There is sense to it, once you understand it. The module at the negative cable is just the measurement device. The "BMS" is in the various modules on the network that interact and share what the module reports. I know that includes the ECM, Central Junction Box and Instrument Pack. It's in the negative cable basically for convenience. It's at the ground connection, the B+ it gets is the power for it, and the voltage reference too.

If you understand current flow, you know that total current is the same at all points in a given circuit, so for that the module could be in either the positive or negative cable.

I just happened to run across this:

The BMS module measures battery voltage and current
which provides information about the battery state of
charge (SOC) and state of health (SOH). SOH measurements
provide an indication of battery condition. The
BMS compares this information to new and used stored
battery values.
Battery information is then communicated to the rear
junction box (RJB) over a LIN bus connection. The RJB
transmits the battery information to the instrument cluster
via the MS CAN bus.
The instrument cluster displays battery charge warning
messages to indicate generator or BMS faults. The
instrument cluster also acts as a gateway between the
MS CAN and HS CAN bus networks to transmit battery
condition information to the ECM from other modules
(audio and climate control, for example).
Based on the information received from the BMS module,
the ECM will control the output from the generator
via LIN bus. The ECM can also request the switching
off or reducing of power to electrical loads if necessary
and override the BMS signals if a fault is detected.
The BMS also monitors battery status with the engine
switched off, sending a signal to switch off the infotainment
system if necessary to protect the battery condition.
Once triggered, the engine must be run for at least 5 minutes
to charge the battery before the infotainment system
will be allowed to operate with the engine switched off
for a second time.
Periodically the BMS module will instigate a self-calibration
routine. To self-calibrate, the BMS first charges
the battery to its full condition.
Once the battery is fully charged, the BMS will discharge
the battery to approximately 75% of its full state
of charge, but never lower than 12.2 V. The time taken to
complete this part of the routine is dependent on the
electrical load on the vehicle and the length of time the
vehicle is used.
When the second part of the routine has been successfully
completed, the BMS will return the battery to its
optimum level of charge. The optimum level of charge
will be between 12.6 V and 15 V, depending on battery
condition, temperature and electrical loading. This process
is run approximately twice a year.


Hope that helps! I know......that makes it as clear as mud!

Welcome to our world.

 

^got it, clear as a bell.

but would the driver know when the BMS was in 75% discharge mode and therefore need to avoid switching the engine off right at the very end of the discharge period to prevent leaving the car with a discharged battery?

Not so bad, if just parking for work because I guess that there will be enough life left to start the car at the end of the day.

But, if leaving parked for a week at the airport, might not be so great.

 

Can the car be left a week at the airport without the battery going dead? Has anyone let the car set for a week without starting engine? If so, did it start after that timeframe?

 

Should be fine. I rotate between cars and go 1-2 weeks without driving the Jag with no issues as long as I keep the car locked and the keyfob away from the car. 2 weeks seems to be about the limit. After that, the low battery message will show up on the screen, but the car still starts. Depending on how low the battery is, the nav screen may not start but once you start the car and charge the battery, all will be fine.

 

My car has gone unstarted for up to 3 weeks while on vacation with no problems. I have never gotten the low battery warning. I do keep the fob well away from the car.

 

I have also left the car for two weeks, locked, key fob well away from the car, with no start up problems or warnings upon return.

 

I am having the same issue as the OP. The local dealer has replaced the battery and reset the BMS twice but the warning still appears after normal driving (no watching movies, listening to the radio, or all doors open for a period of time).

They tell me that the new battery is good and they ran all the tests that were recommended by Jaguar. The Jaguar tech representative told the dealer that this warning was normal. Not so sure I believe this after reading the posts from this thread because there seems to be a big change for those having the car properly updated.

For now I'll keep driving it and document the occurrences of the warning. Hopefully there are others out there with this issue and something additional can be done. Until then I'll make sure we always have a set of jumper cables in the car.

I'll provide updates as they occur.

