DoD vs. battery life

The prius battery is more like a booster battery. Sure, it's powerful.. but not as powerful as the Rav4 EV battery.

 

Say what??? Are you're talking about the difference between high power and high energy cells?

Nevertheless, I've seen the Scan Gauge screen that shows the 80%-40% SOC range. I just don't believe the 40% number.

The NiMH chemistry can't go to that low of a SOC and produce the number of cycles to get to 100,000 miles. No current battery chemistry can.

 

Except that we have proof to the contrary. Yup, it surprised the folks at Toyota as well. There are now a large percentage of Rav4EVs that have over 100k miles on them, with little to no detectable battery degredation. We even have many Ravs with 150k+ miles on them that are still going strong. And these batteries are absolutely *abused* when compared with how the Prius treats the traction battery. We charge to 100% DoD almost every day, and on a somewhat regular basis, we drain them to close to 0% Dod (depending on where you set zero). Many of us have driven the cars until they can go no more... then after waiting a few minutes, drove it a few more miles. This isn't the electornics stopping us, it is the capacity of the battery. The Rav lets us use all of the battery that can be used, and the batteries just keep on coming back for more. These things are amazingly robust, and nobody thought they'd last this long... but the fact is, nobody has had to replace modules (beyond a few mfg failures) in TEN YEARS and many millions of miles of on-road use in these cars.

 

I wonder if Toyota is being really cautious with this for marketing/image reasons. If they succeed with hybrids they may capture a HUGE market share of future vehicles, being so far ahead of most auto mfgs in hybrid technology. On the other hand, numerous stories of traction batteries going TU and hybrids could suffer a tremendous setback, particularly Toyota hybrids. I think Toyota very badly wants it to succeed because of the huge competetive advantage that they will gain.

Just a guess. It's good to hear that the NiMH batteries seem to be so robust. My question is why is this a surprise to the manufacturers? Seems like they'd at least have had a sense for it with their testing.

 

Ten years? do you mean the EV1's or the Rav4?

 

If the batteries are so robust, wouldn't Toyota have saved a lot of money with a smaller capacity battery but use more of it? They must have done testing to come up with usable SOC.

The EDrive PHEV reports battery life of 6-8 years using 80% of the battery, decent but not life of the vehicle. Why are the manufacturers so conservative on the batteries if they could be used more? Surely Toyota had this data by 2003 for Prius or later for Highlander or Lexus?

 

It is called "over Engineering".
You never Engineer anything close to the point of failure. That is never a good Engineering rule. You usually put safe points so that your solution cannot fail. In the case of Hybrid, failure is not an option with the way Toyota has Marketed its product.

 

Darell,

I'm curious how those numbers are accurate.

We know it becomes hot when the NiMH charge to 100% SOC.
Have you ever measured the temperature when the 100% SOC?

Also, the cell voltage becomes less than 1.0V when the 100% DoD.
Have you ever measured the voltage when the 100% DoD?

Ken@Japan

 

A quick search shows the EV Rav4 has about a 105 mile range. So, 150k miles = 1500 cycles on high energy batteries. That sounds like more than testing supports for 100% DOD. The explanation must be that there are some 100% DOD mixed in with 80%, 60%, 40% etc. DOD runs. The batteries can't be depleted each time the car is recharged unless the charger does so and there's no reason for that.

Also, driving until the car can't go any further and then resuming after waiting a bit is a product of internal battery resistance and heat. The batteries cool down, their internal resistance goes down, and electricity can flow again.

BTW - High energy cells are different from high power cells. For the same chemistry, and even footprint, their construction is different. High energy cells are "better suited" for deep discharging.

 

hobbit,

I believe the cell voltage < 1.0V means almost 0% SOC or 100% DoD.
It is not good for the NiMH life.

Ken@Japan

 

Rav4EV. The EV1's were not on the road for ten years before GM called them all in and crushed or disabled them.

 

They are accurate because of the very last part there, "(depending on where you set zero)"

Yes. They get hot. I have real-time max min and average temp for each module in the pack.

Yes I have. I also have real-time readings of pack V, max, min and average module V. When I drop below an average module V of 10 or so, there's no more go left in them. And here is where that last bit that I typed before comes into play. Maybe I should use a term like "usable DoD." At 1V per module, I'm not going anywhere. At 2V per module, I'm not going anywhere... basically anywhere below 10V per module, I don't have enough usable energy left in the batteries to provide traction power, and I call that zero DoD. Fact of the matter is, I cannot take the batteries down below that unless I first drain them through traction power, and then let them sit and self-discharge the rest of the way.

