Charging habit: wait til empty vs charge at every opportunity

[hmmwv]

I have a 18 mile one way commute to work and there are free L2 chargers near my office. I use the long battery life mode so I only charge to 80%. I'm trying to find whether I should charge it at work when the battery is down to 55%, or wait until I get home and charge it .

Which way is better for the battery's long term life? Two charges per day (at work and at home) of roughly 25% each, or a single 50% charge every night? All charges are done with L2 in relatively similar environment.

 

[Mottyski82]

My vote is for the 50% charge @ home. I think people are not putting enough weight on capacity loss due to cycling.

 

[Levenkay]

If you're comfortable with the range remaining at the end of your roundtrip commute, I'd say go with the home charging option. And not because I think there's any appreciable "cycle loss"; but because the battery will spend more of its time at a low state of charge that way. I buy into Prof. Dahn's presentation on the degradation mechanisms in Li-ion cells, and believe that in general, you should always try to minimize the product of State-of-charge and temperature (though I have no idea of how to trade them off proportionally).

 

[keydiver]

We don't really know enough yet about the new 2015 battery chemistry, but all battery life charts I have seen seem to indicate that you get much, much more life out of a battery with more shallow cycles than deep cycling. Not that this entirely fits in your case, as you aren't really "deep cycling" the battery, but my gut feeling is that you will get more than double the number of 80-to-55% cycles as you will get 80-to-30% cycles. I think this is part of the reason why I have yet to lose my 3rd bar at 36,000 miles, when another Leaf owner in the same town has already lost all 4 bars and had his battery replaced by 33,000 miles. He was completely cycling the battery every day, whereas I was often charging 50-to-80% twice a day. But, in his case I also need to factor in that he was driving fast on Route 95, which also heated his battery up more than me driving at 55 mph.
Another factor may be the heat generated during charging. By breaking up the charging into 2 separate cycles, it may give the battery time to cool slightly in between, which would definitely help the older chemistry, but may not make much difference on a Lizard battery.

 

[hmmwv]

So what do you guys consider "deep cycling?" From VLBW to 80% or full? I think for temperature's sake I'm gonna try to charge it at home in early morning hours (2-4 AM) so I get a 80% charge right before I go to work. This way the garage will be the coolest and minimize the battery's temperature. Even though when I charge at work it's in the shade as well the temperature is still in the low 80s so significantly higher than the high 60s low 70s during early morning hours.

 

[LeftieBiker]

A deep cycle for a lithium pack is usually considered to be from less than 20-25% back to 100%. The topic title is misleading, as neither option it proposes is very good. Charging from, say, 35% to 80% should work just fine, as long as the pack is charged to 100% right before use occasionally, to equalize it.

 

[hmmwv]

I guess a better way to ask is: Is it better to have many shallow cycles or few deep cycles?

 

[xmetal]

I got a 2015 on a lease, so to be honest I am not terribly concerned with maintaining a perfect battery and keeping it optimal for 10 years. That said, in our usage it seems that running it all throughout the day and then charging once at night seems to make the most sense and works fine. We do not need the car for more than 50 miles on an average day so letting it get down to 30-some% and charging once is reasonable and easy to manage.

 

[GetOffYourGas]

I guess a better way to ask is: Is it better to have many shallow cycles or few deep cycles?

By most observations, the battery temperature seems to be a bigger killer than cycle depth, especially given your use pattern; your "deep" cycle isn't really a full cycle. That would be from below VLBW to 100%.

For this reason, I would recommend charging only overnight. This allows time for the battery to cool off, and you are charging when the ambient temperature is coolest.

 

[hmmwv]

Yeah that's what I'm gonna do, even though it's nice to be able to charge for free at work it's really not worth the risk of potentially shortening the battery's lifespan.

 

[mjblazin]

It could vary by weather. In summer, charging at night makes sense. If in winter or fall, and temps are in 50's or less, the negative impact of charging at work could be much less or none. You would not want to start charging immediately after arrival and not charge right up to time you leave for return home. If it is really cold and pre-heating, then the pause before return would matter less. That behavior also fits the reduced range you get in colder weather. You would like extra margin on trip home.

As long as you understand the important driver, battery temperature, you can tweak your process to need.

 

[Valdemar]

Heck, if I lived in Seattle and had free juice at work my home plug would get very little use, and I wouldn't lose any sleep over losing capacity from 20% to 80% charges at work.

 

[Yogi62]

If you can plug it in at work especially for free, get back to 80%. If you never hit LBW and rarely need to charge to 100% your battery should last a long time. 26K miles and 12 bars...91% SOH so far... Also free KW is free KW. Here in Boston that is a savings of 23 cents per KW from Jan to June. Most days I will charge it at work and very little at home unless it is below 50%.

 

[keydiver]

This is typical of the DoD (Depth of Discharge) cycling chart I was referring to. As you can see, the shallower the discharges, the more you can cycle the battery before the same capacity loss. For example, if you use 20% of the Leaf battery capacity to get to work, you will get 2.2X the cycles out of it by charging at work, instead of waiting until 40% discharge at home. That's an extra 10% lifespan. But, as I said in my other post, we don't know enough yet about the 2015 Lizard chemistry to know what is going to cause the most significant degradation of capacity. DoD may be a very, very minor part, or it may be the major factor, we just don't know. Personally, if I had a 2015, I just wouldn't worry myself about it.

 

[hmmwv]

So it appears that most people think if temperature is not an issue, then in terms of battery life many shallow cycles and few deep cycles are a wash. I'm not really concerned about electricity cost but more about preserving the battery. The temperature in Seattle has been crazily high for a few weeks now, since most houses, including mine, don't have AC, at night my garage stays between 80 and 85F. At that temperature I might as well charge at work when I can as the charger is under the shade and even during the day the temp is not much higher than my garage's.

Here in Boston that is a savings of 23 cents per KW from Jan to June.

The rate is $0.23/KW in Boston during the first half of the year? That seems to be pretty high.