Idea: Parasitic battery expansion for Prius pack

[The Atomic Ass]

As the title implies, I'm throwing around an idea of a battery expansion for my new Prius Two Eco.

The Two Eco comes from the factory equipped with a PALTRY 745 Wh of Li-Ion batteries, unlike the previous versions of the Prius, which had NiMH.

The Two Eco also has a space advantage, in that there is no spare tire. Having checked the well, I find a tire repair kit mounted into a GENEROUS foam space-consumer. Plenty of space for an expansion pack.

There exists plug-in conversions for this car, sure, but this is not what I'm after: it does not fit my driving, as I work as a contract delivery driver, and so drive 150-300 miles daily. Running pure EV mode at the expense of a higher power bill without being able to use it thereafter doesn't justify the price tag on systems like the unit from Plug-In Supply.

My goal is based on a theory I have about the car's operation.

The EV stage on the eco meter is variable based on SoC
This meaning that the available torque before kicking in the ICE is hugely dependent on voltage sag. Less sag, more available torque in pure EV mode.

If my theory is correct, then what I would like to attempt, is to "parasitically" attach larger Li-Ion cells. By this I mean, cell-by-cell, connecting them into the existing system, such that the car's BMS handles charging, discharging, and cell balancing, without me having to have separate systems of my own.

I am aware I'd have to get cells that are in the same voltage range for this to work, and that not all Li-Ion cells are equal.

The end result hopefully being the car has greatly improved EV driveability, more torque, more staying power, and hopefully, increased fuel economy as well.

Thoughts, concerns, obvious problems?

ETA: I would have fill-up results for it, but I've only driven ~350 miles so far, and still at half a tank.

[redpoint5]

You won't get noticeably better fuel economy doing this. The main reason to have the battery is to capture braking energy and allow the vehicle to turn off when stopped. Being able to drive a little more in EV isn't going to improve fuel economy.

[S Keith]

red is correct.

Major technical challenge and high cost. Of the desired outcomes stated, negligible benefit at any level except for slightly improved economy on mildly hilly terrain due to increased capacity.

[mwebb]

it is already being done
with
mallory ultra caps

info is scarce , however it should not be difficult to put together
it will not be cheap

it should greatly extend battery life but i dunnno if that will make the toyota HV systems angry

[teoman]

It really depends how big of a battery you attach to it.

Also how smart the battery management system is.

If for a 1kwh battery pack it calculates 100kwh input, then somethin MUST be wrong. The energy is going somewhere but where?? So it should shut downt.

This is like you drinking 100 gallons of water to quench your thirst and the pissing out 100 gallons of water. If it were to occur I am pretty sure you would go to a doctor. The bms will also stop functioning. Then again, it may not be so smart and be completely ok with it.

[teoman]

How much will the cells cost?

Check out other members posts where they state their power consumption while driving is. After that you can calculate what kind of EV range increase you are getting at what cost. If it makes sense for you, go for it.

If you live where there are lots of hills, does the charge run out before the peak of most hills? If so the you can get a good gain in mpg, if not, in my opinion it would not really be worth it.

[teoman]

Ps: for lithium ion batteries, the energy remaining is not really calculated based on voltage like lead acid batteries.

Most mobile phones have an energy in vs energy out calculation. I do not know how the bms of your car does this. Could be voltage sag as you say or it may only be a part of the equation.