My guess is that your main battery (left side, bigger) is dead.

FYI, turning the key clockwise is the normal operation. If the main battery is dead and you turn the key anti-clockwise, there is a relay in the trunk that will switch over to the backup battery (smaller, right side).

Once the computer switches to backup battery, the only way you can get it back to working on the main battery is by charging the main battery or replacing it. The computer senses the voltage of the main battery and switches back to normal operation automatically.

I would advice keep some paper towels near the trunk lock so that you don't lock the trunk. Since you indicate your backup battery might be on its way out, you don't want to be caught with no access to the trunk.

Emergency manual access lever is behind right rear seat.


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slightly confused
 

Thank you. Under normal operation, both batteries working, when you start the car normally, which battery is starting the car the main left or Backup right ?

The left is the house battery, and the right is the starter battery.


There is no "backup battery", the juice from the starter battery is always live at the starter, then power is sent from the house battery to the starter solenoid via the start button or ignition switch.

Forgive me expanding your reply... under normal operation when I turn the ignition key to start the car it's the right side battery doing the work to 'start' the car ? Then, the left side battery takes over to keep car running. Did I get it right ?

Yes, The right side takes the load of starting the engine, and then the vehicle runs off the left along with the 190 Amp 2660 Watt alternator keeping both batteries charged.


Just like when you open the doors, roll down the windows with the remote, turn on the lights, or play the stereo without the engine running, or turn on the ignition and dash come to life before starting, that is all the left side supplying the juice.


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So a week ago when my car had sat unused for 3 weeks the remote didn't work and no lights at all came on when I opened door with manual key, no life at all. from your comments this implies main left side battery was dead ? Then, car wouldn't start on normal key (clockwise) operation, does this mean right-side battery was dead too ? I then had to rotate key anti clockwise and then clockwise to.force a switch of battery to start the car and it started. This is my confusion, if car lights and remote etc were dead (main left side battery dead) and car wouldnt start on normal clockwose key operation (right side batt dead) then how come the car started using reverse key operation as we just determined both left and right batteries were dead ?

Regarding the extra equipment attached to both batteries ...pictures attached 1st pic is left side battery. The yellow mark to the left is the main battery terminal. The yellow mark to the right is part of this small box that is attached to the rear side.of the left battery. I think it's a sensor and I'm unclear exactly if / how to remove 2nd picture: this is the "stuff" on top of the right side battery. Also unclear how to remove.
https://cimg1.ibsrv.net/gimg/www.6sp...fa00fa238f.jpg


https://cimg1.ibsrv.net/gimg/www.6sp...722aaa4c12.jpg

I am sorry to say but, you are really not grasping the operation here, who is "WE", I did not determine that "both batteries were dead", if both were dead then it would never have started, UNLESS you charged it or jumped it, which you have not said, SO, Please follow me here :), when you tried to get in the vehicle and all was dead and no start available because your left {HOUSE BATTERY} was dead, you turned your key CCW to activate emergency start, this then sent power form the right {start battery} which obviously had juice to the left {house battery} allowing now for juice to flow to the vehicle and supplying all circuits with power, you then turned the key to the right and she started, the SAME would be if the right {starter battery} was dead, then you would have gotten in the vehicle via remote and all would seem fine as far as lighting and dash, BUT you would have a no start condition, you then would have turned the key CCW to send power from the left to the right, I believe that the misconception some people have is that it switches to one battery during emergency start, that IS NOT the way I see it, look at my GT build thread and see why, the way I see the setup is the emergency mode puts both batteries in Parallel, NOT series, but Parallel, with parallel you still have 12 volts [and double amperage with two good batteries], were as with series you would end up with 24 volts [the amperage of one battery] and a fire.


I hope this helps, now onto your next post.... hilarious

When you loosen the positive terminal with the yellow mark as you have pictured, then that whole unit with everything lifts off of the left battery,
as to the right battery, you loosen the nuts in the red circles and remove terminals from battery, and at the same time you pull toward you the tab in the green box and lift, this will get those pieces out of your way, in the purple square is the yellow connector box for the pyrotechnic device shown in the purple oblong, you DO NOT need to touch these, just disconnect the terminals and lift all together, in the blue oblong is the battery parallel solenoid that ties the positive cables from both batteries together during emergency start.


