Questions about normal Prius & PHEV Prius HV batteries

Grumpy has a great question and usb has very useful replies!

Part of my related question remains unanswered (at least I couldn't find it in the source which was provided).

The early li ion batteries degraded from time which was not related to use. Since this property was not emphasized in the above post, is it proper to assume that the li ion battery included in the analysis and the NiMH battery in the present Prius are now similar in their time related degradation? In other words, is time related degradation no longer an issue with the battery which is the subject of analysis?

 

The paper does not describe batteries made by Panasonic. It describes experimental cells made by the university researchers themselves based on chemistry that may be somewhat similar to what Panasonic is using. What is your source for the claim of which batteries Toyota is using in the production PiP? Tesla is using two different cells in the Model S, apparently. The cell going into the 300 mile pack is using a new silicon-based anode, not a basic graphite anode as described in this paper. Silicon anodes have excellent energy capacity but have had cycling life problems in the past which Panasonic claims to have at least partly overcome. I don't recall the details on the cell which will apparently be used in the smaller packs.

Except these aren't the literal cells which will be made by Panasonic and all kinds of small changes can cause large changes in cell characteristics including cycling life.

Meanwhile, the actual battery in the Volt is warranted for 8 years (and will soon be warranted for 10 years or 150,000 miles) and was presumably designed to exceed the warranty period. GM has done advanced pack aging/cycling in the lab to 200,000 miles.

I do understand that you may expect it to last 8 years (probably less).

 

I doubt university researchers made their own automotive grade batteries to test them.

There are numerous reference to the PiP prototype using Lithium battery from Panasonic. This one specifically is for the production model.

The higher capacity (3.4Ah) Panasonic cells are scheduled for mass production in March 2012 which is also when PiP will come out.

The modules in the video (post#13) looks similar to the modules that Panasonic sells. Yes, we still need an official confirmation (couldn't find one) from Toyota that they are using Panasonic lithium battery. I heard that PiP prototype has prismatic cells. Even if the packaging is different, the chemistry should be the same.

Relax, I simply stated what it said in the GM's warranty documentation. 70% is lower than industry standard for battery end of life.

 

I think the calendar life depends on the chemistry used.

I am intrigued by the 4 separate battery packs. PiP only use one subpack at a time while the other 3 are disconnected. Why did Toyota design it that way? In the past, I thought it was done to easily offer different range.

It is possible that disconnecting the subpacks actually extends the life. Think of it like in hibernation mode as there is no current going in and out. It is like sitting on an air conditioned shelf (A/C cabin temp while driven), not counted toward the mean time before failure (MTBF).

 

usbseawolf2000

Your last words above suggest that the analyzed battery could last "15 to 16 years"

That assumes that its time degradation would permit that. Early li ion batteries "timed out" in three years.

Page nine of the Nissan Leaf Warranty contains the following text:

"GRADUAL CAPACITY LOSS
The Lithium-ion battery (EV battery), like all lithium-ion batteries, will experience gradual capacity loss with time and use. Loss of battery capacity due to or resulting from gradual capacity loss is NOT covered under this warranty (emphasis supplied)."

Since your post makes no reference to time degradation and only to use degradation, are you suggesting that time degradation is no longer an issue?

I look forward to your reply.

Allan de

 

I honestly don't know the answer. I need to do more reading.

Keep in mind that the PiP prototypes were manufactured in 2009. They have been on the road for more than 2 years. Toyota is collecting data from them and it is the best "accelerated" test I have ever seen.

 

There are university and national labs doing battery research that have their own commercially available equipment for making test quantities of 18650 cylindrical cells. It's even more common to make coin-style cells for testing new chemistry variations.

Here's what Tesla says to expect from their state-of-the-art pack for the new Model S which is based on cells from Panasonic:

"Tesla expects the battery to retain approximately 70% of its initial capacity after seven years or 100,000 miles."

