.

I have experienced slow battery charging on my car since I bought it in 2019. I have changed the battery, changed the alternator, and installed a new BMS cable to no avail. I've also had the dealer look at it twice and they said the electrical system is functioning properly. However, this new device which connects directly to the battery tells a different story. The battery from being fully charged goes to 47% and seems to be holding with minimum charging. I'll drive the car more in the next couple of days without putting it back on a ctek to see if the battery recovers by being charged. The Antigravity Tracker just might resolve this problem.

 

Something in those numbers doesn’t make sense.
12.25v = 45%
12.03v = 53%
Lower voltage = higher charge?

 

Something in those numbers doesn’t make sense.12.25v = 45%12.03v = 53%Lower voltage = higher charge?

The different temperature of the two reading might be the reason

I'm interested in seeing what's discovered..

I think the BMS program could be partially responsible for the cars widely reported sensitivity to state of the battery. The BMS aims to maintain an 80% state of charge equating to a rested battery voltage of 12.5 V.
The car modules start playing up below 12.2 V, which equates to a 56% state of charge, but when discharging 12.2 V occurs before that point of discharge (due to electrical resistance).
If 65% state of charge is when you start getting problems, you only need a perfect battery to discharge by 15%. In practice, if you sit in the car, adjust the seats, turn on the radio, wind the windows .... in very little time a warning pops up telling you to start the engine.

For instantaneous monitoring I use an led volt meter plugged into the lighter socket just to keep an eye on battery voltage. It powers off when the car is locked.

I think the following findings demonstrate the normal battery voltage behaviour:
Voltage when first unlocking is 12.6v if recently driven, reducing to 12.2 if the car hasn't been used for a few weeks.
During cranking, 9.5 to 10.5 is normal.
Once started 14.5 to 15.0.
During a run the voltage can drop to 13.5 or less when the battery reaches 80% charge.
Sometimes, particularly after a battery disconnect and/or a full charge the BMS runs a discharge test whilst driving. Voltage is just 12.4 but can increase if coasting or braking. It can do this for a couple of hours and I've seen the voltage drop down to a worrying 12.2.
Next time I used it was back to normal.

I would like a work around to provide the full 100% charge. That would increase the useable battery charge from 15 to 35%.
The BMS program seems stupid to be saving a few pennies worth of fuel at the cost of unreliability or the early replacing of batteries.

 

I'll have the car out for a drive today. I expect the charging algorithm to ramp up as the voltage drops further.

 

Tracking this morning...

 

Looks to me that your battery lacks capacity but not initial voltage. Maybe caused by a questionable alternator?

 

Alternator has been changed. Still going down...

 

It does start charge at 13.4 volts eventually. Everything was checked over by Jaguar to be functioning properly.

 

Yeah, that was thrown out there as a possible item that may have caused what I think may be a problem....... a low-capacity battery. When is the last time you had that battery load tested? Just because volts start okay doesn't mean it has the amp storage required.

 

When the BMS is doing it's discharge test/calibration thing where it turns the generator off you will see the battery voltage reduces as low as 12.2. Perhaps this is happening, however, immediately after starting the engine it will still allow the generator to produce 14 to 15 volts for several seconds before turning the current down with the resultant voltage dropping suddenly to 12.5 and lower.
I'm not seeing that pattern in your graphs but it could be because your monitoring device takes instantaneous sample voltages at certain intervals and might miss transient conditions.

I would connect a voltmeter across the battery and watch what happens during engine start up. An old analogue meter is best to see varying voltages reasonably accurately but a DVM will show steady state readings more accurately.

Obviously be careful not to short wires connected to the battery together or allow the positive wire to contact the vehicle as the battery current will melt and burn them.

 

I have an AGM acting in the same way but not showing signs of low power. If I use a 2 amp intelligent charger in the winter the float voltage rises from 12.1V to about 12.3V and spins that V8 over during start up, as if its summer again! AGM, H8 Group 49.

 

I have the same type battery installed. Although, the VIN calls for a flood battery of slightly smaller size.

 

Going AGM is going better, and the Group 93 is hard to find. Possibly the H8 is an alternative to the H9.

 

Below 12.2 volts now. When will the BMS allow the battery to charge normally? I'll be going away Wednesday and may need to connect the ctek to avoid a dead battery when I return; and, it will start all over again. Is this happening because I never give the BMS time to complete a discharge cycle?

 

I can't tell from your charts whether you are running the engine or how long the time periods are. They are a bit meaningless without knowing these things but what you post concerns me that your battery could simply be going flat.

As I said before, I plugged an led voltmeter into the lighter socket so I could safely see what happens with a quick glance while driving.
I experienced the BMS discharge test continued over a journey of 80 miles (about 90 minutes) and upon starting up after parking for a few hours the battery was being recharged again at a healthy 15 volts. During this discharge test the voltage as read at the cigarette socket dropped down to a lowest voltage of 12.2 by about 30 miles. Mostly it read 12.3 or 12.4 but would be seen to boost to 13.5 or more when braking from a fairly high speed - a sign that the BMS and generator were working correctly.
Also, immediately after starting the engine, the voltage was 14.5 to 15 volts for several seconds before dropping to 12.4.

I suggest you look for similar voltage behaviour to above with a basic voltmeter to verify your charging system is working properly.

If all you see is a steady voltage decline after starting and the battery gets down to less than 12.2 volts it would seem you have a fault.

I've heard that when the generator fails completely it takes roughly half an hour for the battery voltage to drop so low things stop working properly and engine stops. Sorry I don't know the critical voltage when this happens but it could be just below 12.

I'm a rather old electrical engineer and worked with lead acid battery systems on occasion over the years, 48v telephone exchange stuff so I know a bit about their behaviour in a static environment but the BMS program is a mystery deep inside the ECM.
Also, don't take the voltage readings I've provided as being dead accurate, they are provided for guidance. Cheap Led cigarette lighter voltmeters will have an error of +/- 0.2v. A good meter +/- 0.1v on the 20 V scale. There are also voltage drops due to circuit resistance that means that voltage read at the cigarette socket will be less than the battery voltage. Probably by between 0.1 and 0.3 volts depending on how much current is being drawn by the rest of the car.

 

I told him this many posts ago but I got no response.

 

What does this mean going flat? My car's charging behavior has always been like this, even after replacing the battery twice. It appears to have ended the discharging cycle at 27%. I'm now seeing the voltage go to 13.

 

Flat battery... (Not a mechanical change of it's dimensions)


The generator producing 13 Volts will not recharge your battery, it will stop it discharging further but it needs to produce well over 14 volts for an appreciable charging current.

 

My car has never produced more than 13.4 volts. The dealer checked it twice and said nothing is wrong, it is functioning as it is designed. I changed the battery twice, replaced the alternator and the BMS--no difference.

 

Has anyone explored the elimination of the BMS module? Let the car charge like cars have forever? Thanks JLR for reinventing the wheel.