I drive 5-8% of the battery per day. WHICH 5-8% should I use?

[nwdiver]

This chart says absolutely nothing about what it means for cycle life if you go from, say, 75% to 25% as it DoD charts always assume starting 100% SOC (state of charge).

As long as you're avoiding the extremes large cycles will always cause more degradation than smaller cycles. Sure... 100% - 90% - 100% might be worse than 80% - 65 - 80% but 80 - 50 - 80 will be worse than 80 - 65 - 80.

The point is that frequent charging is the best thing for your battery.

 

[ajdelange]

So let's see what the chart actually says. If I discharge 20% I will get 20*3E4n = 60E4 percent out of the battery whereas if I discharge 50% I will get 50*1E4 = 50E4. A difference, yes, but not enough of one to get me to change my practice. Which practice, BTW, is not motivated by a desire to prolong battery life so much as it is to reduce the error in estimates of battery capacity which are dramatically improved as one charges over a larger percentage change in SoC.

 

[nwdiver]

So let's see what the chart actually says. If I discharge 20% I will get 20*3E4n = 60E4 percent out of the battery whereas if I discharge 50% I will get 50*1E4 = 50E4. A difference, yes, but not enough of one to get me to change my practice. Which practice, BTW, is not motivated by a desire to prolong battery life so much as it is to reduce the error in estimates of battery capacity which are dramatically improved as one charges over a larger percentage change in SoC.

Calibration is recoverable with a couple large charge cycles. Degradation is not. I drive from NM to WA at least once a year. I typically gain ~5 miles of 'rated range' back as the pack comes back into calibration. The point is that frequent small cycles are better for pack longevity than large cycles.

 

[ajdelange]

Yes, but not appreciably so (assuming your curve representative of the Tesla batteries). And my comments have nothing to do with calibration but with the "dilution of precision" that comes from trying to estimate charge capacity from a 2 - 3% change in SoC.

 

[nwdiver]

Yes, but not appreciably so (assuming your curve representative of the Tesla batteries).

Depends on how extreme. If you're charging once a week 80 - 20 - 80 instead of 80 - 70 - 80 it can be significant.

 

[ajdelange]

Yes, I am sure we can construct scenarios where it can. I did not suggest charging 80/20/80. I suggested 75/40/75. And that's because of DOP. But let's look at 80/20/80: 80*0.5E4 = 40E4 vs 20*3E4 = 60E4. Yes, that's more. I get your point. Again, assuming that curve has some relationship to Telsa's batteries.

 

[nwdiver]

Again, assuming that curve has some relationship to Telsa's batteries.

~ALL batteries display the same rough characteristics. Large cycles always cause more degradation per % vs small cycles.

 

[KenC]

A 35% cycle causes significantly more degradation than 7 5% cycles. Smaller cycles reduce degradation.

That's an interesting chart. I just want to know if I'm understanding it correctly. If we look at the dot in the upper left corner, it's at 30,000 cycles vs 20% dod. So, for the Model 3, that would be ~1.8M miles of driving, and lets look at the 2nd dot in the middle. It's at 10,000 cycles vs 50% dod, so 1.55M miles of driving. And the 3rd dot would be 5000 cycles vs 80% dod for 1.24M miles, and the last dot appears to be 775k miles.

The only thing is the chart kind of lacks context since 775k miles seems like plenty of miles for the life of Li-ion battery.

 

[ajdelange]

~ALL batteries display the same rough characteristics. Large cycles always cause more degradation per % vs small cycles.

Have to have the last word, don't you? I've learned over many years in engineering that it takes a bold man to say "always".

 

[nwdiver]

That's an interesting chart. I just want to know if I'm understanding it correctly. If we look at the dot in the upper left corner, it's at 30,000 cycles vs 20% dod. So, for the Model 3, that would be ~1.8M miles of driving, and lets look at the 2nd dot in the middle. It's at 10,000 cycles vs 50% dod, so 1.55M miles of driving. And the 3rd dot would be 5000 cycles vs 80% dod for 1.24M miles, and the last dot appears to be 775k miles.

The only thing is the chart kind of lacks context since 775k miles seems like plenty of miles for the life of Li-ion battery.

Don't read that much into it... every chemistry is going to be a bit different. I posted that just to show that large cycles cause more degradation per % vs small cycles... that's all. That part is almost universal. You're going to be better off charging everyday instead of once a week.

Have to have the last word, don't you? I've learned over many years in engineering that it takes a bold man to say "always".

That's why I said '~ALL' instead of 'ALL'... but I've never seen a DOD vs life cycle chart where large cycles don't cause more degradation per % vs small cycles...

 

[hugh_jassol]

That's an interesting chart. I just want to know if I'm understanding it correctly. If we look at the dot in the upper left corner, it's at 30,000 cycles vs 20% dod. So, for the Model 3, that would be ~1.8M miles of driving, and lets look at the 2nd dot in the middle. It's at 10,000 cycles vs 50% dod, so 1.55M miles of driving. And the 3rd dot would be 5000 cycles vs 80% dod for 1.24M miles, and the last dot appears to be 775k miles.

The only thing is the chart kind of lacks context since 775k miles seems like plenty of miles for the life of Li-ion battery.

Exactly right... even if you charged 100-20-100% every single day, the battery would "last" (I don't know what they use as end-of-life) for ~5,000 cycles (which is 6,250 charges, which is over 17 years, which would be about 1.5 million miles), so perspective is needed here.

These charts get thrown around a LOT, but what needs to be remembered here is it's very hard to get more than 1000 cycles or so on an EV pack just due to the fact that it represents arguably a couple of decades of driving!

So everyone gets bent out of shape about what happens in the tails of the curves... It's like arguing about how to save 5 minutes on a drive from New York to Los Angeles. Does it technically make a difference? yes. Does it matter in any practical way? No.

 

[hugh_jassol]

...The problem with showing that chart as evidence to show that many small charges is better than 1 big charge is that, if you look at the chart, you're lead to believe that if you cycle 100-90-100 that's the best thing you can do! I don't think anyone here would buy that.

Again those charts made for batteries that are charged to 100% every time they are charged. It is NOT the same as a chart showing 100-50-100 compared to 75-25-75.

 

[jebinc]

I'm in the same boat. I just set it to 78% because I don't want to hassle with changing the number on the weekends when I do travel more. For what it's worth I'm still at 310 "rated miles" (Austin Power's Dr. Evil air quotes) at 100% after 1 year and 16,000 miles.

Same 75-78%.

 

[overkill]

OP here - thanks for all the useful info. Sounds like what I am doing (charging to 60%) is about right for maximizing battery health. Good to hear it seems that "shallow" cycle charges (5% many times vs 50% less many times) is better for the battery.