Traction Battery size

Unless your commute takes you across Stevens Pass -- unlikely from your location, on the opposite side of the Sound -- a larger traction battery cannot help your MPG. The extra weight will actually hurt it a bit.

Installing a plug-in kit, without plugging in, defeats the entire purpose of the kit. Unless you displace gasoline energy by actually plugging in to get some of your energy from the electric grid, the kits are pointless. And I believe useless as well, as they still can't use regen to recharge the plugin batteries.

 

To put it simply it uses more fuel to charge the battery than you'd gain in electric propulsion. As others have said, there is also the extra weight, extra costs for minimal gain.

The plug in Prius is a plug in as grid electricity is cheaper to charge the vehicle than burning petrol to charge.

 

In addition to what others said (the extra weight will negate any gains and probably hurt overall MPG), you really won't gain any additional battery travel time. Remember, at 30 MPH, you may be able to get 6 to 10 minutes (maybe more) on battery before the engine fires up to charge it, but the engine will have to run longer to charge to capacity (in the absence of any regen braking, etc).

Thus, there's really not much benefit to the larger battery. Toyota has engineered a nice compromise between battery capacity, weight, and charging system requirements to tweak the greater number of MPG for all driving conditions.

 

You would expect the Toyota engineering to be designed to cover most situations. I'm guessing that there may be driving situations for some people where they frequently hit the charge limit of the HV battery while also frequently draining to the lower limit. For them, having some extra battery capacity may help?

 

Any one who lived atop a descent of over 600 vertical feet (200 meters) would in theory capture more energy at the beginning of their drive and deplete it at the end. The OP lives in a city named Port Orchard, a surprising number of ports are at sea level. There are low hills south of Port Orchard, but nothing that would fill the HV battery. The Cascades are not close, but the are the closest range s/he may need to get over.

 

The other problem with running a larger capacity traction battery is you'd also have to change the way the computer operates with it. If the traction battery's state of charge drops down to around 53%, it changes the way the ICE runs-- higher RPM and less torque to the wheels so it can charge. As the SOC drops even lower down to around 46-48%, it enters a "panic-mode" that increases that effect. Your mileage drops quite a lot regardless of your driving in that scenario, and with a higher capacity battery, it would spend a lot more time running in that mode.

Conversely, when the SOC on the traction battery gets up around 70%, it enters an electric-rich mode in which the ICE runs at a much lower RPM and the traction battery provides a lot more of the drive torque to the wheels to improve mileage. That's why the plug-in conversions work so well-- because by being just an external power source to keep the factory traction battery at a high state of charge, the hybrid system gets to spend a lot more time in its most efficient state of operation. But the benefit only really works during the electric operation, and would be negated by having to recharge the extra battery capacity. By the time you convert battery voltage to MG2 voltage, then convert that to motion, then convert the motion back to electricity through MG2 and convert that back to battery voltage (or gasoline energy through MG1), you're only gaining something like 37% of the spent energy back. The rest of that energy is all lost to the various conversion losses along that path.

I read in a thread a long time ago (don't remember if it was here or cleanMPG) about how Prius gets such great mileage. It went something like this: Drivers of normal cars get poor mileage because they waste 100% of the energy to get them moving every time they stop. Drivers of the Prius think it gets better mileage because of its regenerative braking recapturing some of that lost energy every time you stop, and using it to get you going again. Hypermilers get the incredible mileage they get because they drive differently, and don't use the brakes much at all if they can avoid it. There was a proverbial quote about those hypermilers where their dominant driving philosophy is, "Prius has brakes?" Regenerative braking is better than no regenerative braking, but no braking at all is even better than regenerative braking.

Plugging in to the wall will always be more efficient than regenerative braking because it eliminates one set of the conversion losses in a hybrid system like the Toyota's HSD.