After weeks of diagnostics, voltage logging, and some real head-scratching moments, I wanted to share findings about the Emira’s 12V charging behavior which is probably responsible for some of the battery related issues people are having on the AMG version. My car was at the dealer for a long time for a power steering ground fault, but remaining was strange battery charging behavior:
Battery State of Charge (SOC) drops quickly from 100% to ~70–75% while driving, in about 15-20 minutes
Voltage while driving often hovers around only 12.6–13.0V, with occasional brief charging surges (should be 13.6V+)
App-based battery monitors (like AntiGravity’s) frequently report 60-80mV+ diode ripple voltage (that's noisy and higher than ideal of basically 0.1mV)
SOC rarely returns to 100% while driving, and only when plugging in a charger
I assumed this meant continued eletrical issues, alternator failure, etc.
Here's what's going on. Just like the fake engine noise, the Emira inherits its 12V charging logic directly from the AMG alternator system, including Mercedes’ BlueEFFICIENCY energy management strategy. In order to make the engine more efficient it intentionally delays or minimizes charging during driving to reduce load on the engine. The target SOC is ~80%, not 100% to allow for kinetic energy recovery and reduce fuel consumption. I usually see it get as low as 70% before it starts charging again.
The alternator only charges during:
Throttle lift
Braking
Cold starts
Heavy accessory draw
or when the battery SOC gets below 80% (more like 70% in my car)
and the ripple seen in apps during idle/off-throttle = ECU sending field pulses falsely read as ripple
@docron confirmed this is happening exactly on his car the same as mine using the AntiGravity battery monitor.
So basically, by the time you get home from a drive your battery will be somewhere around 70-80% SOC. Then the fan kicks in, and then the pumps continue to run for another 30-45 minutes and at this point your battery is down to around 65% SOC. If you then do NOT trickle charge your battery and also do NOT drive far on your next drive you will be in further deficit...and eventually the battery will go under the voltage it needs to run, or your car won't start.
So for now until this alternator behavior is changed or user selectable (I think some Mercedes cars it is user selectable) I would just treat your car like an EV and just plug it in when you get home. It's kind of crazy, and also not unheard of for a sportscar, and I assume this legacy alternator programming is just part of the emissions package that the whole drivetrain comes with. Anyways if you see some weird behavior with your battery and alternator this is probably what's happening.
Last point, the system seems to not like lithium batteries and at least on my car does not really charge the battery even when it dips below the 80% SOC threshold, I will do some further testing but the stock AGM was properly charging off throttle and in the conditions listed above, the lithium is not doing that. I have a Braille AGM that I will be testing this week will report back anything of note.
Battery State of Charge (SOC) drops quickly from 100% to ~70–75% while driving, in about 15-20 minutes
Voltage while driving often hovers around only 12.6–13.0V, with occasional brief charging surges (should be 13.6V+)
App-based battery monitors (like AntiGravity’s) frequently report 60-80mV+ diode ripple voltage (that's noisy and higher than ideal of basically 0.1mV)
SOC rarely returns to 100% while driving, and only when plugging in a charger
I assumed this meant continued eletrical issues, alternator failure, etc.
Here's what's going on. Just like the fake engine noise, the Emira inherits its 12V charging logic directly from the AMG alternator system, including Mercedes’ BlueEFFICIENCY energy management strategy. In order to make the engine more efficient it intentionally delays or minimizes charging during driving to reduce load on the engine. The target SOC is ~80%, not 100% to allow for kinetic energy recovery and reduce fuel consumption. I usually see it get as low as 70% before it starts charging again.
The alternator only charges during:
Throttle lift
Braking
Cold starts
Heavy accessory draw
or when the battery SOC gets below 80% (more like 70% in my car)
and the ripple seen in apps during idle/off-throttle = ECU sending field pulses falsely read as ripple
@docron confirmed this is happening exactly on his car the same as mine using the AntiGravity battery monitor.
So basically, by the time you get home from a drive your battery will be somewhere around 70-80% SOC. Then the fan kicks in, and then the pumps continue to run for another 30-45 minutes and at this point your battery is down to around 65% SOC. If you then do NOT trickle charge your battery and also do NOT drive far on your next drive you will be in further deficit...and eventually the battery will go under the voltage it needs to run, or your car won't start.
So for now until this alternator behavior is changed or user selectable (I think some Mercedes cars it is user selectable) I would just treat your car like an EV and just plug it in when you get home. It's kind of crazy, and also not unheard of for a sportscar, and I assume this legacy alternator programming is just part of the emissions package that the whole drivetrain comes with. Anyways if you see some weird behavior with your battery and alternator this is probably what's happening.
Last point, the system seems to not like lithium batteries and at least on my car does not really charge the battery even when it dips below the 80% SOC threshold, I will do some further testing but the stock AGM was properly charging off throttle and in the conditions listed above, the lithium is not doing that. I have a Braille AGM that I will be testing this week will report back anything of note.
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