Battery Degradation - Capacity and Efficiency

[ChrisJConklin]

A whole lot of interesting discussion on this forum regarding the dealer stated and expected degradation of the LEAF battery system with time. I see a general expectation of a degradation to 80% of initially rated capacity in 5 years, further degrading to 70% after 10 years...though certainly driving style and charging protocols amongst other things mean "your results may vary"...
I'm interested in the issue because I am collecting data on my 3 month old LEAF electrical power consumption (L2 home charging system, with individual TED charging circuit monitoring), with the intent of upgrading my home solar PV system to zero out my net energy use again (the LEAF has pushed us from a net generator to a net consumer....). With the price of power in Hawaii now $ 0.35 per KwH for a typical residential user on a 7% annual inflation curve, getting it right before doing the upgrade is important.
My key question, for those battery experts out there, is whether I can expect the efficiency of the battery to degrade over the years of operation - or just the battery capacity. Since our short commute doesn't make the expected degradation in battery capacity (e.g. range on a single charge) a major issue, what about efficiency? Can I expect the wall-to-wheels efficiency (currently about 4.00 M/KwH after some short use data) to degrade with time, even at a lesser rate? Some say no, but it seems that as a battery's capacity to carry power gets reduced over time, it's ability to receive that power and to convert it to mechanical power would be less than when it is brand new. Would the CARWINGs reported ratio (which is of course higher than the "wall to wheels" number because of losses with charging...), degrade, reflecting the ability of the battery convert electrical energy to mechanical power? Even if it didn't, would the Walls to Wheels efficiency degrade, reflected that the battery pack is less efficient in accepting a charge, as well as less capable of getting the full KwH of charge in comparison to a new one?
My gut tells me that I would see some degradation in the WtoW efficiency, but that it would be very unlikely to be as a high as a 30% less over 10 years. But that is not based on any real battery engineering based assessment......
thanks,
chris conklin
Honolulu

 

[HighDesertDriver]

I don't think you will see a drop in "efficiency". Rather, you will see a gradual reduction in the capacity of the battery and perhaps a reduction in the maximum amperage the battery is capable of supplying. The Rav-4EV drivers posting to their sites report (Ni-MH chemistry) gradual loss of range and reduced acceleration, but not reduced efficiency. In an ICE vehicle it would be the equivalent of a smaller gas tank and smaller supply line to the engine, but the same MPG.

How much PV you add to your home will depend on whether you are NEM or TOU and your flexibility in selecting charging times. Search this site for "PV", "solar", and "SCE" for a number of good threads on solar electric. Good luck. You have started at a good place.

 

[SanDust]

You should be fine. Recharging and discharging are the same reactions run in different directions. Most of the losses are conversion losses and those should stay the same.

Your PV panels will, however, degrade.

 

[RegGuheert]

Some say no, but it seems that as a battery's capacity to carry power gets reduced over time, it's ability to receive that power and to convert it to mechanical power would be less than when it is brand new.

Hi Chris,

There should be a very small effect on efficiency. The fact is that the resistance of the LEAF battery pack is VERY low, on the order of 90 mohm. So there is little resistive loss to start with. You can estimate that this resistance may double or slightly more over the life of the car. So the resistive losses will double or so.

Now there are other losses involved in charging, particularly as you approach 100% of charge. If you only charge to 80%, you won't see as much of the other losses. I'm sorry, but I don't know how much these other losses move around with aging. One effect I do think you will see with age is that the cells will become more and more different from each other, which may require more balancing. This may result in slightly more power loss as the shunts become more active.

But to put a number on all of it would be a challenge. I doubt the efficiency will drop much, personally.

 

[nater]

As my RAV4 EV began to lose capacity, it used more electricity to go the same distance. It was the last thing on my mind, however.

The issue had to do with impedance going up in the battery pack.