Model S Facts | Tesla Motors

 

It depends on how you use (or recharge) it. Keeping the 8,000 cells balanced is a lot harder than a few hundred. The way each cells are connected also affect the life.

The application (PHV vs EV) is also different. For EV, there is no ICE to cover it's weakness. PiP has synergy going for its hybrid drive that covers each other's weakness. It is easy to criticize PiP turning the ICE on but there are many good reasons behind it.

We know Volt doesn't have synergy drive. The ICE piggyback ride the battery pack until it begs for mercy. It begs when the battery is low or in the extreme cold when ICE need to warm up the battery (not drive together). The strategy is very anti-synergistic that I have a very clear opinion about Volt's battery life.

 

Here are the data sheets for Panasonic's current generation cylindrical lithium cells:

http://industrial.panasonic.com/www-data/pdf/ACI4000/ACI4000CE2.pdf

 

Those are not it. I think this is for the current 2.9 Ah cell. Life cycle is not impressive because it appears to have tested with 100% DOD.

https://industrial.panasonic.com/www-data/pdf2/ACI4000/ACI4000CE25.pdf

 

The link I gave is a superset of all of the current generation Panasonic Lithium cylindrical cells. It includes the 2 page data sheet that you gave a link to. The highest capacity cell it lists is 3.1 Ah.

 

Yes, it will still have those 100,000 small charge and discharge cycles, but they'll be the same amount of energy as in the regular Prius - and thus a much smaller percentage of the battery capacity in the PHV. Because of that, the battery can handle many more of these even smaller charge cycles, while also handling a few deep discharges as well.

Given Toyota's need to make a reliable car, I don't think a cab charged twice daily is likely to have any problems, otherwise some consumers would likely have problems too.

 

Ok, keep in mind that they are consumer grade cells (for laptop). They are not the automotive grade cells that Tesla and PiP would be getting. The spec sheet just give us the hints/benefits:

• Superior cycle life
• High energy density
• Durability
• Excellent shelf-life

 

Found this piece while reading this SAE paper. In additional to controlling thermal and state of charge, PiP is also controlling the pressure in the cells!

One of the big issues for Lithium-ion batteries is battery capacity drop due to aging. However, Toyota has managed to improve this by introducing a constant pressure structure, upgrading cooling performance and preventing very high SOC condition.​

 

I don't have time to download that paper right now, but "constant pressure structure" sounds like a bunch of prismatic cells together in a battery module that are pressed up against each other using springs to allow some amount expansion and contraction under constant pressure. This is already a well-known technique. I believe this is effectively done in the Volt as well.

Am I guessing right, or is this constant pressure thing somehow different?

 

This is very helpful information.

Grumpy started some important issue discussion! As your recent exchanges have revealed, batteries are complicated devices especially when compared to the simple petrol tank. I am not surprised, at all, that Toyota is quietly working to master the issue of battery aging. This has seemed to me to be the Achilles Heel of the Lithium-Ion battery since I became an EV owner in 2007 and drove them (2) over 4000 miles before deciding that it was time to stop doing that. Those who converted to Lithium-Ion of which I was aware did have problems with battery swelling which appeared to connect with cell failure. Pressure control seems an obvious response.

It is a shame that so much good work is going on but that it is not out in public view where we can benefit from the confidence of knowing about it. I declined the Leaf which I ordered because I had so many unanswered questions about the traction battery and its performance. Toyota appears to be taking a more careful approach to the issue, but we still must guess and hope that the issues will be found and addressed.

I suppose that we need to infer a lot from the gamble that Toyota is taking by simply producing the PiP. I am trusting that the excellent experience with Prius so far will be duplicated in the PiP. It seems to be a wager worth making.

 

Several of you below are of the impression that the lithium ion cells in cars are cylindrical.

This video shows how the battery cells are made as flat plates. The finished battery packs in the video are similar in shape to the those shown for the Prius Plug-In and the Leaf.