Prius Prime - 1 year later

One year has passed since I picked up the Prius Prime and I wanted to know how well it was holding up. The car tracks a lot of statistics and with the charging data I wanted to see what I could conclude. I’ve driven 19k miles in a year, I charge the car up every day and fully use up the energy. I run the car in "EV Auto" whenever it is charged.

Specifically, I wanted to know:

1. How many charge cycles have I put on in a year and is there any noticeable drop in capacity?
2. What sort of gas milage am I getting? (assuming no electricity)
3. Based on my miles/kwh and the price of electricity, at what price is it cheaper to use gas?

Assumptions (I'm just guessing here):
-The Prius prime knows exactly how many kWh are remaining in the battery and switches from EV mode back into hybrid mode at a fixed energy level (ex: 2kWh). As the battery capacity degrades, this fixed energy level gradually becomes a larger share of the battery. This means that we can track battery capacity loss by looking at the maximum energy delivered during a charging session. The exact switchover point varies a bit, so I look at all the charge cycles in a month and only use the maximum to determine capacity.

-I can trust the mileage & miles/kWh reported by the Prius Prime's dashboard.

-The Prius prime battery is 8.8kWh total. A charge cycle of this battery is equal to putting in 8.8kWh of energy into the battery (the battery is never fully empty, so you can usually only put in ~6 or 6.5kWh at a time).

Summary:

* 223 charge cycles so far. 0-2% capacity degradation (effectively no change).
* 54 MPG average. In 19000 miles my average speed has been ~35 MPH, with my typical highway speed being ~65-70mph.
* I average 4.1 miles/kwh and electricity costs me $0.25/kwh (California). This means my break-even point is $3.31 assuming 54mpg (if gas is $3.31 or more, it's cheaper to use electricity).

No complaints so far and the car drives just as well as it did last year. Has anyone else noticed any electric performance since they got the car?


Raw Data & Analysis:

 

They don't report it directly on the car so I calculated it from the charging data. The battery has a rated capacity of 8.8kWh, so every time I put in 8.8kWh of energy during charging, I call that one charge cycle. The actual number of charge cycles is greater since there is some regen during normal use (for me it's ~500-1000Wh per day), so that would add on 20-40 charge cycles in a year (0.5*365/8.8 or 1.0*365/8.8).
The regen numbers will pad the miles/kwh a bit so to get a true read on the efficiency I'd have to log the regen Wh each time I shut off the car, but that is way too much work for me. I'm sure that the Prius keeps track of the charge cycles somewhere, but I'll have to wait until my warranty is up before I try and reverse engineering.
Regarding the limited charge rate, the Prime will go a lot higher than that during braking regen. My record going down the grapevine was ~200Wh per minute, which is ~12kW. The electric motor in the car is rated at 53kW, so this isn't too surprising. I'd guess that the batteries can be charged at a max rate of 2C (2*8.8=16.6kW), so that would be a max of 276Wh per minute. Has anyone seen any higher rates of regen?

 

Right, you can't normally charge 8.8kWh in a single charging session, but one way to measure charge cycles on a battery is to add up all the portions of charging. So if you charge 20% of the battery on 5 separate sessions, that counts as one full charge cycle. Each charge cycle you put the battery through causes some damage to the battery and eventually it won't hold a charge.

For the 8.8kWh battery, the most I ever put in was 6.6kWh, which is 0.75 charge cycles (6.6/8.8 = 0.75). If you add up all the energy I put into the battery over a year, you can figure out the total charge cycles on the battery and get an idea of how it will perform. There is a maximum number of times you can charge the battery, but my guess would be that Toyota figured that into the warranty they have to offer (10 years or 150000 miles).

150,000 miles = 6000 charge cycles (at 25 miles per charge cycle, 100% of driving is on electric)

But to go through 6000 charge cycles in 10 years, you would need 1.6 charge cycles per day (6000 / 3650 days = 1.6 charge cycles per day). That's nearly 8 hours on a 120V outlet, which is probably more than what owners of the car would do. To estimate the low end, let's say a typical driver would charge it up once a day, 5 days a week for commuting. This would be 2600 charge cycles (52 weeks/year *5 days/week * 10 years).

So from this information I'd guess that the battery pack would last somewhere between 2600 - 6000 charge cycles.