Adding battery capacity to the Prime

It is a possibility. The BMS may see a different voltage level than expected for a certain current draw (more cells means less voltage sag). Or it could count amp-hours and either just go out of EV mode or even throw a code that the battery voltage didn't drop enough when it reaches a certain amont of amp-hours.

But on the otherhand there are plenty of people throwing in completely different battery chemistries into their old Prius with no modifications to the Prius' electronics and are having excelent results, the complete opposite of "upseting the system balance and causing things to NOT work properly." (Nexcell, Project Lithium) It seems to me, just like with the Nissan Leaf, that our Prii are simply looking for voltages, and as long as everything is within those voltages the BMS is happy. Voltages start droping down below a certain point, the BMS thinks the battery is about empty. They start rising up to a certain point, the BMS thinks the battery if about full. Add more cells in parallel and it should take longer to move between "full" and "empty" voltages.

OR, kind of like you said, the Prime could be looking for more than just voltage levels.

No, I do not own a Prius Prime to test this. As Bisco says, we have to wait and see.

 

Similar discussions have been going on since about the day after the first hybrid hit the street.
And we are still waiting.

In the mean time, we keep having pointless discussions like this.
Mostly because folks who have tried and failed miserably usually don't like to admit it.

 

I added the A123 Hymotion pack to my wife's 2005 Prius Prime. (Or, rather, I had a dealer install it). Based on that experience, there are a couple of aspects of this that should not be overlooked.

The first is crash-worthiness. The Hymotion pack is bolted and braced into the car, sitting in the well formerly occupied by the doughnut spare tire. I'm pretty sure they literally had to have it crash-tested before they could sell it. Just ponder the potentially catastrophic consequence if your 300-lb add-on pack breaks loose during a collision. And ask whether you have the expertise to guess whether or not that will break loose in a crash, without literally crash-testing it.

Second, disposal is an issue, at least around here. Basically, the dealer who installed it no longer deals with them. Last time I looked, I could not find anybody locally who would take that add-on pack for recycling.

Separately, if you are only lacking a few miles of range, and you can't plug the car in, seems like a vastly easier solution would be to buy a (say) 2 KWH portable power station, a cheap countdown timer, and just plug yourself into that every day, at your destination, for an extra hour of charging. Unlike a gas-powered generator, you could, in theory, leave that inside your car as it ran. You might have to set your charge rate down to 8 amps. It's Rube Goldberg for sure. And that's an expensive way to get that last little bit of charge. You would, in effect, be sacrificing the Li-ion batteries in that portable power station, to avoid burning a little bit of gasoline. But I'd bet that would work.

Better still, find a public charging station near your route. At low SOC, it's possible that just a short period of Level II charging would get you the marginal miles you need.A full charge takes, what, 2 hours at Level II. And most of that is when the battery is approaching 100% SOC. If you only need five miles, you might need just a 15-minute top-up.

 

Sorry, that was just poor writing on my part. The "it" that I meant was the portable power station, not the car.

Separately, that's admirably hard-core, using a portable power station as, effectively, a range extender.