Another Battery Degradation Thread

I'm down to 8.4 on a full charge. Temps have been in the 70's. That's not so good. suggestions? reboot (how?)

 

That value displayed is nothing but an estimate. The "reboot" is only cosmetic, resetting the value back to default rather than accurately reflecting a number based on your specific recent driving.

In other words, it doesn't mean anything.

You're still getting the benefit from plug-supplied electricity, as the MPG average should clearly reflect.

 

Try the experiment - do a full charge, check odometer before the test drive, note when the EV runs out and the engine starts, check odometer and see what your actual EV range is relative to your indicated 8.4 starting miles.

 

And remember, the faster you go, the less EV range.

 

shouldn't andy be chiming in here?

 

Can't be anywhere near as high. Since you have the base model as I do, go out to your car while it is plugged in AND recharging. When you turn it on normally, you will see the computer relaying how many kWh is being used for recharging and the estimated time until it is fully recharged. You can't go anywhere obviously because the car will refuse to be put into drive mode while recharging. But as I just tried it, the computer says recharging at a rate .9kWh and recharge time left is 1.0 hours.

 

i find the car's eta to be pessimistic.

 

I haven't had the chance to observe temps while charging yet, but I have collected enough driving data to see a pattern with HV and EV temps.

HV peaks just under 95°F.

EV peaks just over 100°F.

That's much smaller of a difference than I expected.

 

I got that data today. The Prius sat out in the parking lot from morning through lunch.

The surface temperature was 81°F. The air-intake was 90.7°F. The bank temperatures for the battery-pack started at 90.9°F, 91.7°F, 90.3°F

1 hour 10 seconds later, 2.235 kWh of power had been used for recharging and the current rate was 2.257 kW.

At that moment, the air-intake was 98.7°F and the bank temperatures for the battery-pack were 94.6°F, 94.8°F, 94.8°F

 

Here's what we know:

* NCA is generally more tolerant of high temperatures than the LMO-based chemistries in the LEAF and Volt.

* all modern Lithium ion chemistries degrade faster at higher temperatures. The difference between NCA and LMO is one of degree (so to speak).

* all modern Lithium ion chemistries are happiest -- good energy capacity and power with modest heat degradation -- at around 70F. A temperature of 95-105F is high enough to make a significant difference in battery degradation rates.

* battery heat-based degradation is worse during charging/discharging than during stable storage. But, just sitting around for long periods of time at higher temperatures contributes substantially to loss of battery capacity over time.

* battery heat-based degradation is quite a bit worse at high SOC than it is at middle or low SOC.

Taken together, the generally optimal policy is to charge immediately before use so the battery spends the least amount of time sitting around at higher temperatures at a higher state of charge.

95 or 100F is not comfy for a PiP battery but, like humans, the PiP battery can usually tolerate it. Park in the shade when possible.

 

Tell us what "significant" means. We haven't seen any degradation at all in 2 years, nor have temperatures that low ever been stated as an issue for lithium. So, the question is reasonable... as is asking about "not comfy" and "tolerate". They are all quite vague.

Also, keep in mind that SOC influences the aging process too. Only charging to 85% reduces the exposure when dealing with heat.

 

Ha ha, I went back out 20 minutes later and it was done recharging.

 

Battery aging can be characterized in different ways including loss in energy capacity (EV range) and ability to output power.

I have seen a nice table of capacity loss by temperature and SOC which might take a day or two to go find from a book I have.

At least a couple of studies I have read specifically point out the solar load in the Prius from having the battery case right under the large and nearly horizontal hatchback window. By contrast, the Volt battery is mounted within a deep steel tunnel running under the car and added insulation protects it from cabin temperatures due to solar exposure.

These graph temperatures are in Celsius so:

20C = 68F and every additional 5C is an added 9F

25C = 77F
30C = 86F
35C = 95F
40C = 104F

 

As far as I can see, the only actual data points in there are the circles on the 20 degree line on the last slide. Exactly what the storage pattern they are recording is is unclear (permanently charged? To 100%?). Taking them at face value, though, there is good and bad news. The bad news is that the nil degradation we are reporting does not mean we won't start to get some in the next few years. The good news is that, even if the model is right, on John's actual temps, battery degradation is only 10% after five years, and on a shallow trajectory.

 

I believe the reference to 3.7v on that slide implies that the cells are being stored at about 50% SOC. The graph shows projected data based on a mathematical model and the circles show actual empirical test data proving the model.

The real overall takeaway here is that temperatures in the range of 70F vs 95F actually do have a significant impact on battery aging.

It's worth it to park in the shade when possible (especially with a full battery). Charging right before use, when possible, is good because may allow a cool down period from a previous drive and it minimizes the time that the car is sitting around with a full battery.

Here's a chart showing different strategies for how and when to recharge in order to minimize battery degradation. The "late 6.6 kW charge" (2.3 kW for PiP) is about the best strategy and the worst is to just plug in and immediately charge because it leaves the battery with a high SOC for the longest period of time.

 

I always park facing the sun, having the front windshield taking the brunt of the shine. I also always leave the shade in the cargo area extended to fully cover the hatch area. I leave the Prius in the shade until right before plugging in too. The sun here in Minnesota is less direct than in Arizona as well. So basically, solar exposure in back can be minimized... a point often not taken into account.

Yesterday's data collection was an intentional act of leaving the Prius out in the sun and parking with the hatch facing the sun, though the cargo shade was extended. The high temperature during recharging was only 35°C (95°F), a value low on the charts provided. How many owners will recharge during the hottest time of the day anyway?

And I do take issue with those charts. That supposed "significant impact" glosses over the fact that temperatures are not hot year-round. In fact, here, we only see the potential for temperatures above 25°C (77°F) just 4 months out of the year. That means the battery stays very comfortable for the other 8 months.

There's also the fact of SOC level not being addressed. Those charts, from 2006 & 2008, don't make any mention of that. Back then, 100% capacity was the norm. The battery was always recharged entirely. That was years ago. We've learned since then that avoiding 100% is a good practice for battery longevity. Prius stops at 85% for that very reason.

 

I would like to study the source of this graph in post #117. It shows battery "actual lifetime" less than the warranty (CARB state NY) if you use optimal technique or not. I would think that we would have heard the owner outcry if the graph was accurate.

 

I think everybody is comparing single points in time to averages. In all honesty, as john1701a pointed out, you should use these graphs with average temperatures. For instance where I live the average daily temperature for July is 27.2 C and that is the average high. The average low is 15 C which the battery pack also sees. Not something that would differentiate much from the Volt's reliability. In fact I would go so far as to say not keeping the battery pack at a certain temperature may increase battery reliability. Particularly if based on those graphs.


BTW June dailies are 25 C/12.2 C and August dailies are 26.1 C/14.4 C.