Nate

 

[ChrisJConklin]

Thanks all, for the informed perspective on the efficiency loss issue. Seems like the degradation in my PV system output, known to be 0.5-1.0% annually, will predominate any loss in LEAF battery efficiency in sizing the solar array for the out-years. So I'll be conservative on that and won't be concerned with a minimal efficiency loss. Thanks again, Chris Conklin Honolulu

 

[adric22]

The Rav-4EV drivers posting to their sites report (Ni-MH chemistry) gradual loss of range and reduced acceleration,

I had not heard this. I didn't realize there was a chance of lost acceleration. After all, I figured the Leaf battery can probably supply a lot more amperage than the inverter and motor can handle already. So I figured there was no way you'd ever lose that. Can anyone speculate what might happen to the Leaf towards the end of its battery life?

Truth be, assuming I live as close to work as I do now, I'd probably keep drive the Leaf as-is even if the capacity were down to 25 miles. After all, I had a home-built EV that could barely do 20 miles and drove that for a year.

 

[HighDesertDriver]

The Rav-4EV drivers posting to their sites report (Ni-MH chemistry) gradual loss of range and reduced acceleration,

I had not heard this. I didn't realize there was a chance of lost acceleration. After all, I figured the Leaf battery can probably supply a lot more amperage than the inverter and motor can handle already. So I figured there was no way you'd ever lose that. Can anyone speculate what might happen to the Leaf towards the end of its battery life"

The RAV cell-to-cell connectors are not entirely protected from the atmosphere and slowly accumulate grime and slight corrosion. Together with some of the individual pack cell resistances increasing, acceleration decreased while top speeds remained about the same. The sole-source Toyota vendor, BatteryMD in the Sacramento area, rejuvinates the battery packs by repeated deep, controlled charge/discharge cycles (and replacing truly bad cells) and a time-intensive manual cleaning of all the connectors and adjoining surfaces. . . using toothbrushes. The connection scheme and packaging in the LEAF are not comparable, so it is unclear if this will eventually become a problem we will see. Do an Internet search on "evchargernews" to dig deeper.

 

[surfingslovak]

Can anyone speculate what might happen to the Leaf towards the end of its battery life?

Truth be, assuming I live as close to work as I do now, I'd probably keep drive the Leaf as-is even if the capacity were down to 25 miles. After all, I had a home-built EV that could barely do 20 miles and drove that for a year.

If you look at the capacity bars and their spacing, once you have lost all of them, your battery will store only 4 kWh. It will essentially at the low battery warning or below all the time. Assuming that Nissan has tested this, they must think that it's worth tracking degradation this far and that the vehicle will be still usable then.

My best guess is that the internal resistance will have at least doubled at that point. Since the battery apparently accounts for about one fifth or maybe one quarter of the overall energy loss in the Leaf, this will impact energy economy and especially wall-to-wheels efficiency, but it won't be dramatic.

You can see that Nissan is anticipating higher internal resistance when you look at the temperature gauge table in the shop manual. All thresholds are adjusted for degradation, which means that they anticipate more heat loss in cars with an older battery. I believe that the magnitude of this adjustment gives an idea of how much change in internal resistance they expect to see.

I don't know about you, but 4 to 5 degress temperature rise on the pack level seems pretty high to me for a battery that has degraded 'only' 20%.


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[donald]

Hi. I was just wondering ... has anyone yet got any 'compelling evidence' from their own actual measurements of wall-wheel efficiency, for a degraded battery versus the same data when their battery was 'fresh'?

 

[WetEV]

A whole lot of interesting discussion on this forum regarding the dealer stated and expected degradation of the LEAF battery system with time. I see a general expectation of a degradation to 80% of initially rated capacity in 5 years, further degrading to 70% after 10 years...though certainly driving style and charging protocols amongst other things mean "your results may vary"...

Honolulu is warmer than average, and the battery will degrade faster according to the wiki's battery loss model.

http://www.mynissanleaf.com/wiki/index.php?title=Battery_Capacity_Loss" onclick="window.open(this.href);return false;

Honolulu 71% at 5 years, 53% at 10 years.