Leonard

 

If xjrguy's explanation of how this gizmo functions is right it cannot work.
Basically he describes an integrator trying, by dead reckoning, to add up all the current out and in and arrive at the charge of the battery. You can't do that because
1. it will also add up measurement errors and drift. If, for example it is overestimating current drain by 100 mA in ten hours its idea of battery state will be out by 1 Ah.
2. An amp going into the battery doesn't stay there like a penny in a moneybox, wish it did. The 'efficiency' varies between 90% half charged and maybe 60% near full.
3. Even a good battery has some self discharge which the gizmo can't see.
Looking at the long description they seem to use a combination of current flow and battery voltage.
The battery voltage and curren measuremnts are a function of
1. State of charge
2. Current draw
3. Condition of battery plates.
4. Condition of battery electrolyte
5. Condition of battery terminal connections
6. Condition of ground connection
7. Battery temperature
Someone once said that art is a science with more than seven variables. Of this seven, five (3 to 7) are not even measured!
Black box full of Black Arts. Nedes surgical removal followed by a proper exorcism.

 

Just tryin' to be helpful and passing along the technical information and our own real world experiences. You will note in the attached the they claim the BMS will self-calibrate in "a period of time". Jaguar is finding out that the "period of time" is turning out to be weeks, not days, because of the way these cars are driven. Real world is always different than most engineers' supposition.

The experience I refer to is here at the store. Once we convinced the sales dept. to connect a battery charger to the body instead of the negative terminal if they ever had to charge a stock unit, the "Low Battery, Please Start Engine" messages dried up for the most part. That tells me it must be keepin' a tally.

Cheers,

 

@xjrguy
No criticism meant, you have the practical experience and your comments are greatly appreciated.
Reading your pdf the gizmo tries to do exactly what you say with additional wells and bistles.
It actually controls the alternator so the penalties for getting its knickers in a twist are considerable.
The fact that changing the battery requires a reset via IDS is crazy.
I feel this is technology gone too far, folk being too clever for the sake of it and causing us grief.
I'd guess from the size of the thing it's an A/D convertor for current and voltage, the sums being done in the ECM.
A bridge too far iIMHO.

 

"The fact that changing the battery requires a reset via IDS is crazy"
----------------------------------------------------------------------------------
This is standard now for most upper class cars!
Audi also needs dealer recalibration service after installing a new battery!

Nowadays everything much more complicated.

I have had low batt alarm only one time after approx 20 min while storing a cd on the harddisk without engine running.

Storing from cd is too slow. Not acceptable.

Uwe

 

While I don't deny cars are a lot more complicated nowadays, I still think Jag should have outsourced its electrical system to some Japanese company. Yep, I know, sounds ludicrous, but the fact is that the Japanese are vastly superior when it comes to intricate/highly complex electrical circuits and black boxes.

 

I was just notified by my Jaguar dealer that there is what she termed a recall on the battery monitoring system. You may want to contact your dealer..........

 

That is correct. It is NOT a Recall though, it is what's called a Service Action, and will be valid for two years. It applies to XJ's with the last six digits of the VIN number at or below V20566.

I'd advise contacting your dealer about the K099 Service Action and allow them enough time to order in the required part, if they don't have it on hand. Sometimes there is such a run on a part that it might take a few days.

Cheers,

 

I made the 18-mile trek back to Galpin (where I bought the car) to get a couple of issues fixed. Galpin was great (please refer to my post on the gas fill-up problem).

They ended up doing some rewiring (not sure exactly what) and reprogrammed/reset the monitor. Hopefully, this will fixed the problem.

 

Holein1,

Did this fix your issue with the BMS?

 

It seems to have fixed the problem. Haven't seen the low battery warning since.

 

I have the same problem on my 2011 XJL, my trickle charger says green light fully charged and still getting that low battery start car reading. I have the trickle leads coming off the battery in the trunk but someone said to trickle charge it under the front hood and the notice went away. Sounds crazy as battery is in the trunk..
I would love to just plop a new battery in there to start off fresh but all I hear is it's a cluster f*** afterwards because it has to be reprogrammed and some things may not work and am NOT taking it do the dealer. If this is just a load I'm all ears,,, Any advice is appreciated

 

What made you think it was a good idea to resurrect a 7 year old thread? There are now three threads active on essentially the same topic.

 

I'm new on here and basically just reading related topics to my issues without really noticing the dates. Nothing wrong with waking up the dead they need love to