Got a few questions about Prime Traction Battery

It gets even weirder. That 8.79Kwh represents only 80% of the true traction battery capacity. The 20% you don’t see is reserved for regenerative braking and the like.
Don’t overthink things. Toyota knew what they were doing when they designed the Prime.

J

 

Well, the battery isn't actually doing two different operations. It is just doing everything the hybrid does, but with a larger capacity battery. The hybrid has an EV mode that might go over a mile with a full battery if the car is driven slowly. Not only does the battery not have the capacity for longer distance, but it also can't supply energy fast enough to allow faster acceleration and higher speeds. Put a bigger battery into a gen3, and it can go farther on EV only, and at higher speeds.

That is all the Prime is; the hybrid with a bigger battery. Plus a charger to supply grid energy. There isn't a hybrid and EV division to the battery. It is just the system working the same as in the hybrid. It sees there is enough energy in the battery to propel the the car at the speeds the driver asks, so it keeps the engine off. Then when that energy gets too low, on comes the engine.

When the battery is drained down below allowing EV mode, the system doesn't charge it any higher simply because that isn't an efficient use of resources. Coast down a mountain, or use Charge mode, and the car can charge the battery high enough to allow EV mode again.

 

This is incorrect - 8.8kWh is the full capacity. Only around 80% of that is used.

Overall usage is something like

Top 10% unused
Next 70% is EV capacity - filled only by mains charging, regeneration or selection of charge mode
Next 10% is the HV reserve (about the same amount as the used battery capacity in a non-plugin Prius)
Bottom 10% unused

As Trollbait says, the Prius drive system has always been plug-in ready - you basically just needed to add a bigger battery to give that EV range capacity. And the principle of extending life by never fully charging or discharging remains the same.

 

The question you're asking has several trains of thought to help make visualizing how the Primes systems are designed. My way of explaining HV modes (battery reserve) is that it's a conceptual description. Keeping that in mind, there is a portion ( or share if you will ) of the traction battery that is not used. Around 10 to 15% of the traction batteries top most charge voltage as well as around 10 to 15% of the bottom most charge voltage, is not figured into the Primes charge and discharge programming and never sees charge or discharge from the car or the plug.
( And that has to do with accepted battery charge / discharge practice, defined in the cycle life measurement or more commonly a batteries rating of how many charge and discharging cycles can a battery perform within it's expected lifetime.)

The reason some describe the HV mode reserve capacity is that it's a convenient way to explain that the Prime will work the same ways as a Prius after the Primes EV range is used up and the engine starts up.

I say convenient because it can get real complex really quickly to try to explain exactly what is going on, in the Prime ( considering all the things that can effect the cars behavior) when the Prime runs out of EV mode and begins behaving like a Prius. In much that same way as understanding precisely how Prime is going to behave for any one driver, on any one days temperature, driving speed, traffic conditions, battery state of charge, driving mode selected, etc etc etc .....

In short some of the driving techniques learned and used in a Prius still work much the same way when driving a Prime. Other techniques learned in a Prius need to be relearned or adjusted, mostly in understanding how EV mode changes some driving techniques.

 

Here's my take on the battery usage.

There is an app called Hybrid Assistant that shows how much charge there is left in the 8.79 kWh battery. The maximum State of Charge (SoC) that the car will charge to is 85% of that 8.79 kWh. While in EV, the battery will be drained until the SoC becomes 15%, at which point Hybrid is turned on and so is the engine. The SoC varies just slightly at that point. Same if you switch to Hybrid at any SoC between 15% and 85%, the SoC will only vary slightly from that point. The exception is if you take a long uphill/downhill road, this will change the SoC accordingly.

 

Max engine-off speed remains limited by MG1, like every Prius, but it's now up at around 84mph. MG1 reverse limit is now up at 10,000rpm to permit that. (I believe it was 5,500 in the G3).

There are no significant HSD changes since the G3.

Another way to think of the plug-in is having a lot more green bars. The normal Prius with its 8-segment battery display won't try to fill the top two green bars - they're just filled if there happens to be energy to capture, and it aims to keep charge around the green/blue boundary. The EV range is an extension of the green bar range upwards, so when charged, it aims to get down to the green/blue boundary like any Prius ever did.

