How to drain battery

Don't worry about it. Going uphill will automatically lower the SOC as the car works harder. In general you can let the control system do the work.

As for B mode, don't use it unless you need it for control or in cases where the SOC will go full up. Otherwise you are throwing away valuable energy for no good reason. Most highway grades are not steep enough to need B mode. At highway speed it takes a lot of energy to push the air out of the way, so even going downhill you don't get a lot of regen. I hardly ever need B mode at highway speeds even on long 6% down grades. I do find B mode good on slower descents, and in some cases it is simply easier to use B mode to control speed, so that you aren't shifting your foot back a forth between the pedals.

Tom

 

Not entirely true, unfortunately. I live in the foothills of a mountain range, so I do a lot of up and down driving and I run into the same problem as the OP. When you are on the uphill side, not only does the SOC not run down as you would want to make room in the battery for the upcoming downhill regen charge; but since you are forced to run the ICE on the uphill if the speed limit is anything over about 30mph, the controller actually takes the opportunity to charge the battery as you are climbing! Totally counter-intuitive! So every time I get to the top of a long steep uphill, I have only 2 bars empty in the battery and the battery is topped off long before I get to the bottom of the downhill on the other side. There is nothing more infuriating to me than throwing away kinetic energy with the B mode or the friction brakes, but since I cannot find a way to get to the top of the hill with the battery depleted, I don't know what else to do. As a matter of fact, this is my single biggest complaint about the car - sometimes I know more than the car about whether I need to maintain a charge or run it down and there just isn't enough control to let me do it.

 

Of course - the car has to be programmed in a way that best accommodates a wide range of situations - no question. And I'll be the first to admit that my situation is well outside the first standard deviation of driving situations, so I don't expect (nor would I want) the car to be optimized for this particular situation. But I do wish there was another mode that would allow me to impart some of my human foreknowledge to the car by lowering the target SOC. I find that the battery ends up closer to 80% charged at the top of the hills around here. I don't know how steep the hill has to be or what speed you need to be going to actually lower the SOC on the uphill - I've only ever seen the SOC increase as I climb, which drives me nuts. I'm already burning fuel to lift the car up the hill, so the last thing I want to be doing is burning extra fuel to be charging the battery during that time when all that will do is ensure I spill more energy on the downhill... what a waste!

Like I said, I know my situation is outside the norm and I don't think the failure to deal with this situation is a flaw with the car. On the contrary, I think Toyota has done a remarkable job at making the car driver friendly and allowing it to get great economy without thinking about it. But I look at it like a camera - the high end ones always have a series of manual modes in addition to the fully automatic modes. Yes, they do everything when you want them to, but there are times when you need more control and the really good cameras allow for both. The Prius could easily have the same sort of thing by having different manual modes that could be selected... or even different levels of existing modes like Eco1, Eco2, Eco3, etc. Or it could allow you to program custom modes to suit your particular driving style and roads, and store that as a custom mode. Lots of possibilities that don't cost anything except a few more lines of code. I love the car, and as an engineer, I have a lot of respect for what they've accomplished... but everyone has a wish list

 

Thanks for that information. I've actually been wondering why it doesn't do that already. Never mind traffic or route mapping - all it really needs to do is set a lower target SOC on uphills (which it could sense with an accelerometer and not even need GPS) so that it had more room for regen on the inevitable downhill. One almost always descends at some point after ascending for a while, so you could make some assumptions there that should help. Clearly nowhere near as sophisticated as what you just described, but should be something achievable within current technology. Anyway, like I said, everyone has a wish list

 

The car is really good about about managing the SOC. It's like it has been programmed to do it. :madgrin:

 

Almost like that, eh?

 

Ugh, I really wouldn't worry about it people. Purposely "trying" to drain your battery seems counter productive. The Prius is designed to try and keep the Battery within a certain range of SOC.

If you are going up and down hills and mountains you probably aren't going to be able to discharge the battery to the max at exactly the point you may be able to start "B" moding downhill. Good for that.

The whole battery charging within the HSD system has been designed to be as automatic as possible and for good reason. Otherwise you'd have drivers thinking they knew better than the computer system and trying to time out discharge and recharge cycles, which IMO is a bad idea. Just let the system do it's job. If your going down hill and you've recharged your battery to the max before you've reached bottom? So what?...

 

Don't be so quick to disagree with rachaelseven. Having visited her and driven the roads she does a few times now, plus doing the mountains on I-64 in West Virginia, the Gen III seems to do things a bit different. The new Prius will actually try to keep the battery at 6 bars and not use the battery on those uphills. I tried on I-64 to coax the Prius to use some battery by trying to let my speed drop very little on the uphills and/or try to maintain a 55 MPH on the uphills. In neither case did the Gen III use the battery. This needs more study and I'd love to try it on I-70 west of Denver also.

Wayne

 

So your experience reinforces the point that there isn't much you can do about it. Unless you are going to hack the control system, the Gen III is going to keep those six bars come hell or high water - no point worrying about it if you can't change it.

Tom

 

So far that's been my experience. I'd love to hear from others doing mountain climbs and see what their experience is.

Wayne

 

It may not be good for the battery to be cycled so frequently either. It would generate quite a bit of heat, rapidly depleting and re-charging, maybe more than the battery could handle?

 

The battery cycling is held to a very narrow range, just to avoid this sort of damage.

Tom

 

I believe Honda's experience with manual transmission IMAs illustrates this point. If the car doesn't take sufficient control of battery charging away from the common driver, then in-warranty battery failures can become excessive.

Giving drivers this control is a reasonable request, but the auto makers may well need better battery management software before this feature can be ready for market.

 

Simply put, don't use B mode unless you are going downhill and need the extra engine braking. B mode IS WASTEFUL. Its for saving your brakes down long downhills that would overheat them otherwise. It may feel like its doing more to re-gen but its really trying to soak up forward motion and turn it into heat and send that heat out the tailpipe. That is far safer then the brakes heating up to the point of brake fade and causing failure or less braking then you need.

 

Many years ago,in the 1960's when I was a kid, I decided I wanted to manually control the voltage of my generator (yes, generator, this was before alternators in cars). I put a rheosat in the field circuit, and mounted it under the dash. Burned out the headlights and the horn the first time I forgot to turn it down...
Thank god, cars didn't have computers, or electronic ignition i those days...

 

Maybe we should go back to the Model A design where the driver controlled the spark advance using a lever on the steering column. :madgrin:

 

I would love to have had a manual spark advance control in my 1980 Pontiac Phoenix!!...however I'm happy to leave the Prius to minster to itself.