Hi,
The only exception I can think of has to do with regenerative braking. We know that it is better than straight mechanical brakes but you should not think of regenerative braking as being 'loss-less.' The more regenerative icons you have, the lower your average MPG tends to be.

Recently, I started looking at the energy flow and it appears that you can incur very high currents in regenerative braking that exceed the ability of the batteries and control electronics to absorb. Whereas, if you can exercise predictive braking or better still, coasting, you can avoid these high currents and get much better use of your vehicle energy.

There is a type of energy loss called "I squared R", which means the resistance heating is the current squared times the resistance. If you can keep your current loads low by modest driving and easy braking, you'll minimize the heat lost in the wiring, control electronics and overloaded batteries.

Bob Wilson

 

Thanks Bob for your comments.

I have found that with modest driving, slow acceleration (my intention is use the battery as much as possible until the ICE kicks on then, I push the accelerator pedal a little bit more so as to maintain the RPMs modestly high to put the ICE in hopefully some optimum FE mode), and minimum braking, the overall MPG numbers come out great ..

I have started wondering recently though - whether I am putting the batteries at risk by increased charging and recharging cycles and thus reducing its performance by trying to run the vehicle in the EV mode as much as possible, especially under low speed conditions!?

Is it beneficial to preserve the battery power only when absolutely necessary (regen braking, may be warp stealth??) and instead just P&G as much as I can ..and while pulsing, try to always use ICE .. and then glide ... that way, the MPGs would be made necessarily only while gliding - versus in my current driving technique, where I get the MPG advantage with both the glide (free miles) as well as the EV mode ... any comments?

Thanks!

 

There have been few battery failures to use for a pattern. The battery controller is pretty protective. However, as a general rule, avoid heating the batteries is a good idea. Modest driving and keeping the car out of excessive heat is a good idea. Here in Alabama, I park in the shade in the summer when I can find it.

Bob Wilson

 

Thanks Bob. So, I gather that it is OK for me to use the battery at modest vehicle speeds .. and extend the free miles of gliding to a few more in EV mode ...

 

Exactly! The NiMH batteries are well protected and other than heat and over charging, there isn't a whole lot that can be done to hurt them.

Bob Wilson

 

Hmmm .. I can understand how overcharging the batteries can affect the batteries .. but it is hard for me to imagine that the Toyota engineers did not think of a way to protect them from this risk ..

And the same goes for overheating ... if the batteries are being used and as a result, they get heated up, then it makes sense to stop using them and turn the ICE on .. isn't that the case with Prius' computer's logic?

Besides parking in the shade, what else could one do to prevent the battery from overheating? Say in the case where the weather is itself very hot (>120 deg F like in Arizona, TX, etc) ... would that affect the battery's performance as well?

Regards,

 

Parking in the shade and using the AC in hot weather so the battery fan can pull cool air into the battery compartment is pretty much it. In Alabama, I often drive with windows down, in town, up to about 85F. Above 85F, it has to be dry and above 90F, I'm on AC, buttoned up.

Now the only other time I've put a serious hurt on my batteries has been on some hill climbing tests. One hill consisted of climbing a 600 ft., 6% grade, ~1.5 miles at 80 mph. Starting with a high SOC, I was able to measure 68 hp (my ICE is rated at 70 hp) and actually saw a lower gas cost to reach the top. The difference was made up by the battery. But this is NOT recommended, especially because the curve at the top was only banked for 65 and at 80 mph it was an interesting left turn.


The other hill was 980 ft. climbed at ~70 mph on a 95F day. At the top, I pulled off to a side road to walk the dog (she seemed a little anxious) and I first heard the battery fan come on.

On the over-charging, all I can report is seeing momentary battery currents of 100A into our 6.5 AH batteries under heavy braking. If accurate, this would be over a 10C charging rate.

Bob Wilson

 

In other words then, the overheating and overcharging of the batteries is not a serious issue ... and that there are battery fans to take care of the overheating problem. I am guessing that if there is any extra electric current going in the battery, the battery might be designed to reject it ..

 

It remains something I'm concerned about since a local, 2002 Prius had its battery replaced three months ago under an extended warranty at 98,000 miles.

Huntsville AL is a pretty warm place and I spoke with the driver. His mileage suggests he simply drives the car normally (for Huntsville) and every time I've visited him at work, his Prius was parked in the middle of the parking lot with the trunk having a southern exposure.

