Prolonging battery life, temperature, state-of-charge

[abasile]

Does anyone know what exactly constitutes a "cycle" for the battery?

Is it a full discharge - charge, i.e. emptying the battery and then charging back to full? Or do I use up a cycle every time I plug in to charge?

Assuming we have something about ~ 1000 cycles for the battery, does that mean that it will approximately deliver
over its lifetime 21 kWh x 1000 = 21 MWh or (e.g. with 4 Mi/kWh) ~ 84000 Miles of range?

In this context, I believe a "cycle" is a full charge/discharge cycle. However, with lithium-ion batteries, shallower charge cycles generally result in significantly longer lifetimes. See http://www.mynissanleaf.com/viewtopic.php?f=27&t=5331&p=125835#p125761" onclick="window.open(this.href);return false;

Or , if I recharge daily, even for small amounts (I rarely use more than 1-2 bars for my commute), it will degrade after 1000 days ?

That can't be. Every time you utilize regenerative braking, the battery is charged. The battery pack has been engineered to take lots of small charges.

 

[klapauzius]

That can't be. Every time you utilize regenerative braking, the battery is charged. The battery pack has been engineered to take lots of small charges.

Yes, that makes sense.
So I guess it is reasonable to expect ~ 80000-100 000 Miles from the pack? Obviously battery
degradation will be a continuous process, but it would be reasonable to expect some useful range per charge (maybe 40-50 miles) after that?
Does anyone know if the self-discharge rate also goes up with time? Half the battery would still be useful if it could hold a charge and deliver consistently.

 

[aqn]

Does anyone know what exactly constitutes a "cycle" for the battery?

According to my brother, an EE working on the battery for a popular touch pad computer at a major electronics manufacturer, for Li-Ion cells, one "cycle" consists of one cumulative discharge-recharge of 100% of capacity. For example, if I use 25% of capacity, then recharge back up to 100%, and do that four times, that constitute one cycle.

 

[abasile]

So I guess it is reasonable to expect ~ 80000-100 000 Miles from the pack?

Based on the existence of an 8 year / 100K mile warranty on the pack, that seems like a reasonable minimum to expect. I'm personally hoping for 10 years and more than 100,000 miles of use. With shallow cycling and gentle driving, some folks might be able to do much, much better than that.

Obviously battery degradation will be a continuous process, but it would be reasonable to expect some useful range per charge (maybe 40-50 miles) after that? Does anyone know if the self-discharge rate also goes up with time? Half the battery would still be useful if it could hold a charge and deliver consistently.

We could use a real battery expert on this thread. However, Nissan has stated that they expect many LEAF batteries will have second lives performing some sort of stationary energy storage, perhaps to store wind energy generated at night. Thus, it seems reasonable to expect that old packs will still possess the ability to hold useful quantities of energy. On the other hand, over time, we do expect that internal resistance will increase and ability to deliver power will decrease. So, a fairly old LEAF with its original pack might indeed be good for 40-50 miles of range, albeit with compromised acceleration and lower "fuel" efficiency.

 

[klapauzius]

I can see an old Leaf battery indeed be useful to buffer solar or wind, especially since a large capacity is required to make these a viable replacement for 24/7 fossile/nuclear power.
In the meantime I assume when the current battery stops beeing useful in a car, we will have better capacity and price ready to power the Leaf for another 10 years or so. Given that the mechanical parts of the car are so simple (compared to an ICE) and the electrical motor should last forever (or is probably rather cheap to replace), this could be a car that could easily last 20+ years.

 

[highcountryrider]

Crap. :x

Which reminds me of a question. Do we know for sure that cell balancing
only happens at 100% charge? Or is that the only possibility?

When I asked Nissan this directly, I was told the answer was "proprietary".

 

[Herm]

Crap. :x

We dont really need to know everything.. if we need full range we charge to 100% and the cells get balanced automatically (we think).. no need for that otherwise.

 

[highcountryrider]

But, but, inquiring minds want to know! :lol:

 

[surfingslovak]

So I guess it is reasonable to expect ~ 80000-100 000 Miles from the pack?

Based on the existence of an 8 year / 100K mile warranty on the pack, that seems like a reasonable minimum to expect. I'm personally hoping for 10 years and more than 100,000 miles of use. With shallow cycling and gentle driving, some folks might be able to do much, much better than that.

Nissan has been pretty quiet on the battery front, but GM reportedly simulated 200K miles in their lab, which was mentioned on page 7 of this thread. The Volt is using a comparable pack chemistry and design, which means that 100K miles should be readily achievable. Additionally, please have a look at the RAV4 EV 100k club. Since the lithium-ion battery technology the Leaf is using was designed specifically to supplant NiMH batteries, it's reasonable to expect that we will see similar performance and longevity. Another real-world report worth noting is Tom Mologhaney's 65K milestone, which he celebrated last month. Tom did not see any tangible range reduction after 27 months of ownership. His MINI's battery pack has no thermal management and is likely based on NMC Molicels manufactured in mid 2008, which is reportedly the chemistry Nissan will be moving towards in 2015.

Obviously battery degradation will be a continuous process, but it would be reasonable to expect some useful range per charge (maybe 40-50 miles) after that? Does anyone know if the self-discharge rate also goes up with time? Half the battery could still be useful if it could hold a charge and deliver consistently.

Based on the data I've seen so far, internal resistance will rise with battery age, but it should be less of an issue than it was with NiMH. This would imply, that a Leaf with an old pack would potentially have a significantly reduced range, and it might not accelerate as quickly as a new car, but it would still be eminently usable. I'm not sure how the self-discharge rate would change over time, but I don't think that it will be a problem. There will very likely be individual module failures, and once the pack is out of warranty, the owner will have to weigh the cost of module replacement or pack reconditioning against a battery upgrade. Hopefully, battery technology will continue to improve and independent shops will spring up, which should make the cost of such repairs more palatable.

 

[AndyH]

Crap. :x

Which reminds me of a question. Do we know for sure that cell balancing
only happens at 100% charge? Or is that the only possibility?

When I asked Nissan this directly, I was told the answer was "proprietary".

The good news is that is doesn't really matter.

It's possible - and the hardware/software has existed for a number of years - to balance lithium cells during charge and discharge and at any overall state of charge.

But balancing isn't a real big issue with a pack used the way we're using EV batteries due to the relatively slow charge and discharge rate and charge limits.

The folks that need to pay more attention to balancing are the radio control modelers and folks that design power tool batteries (including phones and computers) - because the significantly smaller capacity and much higher charge and discharge currents amplify the effects of cell imbalance (or poor cell matching - capacity and/or internal resistance).

What I've found from my on-road experience is that the only time I 'need' to fully charge and balance my battery is when I expect a maximum-range drive and therefore plan to use every last 'drop' of energy from the pack. Normal day to day driving? No need to even think about it - just plug it in and let it work as designed.