Electric battery degradation

same weather, check your barometer.

 

I did something similar, but I didn't use the switch from EV to HV but the point at which the HV part of the battery is the lowest (three bars left). For me it is very rare that I reach home just at the point of the switch from EV to HV. Besides, I try to avoid that too, because then you still have to let the engine finish its full cycle.

In trips when I completely use my EV range I make it a challenge to arrive home with exactly three bars on the display for the HV part of the battery (at two bars it starts to add charge again). That is the most empty state of the battery you can get. In the two years I have the PiP, from time to time I managed to do so. For these cases charging afterwards provides the maximal values for the kWh input. These maximal input values show a slowly decreasing trend over the two years (with 459 times charging in total). See also the red line in the graph below (horizontal axis: charge number from 1 to 459; vertical axis: input Wh of that charge). When I calculate the slope of this trend line I get a decrease of about 2.2% per year.

Note that over time I changed the distance and speed of my trips a bit (in particular my regular sport location) which explains that in the last year more often I have left more in the battery when reaching home; in this last year it has become more exceptional that the battery is empty when arriving home. This trend in the blue points has nothing to do with battery degradation; it just reflects changing driving habits.

 

@Jan Treur
My measurements above were on those trips that are a bit beyond my EV range (I have it once a week) and the ICE usually starts at or close to a red light, battery icon is filled from 6 to 7 bars and I reach home when it just goes back to 6 bars. I think this is a distinctive point as well.
Anyways, glad to see our results are very similar, this is encouraging. 2.5% decrease each year as compared to the previous one is only 20% decrease (from new) over 10 years.

 

Yes, indeed both our measurements show a number around 2.5% per year, which is not too bad. But it is not clear that it goes like that in all years. For example, maybe after 5 years the percentages become a bit different (lower? or rather higher?). We will have to measure again in later years. Does anybody know whether in general a battery degradation percentage is higher in the beginning, or lower, or the same, as compared to later years?

 

My impression is that it depends on the specific battery chemistry and construction. Some batteries will degrade faster at first and then more slowly for a long period of time. Others are more consistent. I don't know what to expect for these Prius Plugin batteries.

 

pip batteries get so little workout, they should last a long time.

 

What, you mean like getting fully discharged and then fully charged only once a day ?? (within the set limits, that is)
I wouldn't exactly say that is a "little workout".

 

no, i was thinking more of the 99% in the cali hov lanes, who never plug in.

 

No.
But limiting the discharges rates and thus stress on the battery may prove beneficial. PIP does not pretend to be an EV.

 

Right. Whereas the Chevy Volt is marketed as an EV with an ICE to extend its range, the PiP is marketed as a gasoline/electric hybrid which has been enhanced to include some miles from plugging in (a hybrid on steroids, so to speak).

 

The pack isn't fully charged/discharged. I think it's closer to ~30%-85% per plug-in cycle.

http://energy.gov/sites/prod/files/2015/03/f20/batteryPrius8660.pdf

The BMS also appears to have slave units that can set the capacity of each module individually. I've noticed that when I head out at a full charge, whether or not the engine will kick on to burn off some charge is a function of how quickly I dump energy into the already "full" pack.

I imagine that if any individual cell goes above whatever threshold Toyota uses because I've used a lot of regen, it quickly turns on the engine to reduce the SOC. Otoh, if I use regen in a more consistent manner, the onboard BMS can equalize the SOC between cells to the point where the engine doesn't have to kick on.

Patent US20110080138 - Battery pack manager - Google Patents

So far capacity loss seems to be ~.2kWh/20k miles... But that's with minimal EV only use. If capacity losses are dominated by calendar life losses, then we'll lose however much energy every per year, largely as a function of average temperature. If it's dominated by cycling, we'll lose energy as a function of energy stored/ev miles driven.

http://avt.inel.gov/pdf/phev/batteryPrius8663.pdf

Given what others have posted so far, it looks like cycling doesn't affect capacity a whole lot, so capacity loss is probably dominated by calendar losses, which seems consistent with Nickel based chemistries (I think the cells Tesla uses exhibit the same characteristic).

Along those lines, I think owners should play it cool and use it or lose it.

 

the lithium batteries are a nickel based chemistry?

 

I did this calculation in a more systematic manner now. I made a systematic selection of the highest Wh points based on a fixed bandwidth from the trend line and I determined the trend line by a least square method applied to the selected points within this bandwidth (which may seem a bit circular, but it works well). See the new graph below for these selected points only, with the related trend line. This improved calculation provides a degradation estimation over the first two years of 2.6 %, which is only 1.3% per year. This year summer still has to come; this will provide more points that will be selected within the bandwidth. After summer I will make this estimation again.

P.S. If you are interested to see what outcome this method gives for your data, just send me the data (Wh per charging session over time) and I will apply it: [email protected].

 

I think so. I remember Jeff N mentioning that as well, but I can't seem to find anything concrete on the chemistry.

 

I used to be a little confused about this, but all of the references I've seen recently say the Prius Plugin battery is Lithium NMC or Lithium with Nickel, Manganese, and Cobalt. There are at least 2-3 variations in the ratios of the NMC to each other and I don't know what these Panasonic cells use so I don't know how much absolute grams of Nickel are actually used. The Ford Energi cars seem to use very similar but perhaps slightly different cells from Panasonic. The Volt also uses NMC based cells but from LG. I think the Prius and Energi cells are more power optimized whereas the cells used in the Volt are quite a bit more energy dense. That makes sense due to the different sizes of the battery packs.

 

interesting. do you think at 3 years, 20,000 miles ev and 9,000 hv, that i'm not much worse off as hov drivers at 3 years and 29,000 miles of hv?

 

So, Prius versus Prius Plugin? Worse off in what way?

 

no, pip vs pip. me vs a pip driver who never plugs in. you say it's more time than miles?

 

If you have two PiPs driven the same way but one is never plugged in then it's likely that it will age more gracefully since it won't see the degradation from the full charging cycles.

 

Does it really charge higher or discharge lower if it is never plugged in ?
The first I can buy; the latter, I doubt it.
And I doubt the conclusion either because a "full cycle" really isn't anyway.