Even with the best car batteries, I suppose it is inevitable. That day you come out and nothing works and the car won't start. With a McLaren MP4-12C, you can avoid that by keeping the car on a trickle charger, as I have for years when it's not in use. But even then, eventually the battery kicks off. In my case, it was when the battery hit 13 years old, at around 60,000 miles of use.
Before I was good about trickle-charging, I had had the battery get low a couple times and the car became non-responsive. The problem with that in a 12C is that you can't open the door. And if you can't open the door, you can't open the front trunk latch. There are two ways to solve this dilemma. One is to use the manual key in the hidden keyhole under the door "handle" area (There is no actual handle, but the door hides a keyhole under a panel for emergencies). The second method, one I have used a couple times, it to take off the front left wheel arch cover (a few small screws) enough to expose the cable that runs from the cockpit to the front latch (that cable sits at about the 2 o'clock position on the wheel arch). You can then pull on the cable rearward with some needle-nose pliers and the front trunk lid will pop free.
However, in this instance, I didn't need to do either of those, because when I am trickle-charging the car, the front trunk lid/hood stays open a crack. One small win right there.
Because the car was on the trickle charger, I wondered if I just didn't have it plugged in properly, so after trying a few more time and resetting the charger, its, it was clear the battery was kaput. That sent me to the online forums and to Google search and to all the AI bots to find out what is the best replacement battery for the 12C. Well, it turns out there are lots of options on that one. I won't argue the merits of any particular choice of AGM vs Lithium Ion. I'll just tell you what I chose to do.
I decided to buy and install Antigravity H7/Group-94R Lithium which is a direct replacement for the OEM Battery. I bought mine from Supercar Garage ATL, home of the McMedics. And as I learned form the vendor and from helpful forum posts, you also need to install an "emulator" module between the battery and the old harness in order for the battery to talk to the ECU. And yes, there are cheaper options you may find, but make sure you calculate shipping before you decide in my case, I passed on another good option out of the UK because the shipping charges outweighed the cost savings.
So here's my order and some links:
https://supercargarageatl.com/products/antigravity-h7-group-94r-lithium-car-battery-1
https://supercargarageatl.com/products/mclaren-battery-module-coming-soon
OK, enough of that, here's the basic process shown below. You can also find a few Youtube videos on the subject that might be helpful.
First thing to do its to remove all the lining panels from the front trunk. There is a plastic border/seal that runs around the top perimeter, held on by velcro. After you pull that up, you can see the plastic hold downs and plastic screws. I have no idea why mine seems to include an illogical mash-up of Phillips-head, pup-up push rivets, and Torx-head screws. In any case, I took a photo after I pulled up the trim so I knew which ones to put back in which spot.
First step: Pull the small red harness lock to the left and unplug the wiring harness from the left side of the battery, then remove the negative battery terminal from the post. 10MM wrench.
Loosen the strap in the middle using the buckle. This whole thing will be removed from the bottom bolt, so don't worry about whether it's super loose or not.
Now the hard part: Taking off the right side circuit panel from the positive terminal. Hard because you need to move the whole thing out of the way to get to all the hold-down bolts. And the wire harness leading to it is very stiff. Start by popping up the right side of the panel cover, and removing the cover (hinged on left). Then use a 10MM wrench to remove the terminal from the positive post.
The battery bracket is held down by two 13MM bolts on the left and right, and three 10MM bolts on the front, plus two Torx-head screws.
Once all the brackets are free, it looks like this, time to take it out.
About this time I must have made myself a grilled cheese, because these were the next photos in my series... I made this with my home made cast--iron olive loaf and 4 kinds of cheese plus harissa butter. Damn, that was good. Maybe you didn't know that Dave Car Guy is also Dave Grilled Cheese Guy.
OK, back to the buttery. Ooops, I mean battery (sandwich still on my mind).
Installation is just the reverse. At first, I thought I had the wrong battery replacement, because it is a few inches less wide. But I confirmed I had the right one, as the vendor explained that the brackets are wide enough to still hold down the narrower battery. They were right.
Look at the pic below and you'll see that I installed the emulator leads on the positive and negative terminals and then plugged the yellow-lead harness into the red/black harness that came off the left of the original battery. That is so the battery can be read by the ECU.
And the confirmation that it is working is that the battery status on the screen interface shows that the battery as received was at 83%. After running for a bit, it was fully charged to 100%.
Anyway, it wasn't too bad. Try it yourself and send me any questions.
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