"This is gonna be a long post, and I thought long and hard about whether or not I should put it here, but the question keeps coming up so often, that I decided to do it anyway and dive into the decisive question: to charge our little e to 80% or not?
For this I tried to base the calculations on research done by the Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung (BMZ) in Germany. They did some research a while back to find out the amount of degradation caused by charging and discharging. They looked at any combination of discharging to 0, 10, or 20% and charging to 70, 80, 90, or 100%. The research then determined how many full cycles of the full battery capacity it would take before the battery reached its "End of Life" (EoL) - 70% of the battery's original capacity. The research clearly showed that the battery doesn't like being (too) full, but also not being (too) empty.
Just to clarify, one cycle is always the full battery's worth of capacity. Let's say you always charge the battery from 20 to 70% (so only using 50% of the battery), then it takes two full charges, but that would still be considered one cycle (2 charges x 50% = 100% = 1 full cycle).
These were all the tested scenarios and the determined number of cycles til EoL:
A. 0-100% --> 500 cycles
B. 0-90% ---> 1500 cycles
C. 0-80% ---> 3000 cycles
D. 0-70% ---> 5000 cycles
E. 10-100% -> 500 cycles
F. 10-90% --> 1500 cycles
G. 10-80% --> 3000 cycles
H. 10-70% --> 5500 cycles
I. 20-100% -> 1000 cycles
J. 20-90% --> 2000 cycles
K. 20-80% --> 3500 cycles
L. 20-70% --> 6000 cycles
As you can see, not charging and discharging fully gives you a lot more cycles.
But the state of charge (SoC) shown in the car is also not the same as the actual battery's SoC. This is because of two protections Honda built in.
The most important protection Honda built in is the hard reserve of 90%. This means that the car will never use more than the bottom 90% of the battery. This is a hard limit. On top of that, there's a soft limit in the sense that only the TOP 90% of the BOTTOM 90% will be counted towards the range shown on your car's dashboard.
To make make sense of the research numbers, we will have to convert these percentages to the SoC percentages shown on your dashboard. We can 'translate' these research SoC's to the Honda SoC's shown in your car by be using the formula: Honda-SoC = (SoC - 9%) / 81%.
We can now ignore all scenarios that charge to over 100%, because Honda does not allow for that to happen. That means scenarios A, E, and I are out. I'm also assuming no one will be running the battery below 0%, which means we can ignore scenarios B, C, and D as well.
To calculate how much juice we can squeeze out of the battery over the course of its life, we need to multiply the determined cycles with the full battery capacity (35.5 kWh) and divide it by the average efficiency (0.168 kWh/km):
F. 1%-100% --> 1500 cycles x 35.5 / 0.168 = 300k+ km
G. 1%-88% --> 3000 cycles x 35.5 / 0.168 = 600k+ km
H. 1%-75% --> 5500 cycles x 35.5 / 0.168 = 1M+ km
J. 14%-100% --> 2000 cycles x 35.5 / 0.168 = 400k+ km
K. 14%-88% --> 3500 cycles x 35.5 / 0.168 = 700k+ km
L. 14%-75% --> 6000 cycles x 35.5 / 0.168 = 1.25M+ km
Surprisingly, it appears charging to 80% might actually not do much to help the battery, at least not as a single measure on its own. And going full YOLO also doesn't seem to really hurt the battery too much in practical terms.
Warrenty is given for 160.000 km or 8 years. If you always charge when the icon pops up at 15% and charge to 80% only, the battery will probably last you about a million kilometers. I'm not sure the car itself will even last that long.
But even if you are the worst charging psychopath you will most likely still be able to easily get at least a quarter of a million kilometers out of the battery. And, mind you, that would require you to consciously and deliberately and consistently charge from 0% to 100%, always and without exception.
That's why I think Honda has no problems with recommending to always fully charge before driving, because they know on average even a careless person will NEVER fully charge AND deplete the battery CONSTANTLY.
My conclusion is that the battery will be just fine and probably outlast your ownership, no matter what you do."
