HBerman

There is a note in my AMG EQEX4 manual to drain the battery down to 20% every 6 months and then to lock the car overnight for battery reset. What is the purpose? What happens if I don't do this regarding battery life/capacity?

bytemaster0

It recalibrates the BMS so it knows where low pack voltage is. Most helpful if you regularly drive the car to near 0%. Won't harm the battery if you don't do it, but it's useful to have your BMS be more accurate. I do it every 6 moths - bring the battery down to 15-18%. I set a charging pause from say, 10PM until 5AM, plug it in before I go to bed, and then let it resume charging after that so the car isn't drained completely before I have to go somewhere. If you never did this, then after a year or two you might risk running out of energy just prior to 0%. With the above steps it's easy to do, so I just do it around the 6 month mark and it's been pretty happy about it.

nosnoop

It's for cell balancing primarily.
Over time, the battery pack would develop imbalance when different cells have different level of state of charge (SOC). And the overall battery capacity would be limited to the weakest cell in the battery pack.

Cell balancing would transfer energy from cells with higher SOC to those with lower SOC until all of them have the same SOC.
That's why you need to park it overnight below 20%.
Some cars do top balancing when the car is fully charged.
Some others like Mercedes do bottom balancing when the battery pack is low in charge.

bytemaster0

Cell balancing can only happen during charging though. The <20% state is passive and doesn't involve charging, so it should only affect BMS low pack calibration. Balancing happens high-end and requires power input.

nosnoop

That is not true.
BMS can transfer energy from higher SOC cells to lower charge SOC. That's how bottom cell balancing works.
What you said above was for top balancing.

If it were just for calibration, you don't need to park overnight with that <20% charge level, it would get calibrated within minutes once the charge level reached the target level, if not right away.

bytemaster0

Fair point. I've never seen it done before, but that doesn't mean it can't be!

Crito

I was under the impression ours use passive balancing because they were bragging about active balancing for the Vision EQXX like it was a new thing, but I could be wrong.

HBerman

Time of day dependent or just elapsed time? How many hours are needed?

nosnoop

Should be elapsed time.
But we don't have any details how it is programmed to work. I can't imagine if it would take more than a few hours, but it is almost impossible to get a confirmation from official source.

Crito

II think your reasoning is backwards. One of the advantages of active balancing is it's fast, like 15 minutes fast. So why would I need to leave it parked overnight at 20%? But my EQE SUV routinely seems stuck at 99% charge for what seems like an eternity.

Crito

I guess what I'm saying is it seems more likely to me that the sitting overnight thing is to get the BMS to recalibrate. And based on my observations charging to 100% it's definitely doing something up there. But I'm not an official source so...

Bluffton Bob

So it has been a while since I have posted a dumb question - Can I expect to see a noticeable result from doing the low SOC reset? I tried it last night and then took the car to an EA DCFC and charger it to 80%. I drove it afterwards and didn't get any thank you message from the car.

HBerman

I noted the following changes after leaving the car at 19% SOC for 6 hours and then I recharged it to 80%:

1. Max mileage became the same as the mileage range initially.
2. Max mileage decreased by 10% on the first recharge from what it had indicated before the drain.
3. The subsequent range is back to what it had been before the 19% elapsed time.
4. Max mileage increased 8%.

Net, I'm certain that the BMS reset and leveled the cells, but the ultimate effect is unknown because there are NO readouts!

bytemaster0

I've done it several times on our cars and haven't noticed any difference in readout. I did notice a difference the first time I did this, but it had also become suddenly colder outside.