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Old 10-28-2017, 09:41 AM   #1 (permalink)
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Idea: Parasitic battery expansion for Prius pack

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.


Last edited by The Atomic Ass; 10-28-2017 at 10:32 AM..
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Old 10-28-2017, 02:46 PM   #2 (permalink)
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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.
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Old 10-28-2017, 05:50 PM   #3 (permalink)
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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.

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