I recently removed the second battery from my V8 S and replaced the main battery with a LiFePo4 "RE-START" battery from Antigravity. Since they threw in one of their battery trackers for free, I also installed that to see what's going on with the battery.

It all started well, the battery has a higher 13.2V nominal voltage than the original AGM battery, and when I started the vehicle, the voltage immediately went from 13.xV to ~14.5V and stayed there as long as the engine was running (the brief drop in the middle was a stop at a gas station):



Yesterday, I went for a drive again and afterwards noticed that the voltage dropped below 13V at some point while the engine was running, and it looks like the BMS just decided stop charging the battery suddenly:



So today, I started the car and the voltage jumped to ~13.7V for a few seconds before settling below 13V again. I went for a short drive and the voltage went up to ~13.2V briefly every time I dropped a gear, but again soon after dropped below 13V and overall kept going down:



The car still starts fine and everything works normally, but the battery tracker tells me that the battery is losing charge, even with the engine running. I disconnected the battery for a few minutes and started the engine again, but that didn't change anything. I also confirmed that the voltage readings from the tracker are accurate with a multimeter.

Any thoughts what's wrong here? Is this the battery management system confused by the higher nominal voltage of the lithium battery? Is it doing to this to extend the life of an AGM battery? Or did something just suddenly break in the charging system without throwing any warnings? I did see another thread about the alternator failing because of an oil leak, but everything looks dry in the engine compartment.

 

Update: Went for a short drive to see what happens, the voltage dropped rather quickly during the drive and as soon as I was back in the garage and turned off the car, the RE-START battery shut itself down to avoid over-discharging (at ~11.5V).

Since I have to recharge the lithium battery anyway, I put a fully charged AGM battery back into the car for now. After replacing the battery, I started the engine, and the voltage went to ~14.5V for a couple of seconds, before dropping to ~12V again, and stayed there until I stopped the engine. Engine speed seems to have no impact on the voltage. With the engine off, the battery recovered to ~12.7V within a minute.

I still have no warnings about a charging system failure, but I did get a low battery warning just before the lithium battery decided to shut down. I think the next steps are to visually inspect the alternator and then take the multimeter and start figuring out what's going on...

Update 2: The AGM battery was at 12.79V this morning, so it doesn't look like it's draining overnight.

While trying to figure out if there's an electrical issue causing this, I noticed that if I open the driver's door, the voltage returns to >14.5V, and when I close it, the voltage drops again. I immediately suspected the LED footwell or puddle lights I installed a few weeks ago, but unplugging those made zero difference. They also light up when the passenger door is open, but opening that door makes no difference. So, there's a short somewhere in the driver side door...? Any ideas how to debug this further?

 

~14.5V is the charging voltage - I'd guess yours started charging and then discovered the battery was already full, so stopped charging.

Was the engine running when you did this test? If so, I reckon that the interior lights will lower the voltage slightly which will caused the charging to kick in. Shut the door and no extra volts required, no charging. As for the passenger door making no difference, either the voltage was still high enough when you did that, or the driver's door is somehow using more power? That's just weird. Did you try both doors alternately several times to confirm the behaviour?

 

Yes, the engine was running and I opened and closed both doors several times to confirm this. I also turned off all interior lighting I could and unplugged all footwell and puddle lights. The battery still charges only when the driver's door is open.

Note that turning on the infotainment, A/C, or headlights doesn't increase system voltage, so I doubt it's the increased power consumption that causes the battery to start charging again. I would assume any of these consumes more power than the interior lights.

I'm now trying to figure out what systems are turned on/off when the driver's door is closed and maybe start pulling some related fuses to narrow down the issue. Opening or closing the door doesn't seem to significantly impact battery voltage when the engine is not running, so that probably rules out any trivial short circuits in the door wiring.

 

OK, after some more experimenting, let me sum up the findings:

