I4 Battery/Alternator Charging Behavior...batteries shutting off, dying, car not starting, etc.

[Kaz]

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.

 

[docron]

Here was my SOC using Antigravity Battery monitor app after driving to get some dinner 15 min away, stopping at Kroger's afterward.

 

[Kaz]

Crazy how far it drops on one drive...but on the positive side you're showing very healthy charging voltage and it's doing exactly what it's supposed to. Still would be better if the system didn't let the battery get so low in the first place.

 

[okyou]

Thanks for the detailed write-up and information. It roughly aligns to my battery tester that I have used after drives, after parking for a week, and after charger use. Depending on how long the pumps and the multiple fans run after you park the car, that can further drain the battery - I've had my car in the garage keep humming along for almost 30 minutes after a drive while I was working on something else nearby. I have started to keep it plugged in all the time (kind of annoying) and the Telematics error no longer pops up and some other random quirks also seemed to have stopped (except the rear turn signal intermittently being broken is still present).

 

[Kaz]

Thanks for the detailed write-up and information. It roughly aligns to my battery tester that I have used after drives, after parking for a week, and after charger use. Depending on how long the pumps and the multiple fans run after you park the car, that can further drain the battery - I've had my car in the garage keep humming along for almost 30 minutes after a drive while I was working on something else nearby. I have started to keep it plugged in all the time (kind of annoying) and the Telematics error no longer pops up and some other random quirks also seemed to have stopped (except the rear turn signal intermittently being broken is still present).

I wish Lotus would update this part of the software to have the alternator just work like a normal one and top off all the time...doubtful they would change this it might just be part of a locked AMG package. I just timed my car after a drive the pump is still going after 47 minutes

 

[Kaz]

Oh about lithium: so the mercedes system is looking for voltage that drops to 12.3-12.5V since it's designed around an AGM battery and that would be voltage that triggers the alternator to start charging. A lithium battery is putting out 13.1+ at around 60-70% SOC but the ecu sees a healthy AGM battery instead and does not charge. I've verified that on decel and coasting it takes much longer for the system to start charging your lithium battery, and it won't go higher than 13.6V which is not enough to actually charge the lithium battery - it's looking for something more like 13.9-14.1V which is a lot for a car to put out. So yeah, just stick with your stock battery or get the Braille AGM version.

 

[okyou]

I wish Lotus would update this part of the software to have the alternator just work like a normal one and top off all the time...doubtful they would change this it might just be part of a locked AMG package. I just timed my car after a drive the pump is still going after 47 minutes

yeah if you aren't able to let the car really cool down a lot before shutting it down, it'll keep running things - which is good for the car, bad for the battery. Of course in the summer if it's hot out like how it probably is now, I could imagine these things running for an hour or until there's hopefully a power level shut off so you have enough juice to start your car up again (assuming you don't then also leave the car sat for over a week too).

 

[Kaz]

ok after few days of testing the AntiGravity lithium...I am pretty sure the I4 alternator programming is specifically designed for the AGM 12V. I got the state of charge down to 57% and the car still would not charge the battery, because it still shows 12.95V which is normal for an AGM battery and lower for lithium. So the car doesn't think anything is wrong and maxes out at 13.5 off throttle coasting, but that won't charge a lithium battery. So yes, definitely use the stock or the Braille AGM, don't stick on lithium unless you are charging everytime you come home, and I don't know what happens when the lithium gets super low I still don't think the car will charge it.

 

[lynchy73]

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.

Never spotted this thread until now. Good info.

Are there any additional findings much beyond what’s stated here?

 

[Kaz]

Never spotted this thread until now. Good info.

Are there any additional findings much beyond what’s stated here?

Nothing other than lithium batteries don't work well with the i4 because of the voltage programming, the car is looking for a SOC of about 70% on an AGM battery and a lithium's voltage is higher than that so it never triggers the alternator to charge it.

And around every 5th restart of the car the battery will charge to full regardless of the state of charge.

 

[Blue131]

Nothing other than lithium batteries don't work well with the i4 because of the voltage programming, the car is looking for a SOC of about 70% on an AGM battery and a lithium's voltage is higher than that so it never triggers the alternator to charge it.

And around every 5th restart of the car the battery will charge to full regardless of the state of charge.

I didn't realize about charging every 5th start. That's helpful to know. The charge logic is so odd and hoping there will eventually be a fix for it.

The I4 really doesn't like short trips based on my experience. I had it sitting on the tender for a week drove it a very short distance to get gas, didn't lock the car during the time at the station, and when I started it up again, got a rear indicator fault almost immediately that went away with a little bit of driving.

 

[Kaz]

I didn't realize about charging every 5th start. That's helpful to know. The charge logic is so odd and hoping there will eventually be a fix for it.

The I4 really doesn't like short trips based on my experience. I had it sitting on the tender for a week drove it a very short distance to get gas, didn't lock the car during the time at the station, and when I started it up again, got a rear indicator fault almost immediately that went away with a little bit of driving.

