61Wh battery becomes 55Wh battery after 140(?) cycles, voltage drop 0.2V

[Charlie_6]

@James_Jarrett

Here, this is a paper I found about EVs, just searching around randomly: Lithium-ion battery aging mechanisms and life model under different charging stresses - ScienceDirect

Tesla used this as an excuse to cause up to 20% of artificial degradation, multiple, times.

@James_Jarrett

It’d probably be better for an engineer to answer since my entire expertise is hazy notions I got from watching videos about cell phones or skimming explanatory web pages about battery chargers.

I worked as an electrical engineer a while ago and part of my work was with battery. I measured the voltage-capacity curves similar to this one for a variety of make and models under different voltage and temperature. My company has a 5kW electronic load, more than enough to test most batteries for portable devices.

My working experiences with batteries is the reason I discovered the sudden capacity drop caused by the artificial 0.2V voltage downgrade.
In April 2024 I posted my measurement of discharge curve

measurement

@Charlie_6

%, remaining capacity, open-circuit voltage
91%, 3618mAh, 17312mV
80%, 3180mAh, 16742mV
70%, 2782mAh, 16310mV
60%, 2387mAh, 15876mV
50%, 1988mAh, 15518mV
40%, 1589mAh, 15283mV
30%, 1194mAh, 15134mV
20%, 795mAh, 14959mV
30%, 1194mAh, 15209mV
40%, 1589mAh, 15396mV
50%, 1986mAh, 15640mV
60%, 2384mAh, 16011mV
70%, 2783mAh, 16460mV
80%, 3178mAh, 16838mV
91%, 3618mAh, 17331mV
100%, 3972mAh, 17784mV

The 61Wh battery, when charged to 17.6V, is only 91% full. Note the 100%, 17784mV. Due to internal resistance, the voltage drops slightly after charging, in this case, a 16mV drop.

@James_Jarrett

Lower charging voltage will cause the charging current to be lower, which will charge the battery slower, while physically / chemically breaking it down less.

We already have that, it’s “Battery charge limit” in the BIOS settings. Typically, the battery is charged to 16V(4V/cell, 60% setting), to 16.8V(4.2V/cell, 80% setting), when plugged in. Only charge to 17.8V(4.45V/cell, 100% setting) when you absolutely need it.
I set it to 75% in BIOS most of the time for the whole year, only charged to 100%/17.8V in less than 5 times. as a result, the battery degradation is very small, 33mAh, 0.84%, less than 1 percent a year.
My battery’s actual health is 99% or more precisely, 99.16%, but why I only got 91% health on the reading? This is because the battery only charges to 17.6V instead of 17.8V, The battery is only 92% full, 1-(1-0.99)-(1-0.92)=0.91

@James_Jarrett

So the battery engineers have models so that when they’ve got some indication that the battery’s x% degraded, they lower the charging voltage, to maximize its life as much as they can. That’s what you’re seeing.

I, as a battery engineer, maximize my battery life as much as I since from the very beginning, limiting the charge of my battery to 75%(4.15V/cell) and I only charged it to 100% in less than 5 times in a year.
As a battery engineer, I also know that if you lower the charging voltage for 0.2V in a 4-cell battery, the battery will only be charged to 92% full. If the user(or the computer) doesn’t know it, they will believe that the battery has 8% degradation, in my case, 1% actual degradation, 8% artificial degradation caused by BMS misbehaving, 9% displayed degradation. That’s what you’re seeing.