So it's been like almost two years since the last blast about batteries on here. Have we learned anything new?
Maybe not much new, but there's been enough learning (and failures) out there to get a little more nuanced impression regarding what's working and what is not. The greatest depth and variety of information comes (pretty obviously) from the Nissan LEAF universe, since in the U.S. there about a hundred LEAF's running around for every one iMiEV. People will argue that since the battery configuration and chemistry and battery management systems aren't the same we can't make 100% exact comparisons with 'our' car. True, but it's close enough (90-95%?) that we can sure learn stuff from there.
Some people have done 'everything right' and still had their battery packs fail. Hey, there's a lot of cells in each pack. Even with REALLY good quality control you're going to have the occasional outright failure. That doesn't mean you shouldn't treat your pack at least somewhat nicely, just that it won't guarantee zero failures.
No surprise.
Other people appear to abuse their battery pack consistently with no consequences (yet). How can that be? Well, that's probably due to the design engineers not being completely stupid. Their job was to make the pack last through the 8 year 100,000 mile warranty. They know that letting you charge absolutely completely full and then discharge to absolutely completely empty will result in a pack that only lasts a couple hundred cycles at best. So to achieve their goal they have made it so you can't really charge up all the way (even though it says 'full') or discharge all the way. Basically you're allowed to run your pack from about 95% full down to about 10% full. Actually it's about 15-20% when the Turtle kicks in and you only get 80% full if you're on fast charge, but you get the idea.
"So how come it's called a 16 KWh (kilo watt hour) pack if I only get to use 13-14 KWh out of it?" 16 KWh is the minimum that qualifies you for the full federal tax credit. Yes it's really that simple.
Even if the engineering whiz kids have designed the whole system so that making it to 100K miles or 8 years is highly probable, we gotta ask ourselves "How do I get to 12 years or 200K Miles?"
Let's back up for a minute:
If the cells that make up our battery pack are 'rated' for say 1000 cycles (I don't know what the actual number is, 1000? 2000? this is just an example) even that is probably under some sort of 'ideal' conditions that have no connection with reality. This doesn't matter much. Why not? Because we already know the battery is good for hundreds of thousands of 'cycles.' It just depends on how you define cycle. While driving, every time you let off the 'go' pedal or step lightly on the brake the regenerative braking system is charging the battery. Then you start accelerating again, you're discharging. This happens a LOT of times per day for most of us. It's a real world example of something battery engineers have known for a long time: The deeper the discharge, the fewer cycles you'll get. Fortunately much of the 'life reduction' occurs near the limits of fully charged or fully discharged. ...and the engineers already won't let us play too close to those limits, though it turns out we can be nicer than that.
Another thing we can look at is how much current (power) we can pull out of the batteries all at once. We look at this in relation to the total battery pack capacity, around 50 Amp/Hours in our case. This is referred to as one 'C'. For this type of cell it's generally agreed that you can go up to a discharge rate of 3C (three times the 'one hour rate') without causing any damage to the cells. You may have noticed the charge-discharge 'power' meter on the dash. It has three divisions on the power (discharge, right-most) side of the meter roughly corresponding to 50, 100 and 150 Amps. So the system is built so you can't go much beyond 150 Amps (3C) regardless. That's also why the acceleration ramps up somewhat slowly when you tromp on the 'go' pedal. This keeps the peak current draw down to a non-damaging level and also keeps the tires from disappearing in a cloud of smoke ...and you stay on the road. You'll also find the system automatically reduces the current when you reach 'Turtle' mode (near-empty), and that's a good thing. We can, however, be a Nicer Turtle and avoid going above the 100Amp line when we're down below four bars on the battery gauge.
Charging: The amount of charge, how fast, temperature and how long it sits fully charged are all important variables in this equation. Generally speaking a relatively slow rate of charge (8-15 Amps) at a relatively mild temperature (40-85 Deg.F) is kindest to the battery. Since the built in charger is limited to 3300 Watts (about 13A at 240V in.) anything it can do is fine. All we really need to do is control temperature and amount of charge.
Over the last couple years it's become pretty obvious that high charge (or discharge) rates at very high (over 100) or very low (below 20 Deg.F) temps is not so good for the battery. What is perhaps more surprising is that it's also quite bad to leave the car fully charged when it's going to sit at relatively high temperatures for any extended length of time (over a couple hours or on a regular basis) People have managed to knock 20% or more off their battery life (also referred to as 'degradation') in a year or so of leaving their LEAF out fully charged or just completing full charge in areas of Arizona that have high daily temps. (110F or above.) "So what should I do in that situation?" Don't charge above 80% (14 bars) if it's real hot out or you're going to let it heat-soak in a very hot parking lot for a long period. "But I need all of that full charge to get home with the AC on!"
Sounds like you bought the wrong car.
Granted the iMiEV has a certain amount of thermal management (basically running the AC during charge to keep the battery cooler) on models equipped with Fast Charging [CHAdeMO, some 2012's and all 2014 and 2016's.] How effective is it? We don't really know, and it doesn't fix the hot-soak after charging problem regardless. This is something it's at least possible to track, see the CaniOn post.
Another thing that's not completely obvious is that the folks who only drive a short distance and then plug it right in every day and let it charge fully are probably not doing their battery a favor... as you might have guessed from the above. Even in cooler climates having it basically sit all the time at full charge probably won't help the battery life much and may actually shorten the life somewhat. If they were charging to only 80% (14 bars) all the time then there wouldn't be a problem. [The 2012 and 2013 LEAF's actually have a setting that allows that to be the default behavior. This got dropped in 2014 due to some stupidity at the EPA.] No problem though, iMiEV's come with a remote control.
"So to sum up, how do you plan to keep your iMiEV running to 200K miles?"
I don't think I will reach 200K. At 9-10K per year that would be well over 20 years. I strongly suspect it will be done in by simple chemical aging, partially due to unavoidable thermal cycling, before then.
Having it sit in an air-conditioned/heated garage kinda defeats the whole purpose, no?
That said, I'll try to get it to last as long as possible:
Mostly I'm a 'Nicer Turtle.'
I don't run it hard when the battery is low.
I only rarely let it get below 4-5 bars on the battery gauge.
I rarely let it charge over 14 bars (out of 16)
About once a month (or two) I run it down to 2 bars and then give it a full charge. "But you said... uh, why?"
Running it down to Two, where the display starts flashing, causes the battery gauge to reset it's 'expectations' when it gets a full charge immediately after. Granted the factory folks only want you to do that once a year or so, but it seems to happen, just in real life every month or two. ...and you DO want to give it a full charge occasionally (I let it go to 'full' every week or two) since that's when the cell-to-cell equalization system kicks in. See the old battery post for details on equalization.
Newbies:
In fact if I had a new iMiEV that had been sitting on the dealer's lot for awhile, I'd tend to make the first ten or so charges 'all-the-way-to-full' one's (just however much you happen to use it every day then let it charge full) and then have ONE run-it-down-to-two and then full charge during the first couple weeks of ownership just to make sure the battery cells were equalized and the system was properly reset.
Then shift over to staying in the 4-14 bar range as much as possible.
"How do I keep the charge from going over 14 bars?" That's what the little remote control is for. With my particular car and my charger I know I need a bit over 6 hours to go from 4-5 bars to 14. I just set the start and end times to get that result. You'll learn all this over the first couple weeks.
And BTW: Don't freak out, unless you're running it deep into turtle mode every day for weeks, you're unlikely to do serious lasting damage. They did a pretty good job on the design.
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