 

The controller imposes two artificial limits. One is top speed to keep the reduction gears (remember, one ration from 0-80mph!) from flying apart.

The other limit is allowable current at near-zero "usable" DoD. When the vehicle is really close to an empty "tank" your top speed is limited to about 25mph (depending on conditions). Taking high current out of a dead pack is very, very bad! But it turns out that you can sip it for quite some time.

The car (like all the production cars) came with pathetic instrumentation. Analog "fuel gage" and analog Voltage gage (with no gradation). That's it. But through reading the OBD2 port on a Palm device, we can see damn-near everything in real time. We know instantaneous voltages, temperatures, wheel speeds, capacity used and replaced (through regen or charging), etc. If I were to start the modules at 1V, and fill them to capacity (where they get really hot) the batteries would hold about 10% more than I can actually use (as explained earlier). The car is not imposing this limit artificially, that's just how it is. I can use them until the car won't move any longer... and when the car has come to a stop two or three times in a row like this, I call that DEAD. Zero usable DoD. Though there obviously IS still some charge in the batteries - certainly enough to kill you... they just won't push the car any further.

Now I'm not sure if we're talking about modules or cells....

Anyway, at 1V per module, you've already damaged the thing. For their own safety they should be kept above ~10V. When fully charged they are about 14.4V

 

Oops. Almost missed this one!

If I understand your comment correctly, then yes, you are right. EVs are not drained to dead every time they're driven, and so do NOT see a full charge cycle every day. We certainly drain them more than any hybrid does though. In fact, I often stop my charge early to avoid 100% in the hot summer days, to protect the batteries against over-heating. Production cars should make this simple, but allowing a 90% charge by default, and if you want/need a full charge, you just press a button before or during the charge cycle.... but so far, the cars are pretty dumb as far as that goes.

Yup. Same as any large-draw battery device. Dare you do do that in a gas car.

I've heard this before as well. I'm not sure how MUCH better suited they are, however. Significant? Regardless... we solve the problem by changing over to energy cells and allowing us to plug them in and discharge them more!

 

Think satellites ... or even better .... space probes. That's one of the primary apps that high energy cells were designed for. They're made to have lower charge and discharge rates and not for high power apps. No 10C bursts!

The whole thing is a little daunting. All of the data provided by battery manufacturers is presented to make their products look great. "We get 10 million cycles." True, but at C/20 and from 100 to 99% SOC and back at 27 degrees C.

I've seen module testing deform gen 1 prismatic cells with constant charge and discharge rates of as little as 3C.

 

Two other observatons to throw into the question of why Toyota is babying the Prius HV battery --

RavEV was limilted AFAIK to Cali. Prius goes everywhere, and the climate range is more extreme.

2. Stories of NiMH battery failures (complete, or happeining) are not exactly rare in the Honda Insight realm, mostly it seems to me with owners who aggresively discharge the HV. I'm not clear why the RavEV tolerates that kind of 'abuse' and the insight does not, but obviously the chemistry has it's vulnerabilities that can be unmasked in daily driving.

 

Only offered through exactly 25 dealerships, and all in CA. But... now that they're all off lease, and are OWNED, they are all over the country. We have them in bitter cold areas, and even in very hot ones (two in Kenya that I know of). They perform VERY well in extreme climates.

As has been pointed out earlier, all that is NiMH is not the same. And certainly how the battery is managed (for cooling, charging algorithm, etc) all comes into play. But as far as "daily driving" being the culprit, I'm not convinced! I have two Rav4EV friends with cars that are about 3.5 years old... one with 80+k miles and the other with 85+k miles. If that isn't "daily driving" I don't know what is. And this is 100% on NiMH "energy" batteries. Neither of these cars has EVER been in for service. Not even for check-ups. And they're both seeing over 100 miles of range still. Many folks think of 100 miles range as too limiting, but as you can see, for some people, the only limit is the number of hours in the day available for driving!

 

Hi Darell,
Thank you for your comments.

I'm not familiar with the battery meter on RAV4 EV.
So, you're saying that you set zero when you think the SOC is almost 0%, right?

I see. Then, it's almost 100% SOC when it becomes hot.
However, as you are saying on another post, it is a good idea to stop charging at 90% SOC, right?

The EV-95 module has 10 cells.
So, I meant the safety minimum level is 10V/module or 1.0V/cell.
I believe you agree it.
But, you are saying you are not going to discharge to that level very often.
It is not a good idea to discharge to the 10V/module every time, I think.

Ken@Japan