Battery Charge Level Indication
This is a fun one. On a McLaren battery there is a little control module which measures the current going into and out of the battery. This is basically a flow-rate measuring unit and it tracks the movement of energy in both directions.
When the car is running the flow is usually from the alternator into the battery (i.e. battery is charging) - when the engine is off, the flow reverses and the battery slowly discharges into the (countless!) electronic devices throughout the car.
When this works, it's great! There are some things to note though:
- The unit can only see energy movement and 'estimate' the battery level most of the time. Over time it can wander, quite a lot.
- It can read the voltage of the battery, but voltage does not directly equal charge level unless the battery is at rest.
- If the unit and the battery get out-of-sync with each other, you can get stuck in a bit of a loop (see further down).
- The sensor unit is temperature compensated (this is a good thing) - but expect much lower 'maximum' levels in winter.
- The value of the 'Days Parking' display (when you power the car down) is literally just Battery Percentage divided by the average idle load.
You can view the battery percentage currently being reported by the unit in realtime on the dash. Except when it's broken...
When it's working correctly, the percentage value is simply divided by an 'idle load level' - in my case, that's literally the number 2.
So When my battery is at 82% charge, I have 41 days remaining... (spoiler: i do not)... and when at 50% charge, i have 25 days remaining (also, no). But still, that is how it works, so be aware.
Initial after-start charging
I've noticed that in the 15 minutes after a cold-start, the battery level generally charges up to about 10% and then stops. This seems to be an initial recovery charge from the alternator and then the rate is held level for a while. It can take another 45 minutes of driving for the level to start to climb again, and often it begins to drop in between as (what i believe is) some battery conditioning is being automatically performed.
Recalibration
When the two are out of sync, a manual recalibration can be performed. This involves draining the battery quite low on purpose (avoiding the self-disablement that occurs when it's very low) - and then allowing it to charge back up again for a solid 4 hours minimum using the external charger. As this occurs, the sensor on the battery will 're-learn' the total amount of energy being stored. After which, it will much more closely align with the true state of the battery.
I've also found that the charge level will suddenly jump to 100% if it's been between 80 and 90 for a long time while charging, say on a 1hr+ drive. I believe this is a software feature, where after some time of engine-running has occurred without any noticeable absorption by the battery itself - the sensor realises that it must be full, so it literally 'resets' to 100%
Battery Management Modes
When the sensor thinks the battery is low (remember, it may NOT be low, it only has to THINK it is low) then it notifies a number of other systems around the car. These systems then drop into a low-power mode. One of these features is to drop the windows a little, in order to allow the manual override of the door unlock system in case the battery goes too low and shuts down (into self-protection mode).
Other systems like seat controls (if you have the fancy heated seats) and steering column adjustments all stop working also. Plus the ICE can disable itself too.
Obviously the auto-stop start system will also only operate if the battery is above a certain level. I've not got hard evidence on this (as i don't often use that feature) but I've seen it 'not' operate below 75% on many occasions, so i suspect it's somewhere around that point.
McLaren Factory Charger
The McLaren supplied charger connects into the funk 12V socket using the special red accessory connector. It works just fine when you first connect it, but it can get out-of-sync with the battery if left connected for a long time.
The general consensus seems to be that it either crashes (the control circuit inside it) or that it just gets satisfied the battery is charged and that it doesn't need to do anything else.
So, for long term use, there seem to be two schools of thought:
Fit a timer plug that turns it off and then back on once per day.
OR
Get a CTEK Lithium Model instead - like this one
Also, and this is very important, NEVER LOCK THE FRUNK with the charger connected if you can avoid it. Having easy access to this area with the battery right there is key if you do end up with the battery in self-protection mode and requiring a manual kickstart charge.
On that note....
Kickstart charging
If your battery has gone into self-protection mode, you can try to kickstart it gently by repeatedly 'resetting' the Lithium charger. This is true for both the CTEK model and for the original McLaren charger. Some people have also had success using another charged 12V battery in parallel to it, allowing the current to flow between them until the Lithium battery wakes back up again.
Reset Button on Factory Charger:
Be aware though, that allowing this to happen regularly on a Lithium battery is very bad for it's long-term health and should be avoided at all costs.