And I realise that is indeed the way the battery indicator appears - no bars, but the green/blue split is still there.

 

Two nitpicking points:

1. 2017 Prius is Gen 4
2. There is no 'III' in Gen 4, it's called "Three" instead

 

One nitpicking point - he didn't say he had a 2017 Prius.

 

It's not significant but the Prime has a different final gear ratio than the standard Prius. This also helps the car achieve its 84 mph EV speed.

 

I'll need to double-check, but I think the difference in final gear ratio actually is working in the other direction - certainly the overall top speed due to MG2 limit is lower compared to the normal Prius in both G4 and G5.

 

Thanks for the correction.
J

 

You're right. In the regular Prius, it takes 10.83 rotations of MG2 to make one rotation of the tires while it takes 12.3 rotations for the Prime.

 

Yep, that's the G4 ratios. G5 ratios have converged, but they're still different. 11.23 for the 2.0 hybrid, and 11.94 for the Prime. (11.54 for the 1.8 hybrid).

So yes, they've lowered the final drive gear, presumably because they want to have more torque in EV mode, and that works against the top speed both in EV and HV mode.

 

I had to buy a 2019 Prime on spec to figure a lot of this out. We've had a Gen 2 and a Gen 3, so we were also really curious to find out what the differences were with the Prime. The used car dealers were of little to absolutely no help. My wife and I made three separate trips to the Denver area (mainly to visit family, but we also shopped for a suitable car for her daily 100-mile commute to work). Only one of the cars we drove even had enough charge to try out the EV mode. What made this worse was it's almost impossible to figure out the EV/HV modes during a test drive, especially when the salesman doesn't really understand a regular Prius, much less the Prime.

After having the car for a few months, here's my take.

It's a MUCH bigger battery - physically. The battery isn't just under that first foot behind the rear seat, but goes almost all of the way to the back. You have to give up the spare tire, before 2020 you give up the center back seat (to a console), and the floor of the cargo area is a few inches higher than the cargo door rather than lower. There's the "batmobile" style rear window too (only on the Prime), which forces you to lose the rear wiper as well as a carbon fiber rear hatch door. The combination of the battery, hatch, and rear window makes me think if there's any collision involving the rear of the car, insurance will probably just total the car.

The front doors seem to be a little more toward the rear, making it just a little harder to get into/out of, especially in a tight parking lot. This is a Gen 4 versus a Gen 3 issue. I tried getting into a Gen 5 a few days ago, and the front doors there seem to be a bit shorter as well.

It's not all downside. When my wife lets me drive her car on the weekends, I get to explore the EV mode. Since my trips are all in town, they're usually all EV. I absolutely love driving it. The car is super quiet, smooth, and has more than enough power and speed to keep up with all the other cars going from stop light to stop light. If I got onto the Interstate, it also has the speed to get me a few miles down the road (80 MPH) before switching to the regular hybrid mode. The only way I could be happier with it is if it had the 50 mile EV range of the 2023/4.

My Gen 3 was a stripped-down version, and I love some of the extra features of the Gen 4. I don't know about maintaining the sensors that make these happen. The Gen 2 and Gen 3 are very DIY-friendly. I don't think this will be true for all the electronic sensors required to make the new features happen.

One thing to know - check out the IRS website to find out the restrictions for the Clean Energy Vehicle tax credit, unless $4000 doesn't mean that much to you. I got my Prime at the tail end of 2023 so some of this may have changed for 2024. Most of the Primes I looked at were close, but not able to qualify for this - either they were pre 2017 (not enough EV battery capacity) or more than the $25K purchase price limit. Dealers were also no help here (probably because they did not want to delve into the tax code, probably because they wanted to charge more than $25K for their car).

 

Likely to offset the increased weight of the much larger battery.

 

Another difference with the Prime drive train is that it has a one-way clutch that allows MG1 to propel the car. The clutch keeps the gasoline engine from turning backwards while MG1 is contributing to moving the car. In the regular Prius, MG1 cannot propel the car.