Lol if Lotus doesn't care about the fake engine noise they certainly don't care (or even know) about the built in Mercedes alternator logic ...that seems very deeply embedded into the package. And yes to short trips being problematic because most of the time the car is just running off battery power and not alternator -

 

[Blue131]

Lol if Lotus doesn't care about the fake engine noise they certainly don't care (or even know) about the built in Mercedes alternator logic ...that seems very deeply embedded into the package. And yes to short trips being problematic because most of the time the car is just running off battery power and not alternator -

True, I've written to them about the issue as well hoping that if enough complaints come in there is a slight chance they might look at it. But with all these rumors about moving to a 6 cylinder hybrid, I just don't know how much attention they place on an engine that might not be around for much longer.

I've also taken to carrying around a portable lithium ion jumper wherever I go as a safety measure to ensure I have enough to start the car just in case. I'd definitely recommend it if taking the car on longer road trips where you can't rely on a tender at overnight stops where the cooling fans will kick for extended periods after the car is shut off. Not sure if you have other workarounds you'd recommend?

 

[lynchy73]

@Kaz what’s the highest charge voltage you have measured coming from the alternator during regular usage?

I personally left my LFP battery to run low, didn’t charge it, to see what happened. For me it hovered around 20%, charging in bursts, but the car never let the battery go lower than that. Ended up with the battery voltage at circa 12.85V, after the fans had ran.

My concern is that the various systems in the car are not actually so happy running at these fairly low voltages. When I checked my car on Lotus diagnostics there were lots of weird issues, but after clearing and keeping the AGM on a trickle charger, they didn’t return. I found that LC didn’t work in my car for example, but if I cleared codes with a basic OBD reader, LC worked again, even though there were no CEL or errors that could be read with a basic OBD tool. These marginal voltages do cause small issues and less than optimal running IMO.

 

[Kaz]

@Kaz what’s the highest charge voltage you have measured coming from the alternator during regular usage?

I personally left my LFP battery to run low, didn’t charge it, to see what happened. For me it hovered around 20%, charging in bursts, but the car never let the battery go lower than that. Ended up with the battery voltage at circa 12.85V, after the fans had ran.

My concern is that the various systems in the car are not actually so happy running at these fairly low voltages. When I checked my car on Lotus diagnostics there were lots of weird issues, but after clearing and keeping the AGM on a trickle charger, they didn’t return. I found that LC didn’t work in my car for example, but if I cleared codes with a basic OBD reader, LC worked again, even though there were no CEL or errors that could be read with a basic OBD tool. These marginal voltages do cause small issues and less than optimal running IMO.

On my Braille AGM and the stock battery I'll see charging voltage around 13.6-14 which is normal. On lithium the voltage was always around 12.4-12.8 which is just running from the battery not from the alternator. On lithium I had various systems randomly shut off during driving like ESC module, safety systems, etc. then when cleared things were fine just like the behavior you described. But everything works great with the Braille AGM - the car definitely is not happy when the voltage gets low. Are you running the Antigravity Bluetooth module? Super handy and it logs

 

[lynchy73]

On my Braille AGM and the stock battery I'll see charging voltage around 13.6-14 which is normal. On lithium the voltage was always around 12.4-12.8 which is just running from the battery not from the alternator. On lithium I had various systems randomly shut off during driving like ESC module, safety systems, etc. then when cleared things were fine just like the behavior you described. But everything works great with the Braille AGM - the car definitely is not happy when the voltage gets low. Are you running the Antigravity Bluetooth module? Super handy and it logs

The LFP battery voltage will not rise much when it’s hit with fairly high charge current (when it’s also at a fairly low SOC), like you might see with an AGM. Firstly, the AGM is likely at a relatively high SOC when you see these higher voltage, in which case voltage will rise with charge current (over the OCV, open circuit voltage), plus the AGM internal resistance is gonna be higher than the LFP, which again means voltage will rise, rather than absorb lots of current.
I wouldn’t say those voltages around 12.8V with the LFP fitted are purely the battery supplying the power, but that is the case much of the time. At lower SOC on the LFP (like circa 20%) I was observing there was some more reasonable steady state charging occurring, such as 10A or so, for longer periods. At higher SOC on the lithium, I was just getting very occasional short bursts of charging, sometimes over 100A, but the vast majority of the time, the battery was supplying the power.

The LFP battery I have, has Bluetooth built in. Can just see basic stuff such as current, cell voltage and SOC estimate. I don’t have a way to monitor the AGM battery.

My AGM is almost 2 years old. I often don’t drive much, so whilst it never let me down, it does seem like it was causing random issues, that were resolved when I started to use a charger.

I’m gonna be working on this little problem and hopefully can find a way to either get it working better with LFP or use a different battery type that plays nicer with this crazy Mercedes charging system.