main battery left and starter battery right.
If your main battery drops below 10/9 volts even with a fully charged starter battery the
car will not start. this is why you keep your main battery only on charge.
If the starter battery goes flat, you put the key in turn 2 times to the left and then start the car as normal with the key. 2 times to the left tells the car to use the main battery.
I have had NO none starts in 14 months using my tender, hope this helps some of you.

I'm using, YUASA 9000, 2 x 12v 95ah 850a battery's

 

Not sure if you have a special car, but my 2009 CGT Mullinier uses a H8 on the rear left and H5 on the rear right. My car came with batteries from Advanced Auto Parts (BRAND: Auto Parts International).... the dealer dude was cheap.

My main battery was down to 3V (more on that later, will create a separate thread) but I was still able to start the car by turning the key left and activating the starter battery.

For those of you whose main battery drops below 10V, remember that the correct way to charge the battery is to disconnect the negative lead. Leaving the negative lead while charging caused my battery charger to think that the battery was bad and drained the battery further....

The low charge in my battery manifested in several ways....

- TPMS Module error warning
- KESSY module error warning
- Comfort System / LCD Screen constantly rebooting.
--- and finally..... massive smoking out of the engine.... I'm talking fumigator style.... I will be starting a separate thread on that.... but the short story is that when the battery voltage goes really low, the ECUs send incorrect ignition signals to the injector and coils and causes fuel and oil to be dumped onto the exhaust.

 

does your battery light stay on when started?

 

No it does not. It didn't stay on even when we had a ton of electrical warnings due to low voltage.... Go figure.

 

I got 2005 Continental GT, US model. Replaced both batteries with LiFePo4. Car lost about 100lb and became much quicker. No warning lights, everything works like it should. Boy it runs awesome!

 

car always has to be on a tender!

 

I often don't drive the car for 2 weeks and yes, it's not ideal for the battery. Recently car would not start in normal mode, completely dead, no lights on at all. I turned the key the other way to engage the other battery and yes, car started. I charged main battery (rear left of car as you look into trunk. ) but even charging had no effect, still wouldnt start with normal key bUT did start usimg reverse key. I bought a new agm battery but when I was about to install it the car started ok with normal key start. It appears the main battery came back to life. I bought a medtronics battery tester and the main battery is showing "yellow" meaning should replace , but it wasn't red (must repalce) . I then tested the right side battery and it showed "red". Here is my confusion, perhaps someone can clarify, 1) with normal key use which battery, left or right, is being used to start the car ? 2) with reverse key use ie when car won't start, which battery is being used to start the car ? .....I thought the right side battery was the starter battery but after my test showing it to be "red" I'm now confused. I'm charging the right battery as I type. I figured out how to change the main battery and I plan to do that. 3) is there a guide to change the "right side " battery, it looks easier then the left but there are still some devices attached (pyrotechnics I think) so I'm asking how do I remove these devices on the right side battery . Thanks guys

 

if car doesn't start on normal key action , but does start after first Turning/reversing key to the left , then which battery is at fault ?

 

I have just replaced both batteries in my 2008 GT speed. There is no real issue other than the left battery is very hard and heavy to lift out of the bucket. I changed both in 1.5 hours. That includes resetting the windows, seats, steering wheel, and right side mirror. Remember that the memory reset procedure is with the ignition key in the run position engine is off. I would suggest disconnecting the right side negative then the left side. Then remove both batteries positive cables. Every thing will be safe and then reverse the procedure when connecting the new batteries. If you follow the other members suggestion you will find the procedure worth while and save $$$$.

 

Just a quick note: The life of a normal battery is max 5-6 years. Any longer and you are asking for problems.

 

Apologies for duplication. I sent joe44 a pm but thought I'd post here also.
joe44 wrote........[ If your main battery drops below 10/9 volts even with a fully charged starter battery the
car will not start].... my question: do you mean car won't start with normal key position but will start after reverse key 2 to the left ? or do you mean car won't start period, irrespective of key action ?

Joe44 wrote ......[If the starter battery goes flat, you put the key in turn 2 times to the left and then start the car as normal with the key. 2 times to the left tells the car to use the main battery.] ....my question : so are you saying if main battery goes out your are totally dead, no options except replace but if starter batt goes out you can use main as a temporary backup ?

 

Main battery left side . There is a small box, think it's a sensor attached to the battery harness at the back . Does this need to be removed from the harness when you remove the 4 cables....if so ...how is it removed ? Is there a clip or bolt ? I don't want to force anything. ....

2nd battery right side. Also has a bunch of "stuff" on top of the battery. Do I remove this stuff and if so how exactly ? Or...does it all come off simply by disconnecting the two batt terminals ?

 

First, attempt to charge the battery before replacing them. Disconnect the negative lead and charge the battery until full (mine was very bad, took 48 hours). Most new chargers will tell you if the battery is bad ad cannot hold more than 70% of the original capacity. If replacing buy from Audi or VW. Aftermarket batteries just don't seem to last long on these cars. You can expect an average life of 5 years from a battery.

Main battery is left (big) and starter battery is right (small).

Once you cut off to the emergency battery (key to the left), it will always resort to using the starter battery (right side) until the charge on the main battery comes back above a threshold.. meaning, you must either charge or replace the main battery for the computer to switch back to it.

 

I don't recall seeing a small box... pictures will be helpful. I just followed the instructions at the beginning of the thread and it was good. It looks daunting at first, but once you disconnect the 4 or 5 cables from the positive lead, the negative lead and remove the junction box, you will have to also remove the small plate at the bottom of the battery that is bolted to the trunk. That should allow you to move the battery and lift it out.

The right side is more or less similar with fewer wires to remove and a lighter (easier) battery to maneuver.

 

Thanks. Here is what's happening now , I enter car and turn key anti-clockwise (I think this is normal operation) ...all dead. ..I then turn key clockwise it starts just fine. Note..I didn't turn key clockwise to then turn it anti-clockwise to start car...I simply turned it once clockwise and it started with the one turn of key. Given the above, which battery is working and which is not ?