How to extend LIMITED SERVICE LIFE of Prius PHV traction battery.

My 2005 Prius arrived unexpectedly early, two days before a 32 day vacation in Europe. I was worried but my car was just fine when I got back, just like my dealer told me it would be.

 

This is because Tesla has an active thermally managed battery management system, which runs all the time to cool the battery pack, well, it uses pack power to do that, which is why it discharges so fast sitting unused. Tesla had to do this because of the cells they use.

 

At the cell level the voltage is the same so current directly relates to power (In the Volt, I see the 3 small cells connected in parallel as one larger cell).

Now, we both know that the idea of blended CD in the Prius PHV is to ease the burden on the battery (Ice takes the high loads).
Knowing this, you wrote in the original post I was answering:

To me, this is a spin.

 

Not exactly. The correct analogy to running a Roadster's battery down to zero is to have a Prius Plug-in run out of gas. Then the Prius Plug-in is in the same predicament of potentially running down the traction battery; in fact, there have been cases of this with the Prius Liftback causing early traction battery failure.

I think Prius are a little less prone to this b/c the traction battery is disconnected when the Prius is off. There is still battery self-discharge, but I'm not sure how how fast this is for NiMH and Li-ion batteries.

 

Li-XX batteries are usually damaged if they are discharged bellow to 20% SOC on the cell level but balancers balance the cells, so all the cells have the same SOC at the same time.

NiMH batteries have a problem of only having 1.2V/cell comparing to Li-XX chemistry, which has aprox. 3.7V/cell. That makes a balancer for NiMH battery very expensive. That's why NiMH batteries usually don't have balancers, which if not carefully managed, can cause different cells having a different SOC at the same time.
If you discharge NiMH battery to low, the cell with the lowest SOC could go below 0 Volts and the sum Voltage of the whole pack would still be above the discharge cut-off. What happens then is that the cell, which goes below 0 Volts, starts being reverse charged by the discharge of the other cells. This damages the cell considerably and because the cells are linked in series - when any of the cells is damaged, the whole pack suffers.

This was the case of many premature deaths of Vectrix' NiMH batteries for owners, which were replaced in warranty time, because they died so soon. Usually, they didn't last 10.000km. Mine died after 4.000km.
The discharge voltage was set to low. That was the cause.
Software update set the discharge cut-off higher, which reduced the range a bit, but it saved the battery. Some Vectrix owners have driven up to 60.000 km so far.
New software also added a crude balancing. Every 12 hours of driving the charging ends with a 4 hours of C/10 current charging. Even if fully charged, it doesn't damage the battery, but causes the not fully charged cells to catch up with fully charged cells.

 

I would argue 50-80% SOC would be good. Keeping a battery full is hard on the battery as well actually. I think someone here already said drive it until the engine turns on then let it sit. That will probably be ok as well as long as it isn't long periods of time (months), but as was said the 12V could drain as well.

 

That is b/c you don't understand batteries then.

There is something called the C-rate and it relates the current draw the capacity in Ah of the cell. The same thing works for a battery pack.

If you have a 4kWh battery pack then 4kW is about a 1C rate. If you have a 24kWh pack then 24kW is a 1C rate. So you can have 6x the discharge rate with the same pain on the battery if the battery is 6x bigger.

So yes it is true that the "leadfoot" is doing less damage to a much larger pack if they are of equal quality cells and the power demand is the same. That is why the Prius turns on the engine. Otherwise toyota would have needed to put a bigger motor in and far superior cells (which cost 8x as much).

 

This has been discussed at length in THIS thread.

Tesla's own reply is HERE.

Prius owners have ruined their batteries also, by running out of gas and then continuing to drive until the battery went dead.

The issue (really a non-issue) with the Tesla Roadster is that in order to achieve improved battery life, the Tesla battery pack is the most coddled pack in the EV world, but this takes power. Therefore, owners are told over and over again that they must plug it in. We even had to sign a waiver to the effect that we understand that we must plug it in. And even then, it takes weeks to go dead. Those very few who had this happen had weeks and weeks to arrange to get their Roadsters plugged in. Not doing so was gross negligence. This is a high-performance sports car, designed for a niche market, and it has its own requirements. Failing to plug it in is like driving an ICE without checking the oil when you know it has an oil leak.

But according to Tesla, they've designed the Model S so that it will take a year for a full pack to go dead, compared with eleven weeks (nearly three months!) fo the Roadster.

And yes, the battery pack is nearly 40% of the cost of the car. With a 245-mile range, this should surprise nobody who has had experience with EVs. I believe this is actually cheaper per mile of range than the pack in the Gen II Prius.

* * * *

On the subject of power draw and stress on different size packs, think of it this way: In a larger pack, with more total energy, a given power draw is a lower percentage of the pack's total energy in a given time, and therefore puts less stress on the pack, all else being equal. Of course, packs in different cars are not equal, and cells can be designed for high discharge. But in general, a larger pack can sustain a larger power draw.

* * * *

And finally, on the subject of vacation, Tesla has a Storage mode. You leave the car plugged in, set on Storage, and it maintains the car at a lower SoC. Since the car can communicate with Tesla, they can inform you if the pack SoC drops too low, and you can make arrangements for a neighbor to make sure it is properly plugged in.

I'm sure Toyota has considered vacation/storage, and that the owner's manual will provide the correct procedure, which will depend on how the car's systems are designed.

 

Knowing Toyota, I am resting assured they sizes the traction battery conservatively to the job.

Lets examine the Prius PHV battery compared to the Volt battery.

