DoD vs. battery life

[hobbit]

So we keep hearing about how if the Prius HV battery SOC ever gets
down to like 40%, poof, it's toast and will never recover and will
cost 4 grand to replace, yadda yadda.
.
And yet Darell has taken his rav4, same NiMH battery chemistry,
down to dead zero on a semi-regular basis, and has experienced
little or no capacity loss.
.
WTF is up with this? Are we being misled about the Prius pack
capacity/lifetime, or the batteries really different in the Rav?
.
_H*

[V8Cobrakid]

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

[McShemp]

Originally posted by V8Cobrakid@Mar 15 2006, 03:31 AM
The prius battery is more like a booster battery. Sure, it's powerful.. but not as powerful as the Rav4 EV battery.

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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.

[darelldd]

Originally posted by McShemp@Mar 15 2006, 07:23 AM
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.

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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.

[tripp]

Originally posted by hobbit@Mar 15 2006, 01:52 AM
So we keep hearing about how if the Prius HV battery SOC ever gets
down to like 40%, poof, it's toast and will never recover and will
cost 4 grand to replace, yadda yadda.
.
And yet Darell has taken his rav4, same NiMH battery chemistry,
down to dead zero on a semi-regular basis, and has experienced
little or no capacity loss.
.
WTF is up with this?  Are we being misled about the Prius pack
capacity/lifetime, or the batteries really different in the Rav?
.
_H*

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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.

[ScottY]

Originally posted by darelldd@Mar 15 2006, 02:01 PM
...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.

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

[gschoen]

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?

[dipper]

Originally posted by gschoen@Mar 15 2006, 03:33 PM
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?

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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.

[ken1784]

Originally posted by darelldd@Mar 16 2006, 04:01 AM
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).

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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

[hobbit]

Maybe the rav4 controller does impose some artificial limits
after all? Is there enough instrumentation to tell if you're
getting X amount of aH out of the pack, where the supposed full
capacity is a larger Y? Ken, you were implying that < 1V / cell
is just before the really dangerous point for the chemistry,
correct? I would think that it's all about the chemistry, not about
the physical size/shape of the cells or electrodes.
.
_H*