I have also gotten a private e-mail about 'rejeuvenating' a Prius battery. The technique is to add KOH whose loss is determined by checking the weight of the battery. Also, the Dept. of Energy fleet report of the Arizona Prius showed a significant loss of capacity (but not a proportional loss of MPG.)

My engineering sense suggests use, not abuse, and that means trying, within limits, to keep the battery as cool as practical in the summer and avoiding the battery killers, high charge rates and heat.

BTW, high charge rates are not to be confused with overcharging, which kills the batteries stone cold dead very quickly. High charge rates simply means there are more amps than the chemistry can absorb so what is left over leads to heat. I've got recorded data that shows the battery voltage climbing to ~340 VDC, a significant over voltage (for NHW11.) You'd really have to work at it to hurt the battery and probably live in a hilly area, like north Alabama, to do any serious damage over a long period of time.

One last point is there is not a linear relationship between battery capacity and MPG. It turns out the control laws are the key and the actual amount of battery capacity needed for good MPG appears to be less than what is provided. I meant to say the Prius battery has more capacity than is needed for efficient operation. But the extra capacity helps give it a longer service life.

Bob Wilson

 

It certainly is OK or the car would be a big pile of useless. However, EV mode is not the same as battery-only. EV mode is something accessed by a button on Europe and Japan models, and modded American models, that prevents the gas engine from turning on below 35mph. It ensures battery-only operation and allows you to accelerate harder battery-only than D alone would (D would kick the gas on for high torque demand).

The idea is to try to only use the battery when:

A) There's excess battery power available (more than 6 bars)
B) The efficiency of the electric motor would be higher than the efficiency of the gas motor.

If you can use it when both A and B are true you'll get very good mileage.

One example of A occurs without your doing anything special - while you drive at constant speed the gas engine turns, producing more torque than needed to keep you moving. The extra torque is bled off to the battery, creating a surplus leading to ideal conditions for (A). The Prius computer periodically turns the gas off and drives on electric only to eliminate this surplus as seen on numerous of Bob's graphs posted on this forum.

An example of B also occurs without your doing anything special - when you accelerate gradually the Prius computer will use the battery to push you forward because the Prius gas engine efficiency at such low RPMs is terrible. Note that the Prius computer keeps A in mind during this stage - so if you have 8 bars, it might accelerate to 30mph electric-only; if you have 4 bars, it might accelerate to 5mph before engaging the gas engine.

With this in mind you can get a little more efficiency the Prius computer alone cannot, because you know what you're about to do. Examples stated are to attempt to drive electric-only up the last part of a hill if you know you'll be picking up electricity going down the other side. This is an extension of A: You don't have an excess in the battery, but rather, you know you're about to.

Another example is to run on electric-only for the last quarter mile coming home since you know the gas engine is going to run a little extra when you next turn it on to produce heat - may as well make room in the battery since you know what's coming. The Prius computer doesn't know you're done driving for the day, but you do. (If only cars could drive themselves without fear of lawsuits, the Prius computer would actually know this and optimize accordingly).

In any case, on the topic at hand, I've so far found:

Below 15mph with 6 bars of battery, it's best to accelerate electric-only just like the Prius wants to do.

Below 25mph with 6 bars of battery, the best way to accelerate is when there's a red and yellow arrow to the wheels and no arrows to the battery. Accelerating without the yellow arrow to the wheels (red arrow to wheels green to battery) below 25mph appears to get less efficiency at these low speeds, judging by my MPG averages.

Lately I increase my pressure on the accelerator slightly as I go from 15-30 keeping the yellow arrow to the wheels (the ICE is increasing RPM which increases available torque, so the increased pressure doesn't cause a yellow arrow from the battery like it would at 15mph).

Then around 30mph I leave the accelerator where it is and the car continues to accelerate showing iMPG numbers going from 27 to 34. As it moves to 40mph the arrows change to red to wheels and green to battery. This final stage is what I use repeatedly to maintain speed during Pulse and Glide.

On one tank, I used these methods consistently in 50F temps to achieve 78mpg. Then I drove to San Diego (all highway) and my mpg plummeted (noted in my data on this site), but the numbers were impressive while I could stick to the method.

With the EV mode I intend to pull into my hilly, 5mph speed limit complex electric-only and cut the gas engine out during deceleration (for example coming off of highways) to achieve 60mpg even in less than ideal conditions. It's possible that as summer returns to LA I'll get even better than 60mpg with both EV mode available and regular driving temps of 80F.