The Prius PHV uses 4.4 kWh battery comprising of 56 3.7V cells in series for 207V total.
Each cell has a capacity of 21.3 Ah (4400/207).
The max power demand from the battery was not officially published yet, we hear 31 kW and 38 kW. So the burst current demand from each cell is 150 A (31000/207) in case of 31 kW or 184 A in case of 38 kW.

The Volt uses 16 kW battery comprising of 96 3.7V ‘cell groups’ in series for 355 V total, each cell group contains 3 cells in parallel.
Each cell group has a capacity of 42.8 Ah (16000/355) and each cell has a capacity of 14.3 Ah (42.8/3).
The max power demand from the battery is 111 kW. So the burst current demand from each cell group is 312 A which is 104 A for each cell.

So we can see that the stress on the two batteries is similar: the individual cell for the Prius is 50% larger in Ah capacity (and probably same amount physically) but has to deliver 44% more burst current (or 77% more in case of 38 kW).

 

i can deal with the 12v like my current prius, but i guess i'll have to see toyota's recommendations for long term storage. we hope to be in florida most winters for 2-3 months and the pip will sit in the semi-heated garage at around 40-50 degrees. it's never been a problem for the prius as far as i can tell, but of course, nimh batteries.

 

You are right. I forgot about this possibility. For me, though, this will never happen. I have driven over 450K miles in my life and never run out of gas.

(So it isn't completely idiot proof, but it requires someone to not use common sense).

 

Exactly the same as with the Tesla Roadster! It requires gross negligence to run the battery all the way down.

 

I'd have to agree with sxotti on this. 40 - 60% would be the sweet spot although the lower the better. Of course if you have the advanced version doesn't it come with an app to monitor the battery? I would be experimenting when you get it if I had the advanced version to do just this. Of course I may try my own experimenting even though I am getting the base version.

 

We have a winner ! ! !

ergo, Barney Fief saying, 'move on, nothing to see here'. But, something tells me I might as well admit it ... the hand wringing will continue.

 

i also, am getting the base version. i wonder if you could hook up a timer that would come on once a week for an hour or whatever it would take to keep that low state of charge?

 

^ Wouldn't really help, since what you'd need is something that would turn the charging on and off according to how much charge is on the battery.

But I'll bet Toyota has already thought of this, and there will be something about it in the owner's manual. Note that the PiP will never charge its battery to 100%. It might be as simple as that it can be safely stored for extended periods at the maximum charge that the computer permits.

 

It will be safe to do so. However it will also likely result in a slightly greater rate of capacity loss. Remember each individual decision, each minute you drive at 62 mph (or whatever the max EV speed is) will only have a tiny effect. They will add up over time of course, but leaving the vehicle charged when you go on vacation isn't going to kill the battery or anything terrible. It might results in a decrease in capacity that is like 0.002% more than leaving it at a 50% state of charge. For example the leaf manual says not to charge it to 100% every day if you do not need to. That would be way worse than charging it to 100% and then leaving it for a month.

Letting them discharge past the lower voltage limit and over charging can be very damaging, but otherwise we are talking just a little change from one individual event.

 

I don't see 100Ah vs 150ah be the same stress on the cells, if they were comparable cells. But as I think sxotty said, other configuration differences (cells vers pouches, cooling etc) may be more of an issue and not all of that is known.

The other, maybe more important question, is how often they will be at near maximal stress. Since the Prius will kick in the engine at 62 and on hard acceleration, if the max stress is driving at say 60, then it may be very frequent for the Prius PHV. The max demand is 31 (or maybe 38) kw which can occur commonly and for sustained periods of time. Its a partial pedal press to get there and people trying to drive all EV to work could easily put on 8-10 miles of high stress. .

The max demand Volt, however, at 111kw, is very agressive acceleration. Even at 101mph, unless you are going uphill, you are not going to be using that much power for sustained travel. (I was not all the way down on the pedal and was accelerating when I hit the 101 limit, I believe it still had room to draw more).

At the same speed of say 60mph, if the cars were equally efficient, the stress on the Volt's battery would be more like 35Ah per cell vs 150ah in the Prius. Even if one were to say the prius is 50% more efficient, it would be 50Ah vs 150ah.


Because we don't enough about the actual cells its hard to say a lot about how they will each handle the stress. On the plus side Toyota has been good about its design and while people worried about the NiMH batteries, that turned out to be unfounded. I hope/expect they Prius PHV batteries will be good for a long time. (And this whole analysis/discussion makes me feel much better about the Volt's batteries.. since I want to keep mine forever

 

This is just speculation unless you know the actual characteristics of the batteries. And you are assuming that Toyota will not have taken these issues into account, and that you can find a better solution than Toyota's engineers did.

Remember that the Leaf, which you gave as an example, allows owners to charge to 100% SoC. The PiP will not allow that. Next, you don't know (nor do I) the ideal SoC for long-term storage. Then, assuming you've made your own guess of the best level and believe your guess is better than Toyota's, you'd have to build a mechanism that would detect the battery's actual SoC and turn the power to the charger on and off accordingly.

And all for, what? To extend the life of the battery by maybe 1 or 2 percent? And the battery "life" is just when it reaches 80% of its original capacity. So maybe for all that effort it reaches 80% of capacity a month or two later.

Or maybe you guess wrong and your scheme is less than ideal and your battery reaches 80% of capacity a few months earlier.

Nissan has definitely given owners the option to wear out their batteries sooner in order to get more daily range, and GM is IMO not very trustworthy. But I'd trust Toyota to get it right. I'd say the best way to extend the service life of the battery would be to read and follow the recommendations in the owner's manual.

 

Daniel talks a lot about his Tesla now, but he is one of the most experienced Toyota Prius owners on this board.

This is good advice.