• With the engine running, closing the driver's side door results in a significant voltage drop (from a normal charging voltage, ~14.5V, to ~12V), and it seems this is not the BMS deciding whether to charge the battery or not, considering the Antigravity battery I had was eventually discharged enough to shut itself down. I assume the AGM battery would do the same, but take a bit longer due to its higher capacity.
• The voltage drop is not caused by the interior lighting, as opening the passenger door also lights them up, but won't result in a voltage change. I also unplugged the LED bulbs I had installed to confirm they are not causing the issue.
• The voltage drop only happens when the engine is running. There's no overnight power drain when the ignition is off, and the system voltage doesn't fluctuate when the driver's door is opened or closed with the ignition on or off, as long as the engine hasn't been started
• The physical position of the door doesn't make a difference, but as soon as the car detects that the door is no longer open, the voltage starts dropping immediately. I can close the door until it almost latches without the charging voltage changing.
• It doesn't matter if I'm sitting in the car or not, or if the car detects the smart key. The only thing that seems to matter is whether it thinks the driver's door is closed.
• Whether the car is locked or unlocked makes no difference. All the seat and mirror controls on the driver's door work, the mirrors can be adjusted normally, they fold as expected, and the blind spot monitoring lights up. The outside door handle pops in and out, and even the door handle light works.
• Turning on/off other electrical components doesn't change the voltage noticeably. A small initial dip after headlights are turned on, but with the door open, the alternator soon adjusts the voltage slightly higher again.
• Pulling each relevant sounding fuse one at a time from the passenger footwell fuse box changed nothing. (Well, pulling the F56 "remote control module" fuse apparently shuts down the engine...)

Does anyone know what other electrical components are turned off when the driver's door is opened? Does the car deactivate the driver's airbag (there is an occupancy sensor though) or blind spot monitoring, perhaps? Also, any ideas where the switch that detects door position is located? I think I might have to start disassembling the door panel to see if something is broken there.

 

There is something about opening the door turns on the E brake or put the transmission in park I think.
A couple of guys commented that if you open the door while still rolling the car stops suddenly..
I can’t remember for sure.

 

Ah, true. I tested this and it doesn't engage the EPB, but it does put the transmission in park. It doesn't seem to matter for the charging voltage though, and neither does the parking brake.

I removed the door panel and went through all the wiring, and nothing seems to be wrong there. I unplugged pretty much everything inside the door and the only thing that affected charging voltage was unplugging this connector, which among other things, tells the computer whether the door is open (unplugged, it thinks the door is always open, so even when I physically closed the door, the car didn't stop charging):



So, fiddling with the door got me no closer to fixing the charging issue. Here's a graph of the voltage changes:



With the door open, the car starts charging normally and continues charging at 14.3-14-7V as long as the door stays open. In this graph, the headlights are on all the time and A/C is on starting from the indicated time. Voltage starts dropping immediate when the door is closed, and based on the previous experiment with the Antigravity battery, would happily continue dropping until the battery dies.

 

It seems fairly certain that you have a fault somewhere in the interlock circuitry that inhibits driving with the door open. Could be a bad control module that handles that function. If you take it to a tech, leave the AGM in place so they don't try to blame the lithium battery. I've been using a lithium battery for a couple years with no issue other than the ECO stop/start doesn't work with it as the BCM thinks the battery is always low on charge.

 

Here’s a WAG, what about the window regulator?
When the door is opened the window lowers, when closed it raises, could the regulator still be trying to raise the window after the door is closed and therefore be drawing extra current and thus dropping the voltage ?

 

Good thought, but unlikely since that functionality occurs whether the engine is running or not.

 

Thanks for the responses. It also occurred to me that this might have something to do with the window regulator, but like Unhingd said, the engine running shouldn't matter in that case. I did test this with the window rolled down too, and still had the same charging issue.

I'm going to see next if there are any error codes, but it will take a few days before I can get a hold of a reader. I will also test if connecting the secondary battery again changes something. If not, I guess I'm taking it to a mechanic to see what SDD says about the situation.

 

Where in the SF Bay area are you? I have a code reader. You can PM me if you don't want to post it on the forum. I'm retired, so available most any time. I'm in the East Bay, Danville,San Ramon area.

 

Thank you for the kind offer, Wayne! I'm a bit further away in San Jose and I already ordered an iCarsoft reader last week (should be here in a day or two), but otherwise, I might have taken you up on that!

 

Just to throw it out there as an idea. I had a battery draw from the door latch.

 

I looked at the door latch, but didn't see any uncoated metal that would conduct electricity, at least my multimeter didn't think so. Was this in your F-Type? What was causing it?

 

Quick update: My iCarsoft LR V2.0 arrived today, and I used it to read the codes. It found some minor electrical faults, like a door module fault with "local interconnect network bus C," but nothing that looked immediately relevant to the charging problem. I cleared the codes and will see if they come back later. There was obviously a persistent code about the secondary battery missing.

However, I also did a BMS reset using the tool, and the car started charging the battery again normally with the door closed! I went for a short drive and system voltage stayed at normal levels the whole time. I will keep the AGM battery installed for a while and monitor the situation. If the problem doesn't return after the BMS has had a chance to learn the battery again, I'll pop in the Antigravity battery and do another BMS reset. Hopefully the problem was just caused by the BMS getting confused by the battery switch. Still not sure what that has to do with whether the driver's door is open or closed though...