BE SURE TO FOLLOW PROPER PROCEDDURE FOR BATTERY R&R, RIGHT NEGITIVE OFF FIRST AND ON LAST.


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Thanks Jonny..I think you nailed it with information I've not seen before, namely that "emergency start procedure " (key ccw to select then cw to start) activates "both" batteries in parallel, ie NOT a switch from one single battery to another single battery Boom..you're the first person to outline this critical concept and it completely explains my confusion and it also explains my use of the royal "We" as I was, in my last post, working on the (incorrect) basIs that emergency switched one batt to another single batt..we now know it switches from 1 batt to both batt. You even showed me the component (next thread) responsible for the parallel switch.

This makes a lot of sense now, I do believe someone else posted "if main dead then even if starter batt is new then car won't start with normal key operation.

I think the other point worth making is that the term "starter battery" is a slight misconception, it may be providing power to the Starter Motor but it's not providing power to the rest of the electrics needed to actually start the car, starting the car requires power at Starter Motor AND power in main Batt for rest of electrics. Your "sharing" power in parallel across BOTH batteries perfectly explains this.

So to recap ...car sits 3 weeks, no life in lights or remote, manual key entry, = Main batt dead, Starter Batt is good, but, car doesn't start with normal CW key (as main batt, required for all the others electrics apart from S Motor, is dead) , then to resolve, flip key CCW, this "shares" power accross both batteries and car now starts with cw key .

Ok...but there is more confusion... .....being happy the car started as above I drove car 15 miles (but in Manhattan so rarely over 40 mph) thinking this would charge the main batt. I switched off car and and tried to start normally, but it didn't start unless I did emergency key ccw procedure (I recall lights and remote now worked normally so main batt had some charge in it.) I then charged the main batt overnight (but with negative cable in situ which I heard is wrong, should be disconnected) and the status quo remained, lights & remote were on but car didn't start unless emergency key ccw process was used. So now the situation is main batt working , right batt working, lights are on but car doesn't start with normal key. Why is this ? My guess is that while main batt is working for remote and interior lights etc, it is not working well enough to assist in starting car (despite a good Right batt), the emergency key ccw to "share" power is still needed to start car. Am I correct ?

Then a few days later, without any further intervenation, car miraculously started normally and does so to this day, the emergency key ccw procedure is NOT now needed . Why did it now start to work ? My guess is all the "sharing" of power, starting & revving till warm, charging..plus a few days time got the Main back into reasonable shape to work normally. Am I correct ?

I then bought a Midtronics pbt300 which I think is rated for agm. Main batt in situ tested "Yellow" (should replace) starter Batt tested "Red" must replace. (My brand new agm batt , not installed yet, tested green). I also read that capacitance testing in situ can be inaccurate due to croc clips not having access to clean terminal post.

In conclusion, I should probably replace both batteries, its time. As mentioned I already have the new Main Batt ready to install. I would prefer to do one battery at a time ...natural conservativeness.... that way I have one good batt (main) , then I'll buy and replace the starter.

I apologies for the lengthy post but it's all too easy to miscommunication by brief written messages so I tried to be accurate.

Thanks again , on your diagram look above the purple square box (which contains the yellow connector) there is a black box the size and shape of a pack of cigarettes, it's hard to see exactly from diagram. Does this also naturally lift off as I remove the other bolts and clip ?

I'm now ready to do the work, I'm good with the "keep wheels straight" and the window reset procedure. Should car doors be opened or closed during battery replace process where both negative cables removed?

FInally, would someone be so kind to detail the rest of the "reset" process, after battery removal, in detail , ie mirrors, seats and anything else, how do I reset these ?
Thanks indeed.

ctek charger
 

I've read somewhere that to charge a battery one must remove the negative cable 1st, then charge. Is this confirmed ? how does this match with the trunk cigarette lighter quick connector used to charge the battery as negative cable is naturally still connected.

Answered in red.

This is more of a safety procedure, as the battery tender is very low power, were as a battery charger can be high power and may damage the battery control module located behind the main house battery, or any other of the many components, I know on my Audi A8L W12 there is a special hex post for the negative side next to the battery for charging or jumping, this hex post bypasses the battery management module, people on AW have hooked up to the battery instead of the hex post and have blown out the battery management module with to much juice, now our Bentley's DO NOT have this hex post, that is why I remove both negative terminals, or I use a 2 amp charger while I am working on the vehicle and want power.


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