Charging EV portion of the battery going down mountains

1) pressing the brakes gently charges the battery. (panic braking is via disc brakes)
2) In a Prime, B mode may be the fastest way to charge.

I am guessing you can recover 4000 vertical feet of energy. (no part of SC is over 3600 feet)

 

it charges the same way it charges whenever you're driving in any mode. the tranny motor generator is turned by the wheels, generating electricity to the battery. as long as your foot is off the gas or braking, it will keep charging until you are full, or the car reaches 6 mph and less.

 

The answer is different for the PHV and Prime compared to non plug ins.

A non plug in will fill the battery in less than 600 foot vertical drop so use the engine to slow the car.

Your prime has a huge battery by comparison, perhaps a 4000 foot vertical drop to fill it. Those will be rare, so the engine as air pump is less of a strategy.

 

Forget about trying to optimize regen. The M/G just does not have enough of a efficiency power curve to dwell on. E.g,



See how linear the red line is ?
torque * rpm = power
Keep the car at a nice safe speed and the rest will happen automatically.
I find this aspect of electric motors to be one of the compelling features of EV driving: Whether driving at 10 kW or 60, uphill or down, the car's efficiency is not going to change much. All I have to do is watch the road and enjoy the ride.

Earlier, I think you were asking if hybrid mode limits the regen energy the battery will take to some value below it's maximum capacity. The answer is no.

 

You're in SC so you've probably heard of Caesar's Head State Park. I drove up there one day and coming back down my battery was just depleted and I managed to charge up about 6 or 7 miles (out of a max of about 28 miles at the time) while braking (not in B, just in Drive) down an altitude change of about 1000 feet.

It's a lot of fudging numbers, I'm not sure of the exact amount of charge I got or the exact altitude change down the steeper part of the mountain but for around 1000 feet you can say you'll get about 6 miles just using the brakes.

 

Let's see.

Regen is about 50% efficient
Potential energy in Joules is mass*G*meters, so 1600*10*300 = 4.8 * 10^6 joules
Half that is 2.4 * 10^6 Joules sent to the battery

If we figure that a mile of flat driving consumes 220 Wh, that = 8*10^5 joules

48/8 = 6 miles.

Yep

 

I ran a quick sanity test descending a 525 ft, 8% hill at 65 mph. Initially everything looked pretty good until about 3/4th of the way down. It felt like the regeneration-charging slacked off and the car sped up. I plan to repeat the test recording a video and add instrumentation:

• 55 mph - initial test to see if it was the rate that might have led to the early cut-out
• 65 mph - replicate the original test

Bob Wilson

 

No. I typically make the video and then manually retrieve the data into a spreadsheet for analysis.

Bob Wilson

 

But remember, as in all Prius, you can only charge the battery to 80%. It will then stop regen. and use friction brakes. Of course 80% of the much larger battery in a Prime is a lot more than 80% of a standard Prius battery. From the calculations done above in other posts, it appears you won't max. out the battery in SC.

 

OK then, an exercise:

Say you want to finish at the bottom of your final descent of a trip with a full battery and you started the trip with a full battery. When do you change from EV to HV mode ? We'll presume for now that hills in the middle are not a consideration.

It is actually a trivial question ;-)