Frequent small charges vs. infrequent deep charges

[Big Earl]

Fundamentally it due to heat. The hotter, the faster the degradation.
Cooling systems have limits, the very core is not cooled, the heat has to pass though entire battery to sink.

Still open question how much the matrix breaks down during charge/discharge cycles.

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Heat is not a concern with Level 2 (AC) charging; it barely adds any heat at all, so there is essentially no significant different between 48 amp 240 volt charging and 12 amp 120 volt charging.

 

[Boza]

But I can hear the pump starting from time to time when L2 charging above 24A. I doubt it kicks the AC on but definitely starts moving the coolant.

 

[MontyFloyd]

Heat is not a concern with Level 2 (AC) charging; it barely adds any heat at all, so there is essentially no significant different between 48 amp 240 volt charging and 12 amp 120 volt charging.

11,520 watt vs 1,440 watt, 8x more power.
"no significant different"???

 

[Big Earl]

There are several pumps that run anytime the car is charging. It’s normal for them to speed up to handle thermal conditions while charging. This is normal operation and not an indicator that you’re doing damage to your car.

Yes, 11 kW vs 1 kW is a big difference, but in terms of thermal load and stress on the cells, it’s insignificant.

If you want to charge on 120 volts, go for it. Just be aware that in addition to using more energy to add the same amount of range, you’re adding a lot of runtime to various components (computer, on-board charger, coolant pumps, DC to DC converter and contactors) that will ultimately see shorter lives as a result.

YMMV

 

[MontyFloyd]

There are several pumps that run anytime the car is charging. It’s normal for them to speed up to handle thermal conditions while charging.

Yes, however the heat will still need to travel though jellyroll layers to the surface, and then to cooling system. The much larger 4680 will have a substantially larger thermal gradient simply from larger volume.

Yes, 11 kW vs 1 kW is a big difference, but in terms of thermal load and stress on the cells, it’s insignificant.

Have report to back that?
Thermal loads is linear, the more load, the more heat. Of course variable is how well the matrix handles the heat.

If you want to charge on 120 volts, go for it. Just be aware that in addition to using more energy to add the same amount of range, you’re adding a lot of runtime to various components (computer, on-board charger, coolant pumps, DC to DC converter and contactors) that will ultimately see shorter lives as a result.

YMMV

Yes, true. Running other systems, then one must look at replacement costs of those.
Do not have numbers, but an guestimate changing those smaller items will be a net savings over entire battery pack.

And yes, YMMV

 

[Empiro]

I recently read a very interesting Reddit post here that goes into battery heating. One takeaway from it is that AC charging, even at 48A, only heats up the battery about 0.3kw (and about another 0.5kw seems to be lost in AC-DC conversion). So basically unless the ambient temperature is already high, AC charging won't cause your battery temperature to get too high.

This article has some data that is research-backed. However, the research there may or may not apply to EV batteries. Its conclusion is that going from 75-25% offers you the best overall longevity. While 75-65% offers you the most number of cycles, each cycle is so short that you'll reach it more quickly. I think the biggest takeaway from the article is that no matter what your charging habits are, you should avoid higher states of charge unless necessary.

 

[Boza]

I read the second article before; hence, my initial question

 

[Dave EV]

I have an L2 charger at home and I use about 28kW per week driving around in the city. I could charge 28kW once per week or 4kW every day. All the other concerns aside, what would be better for battery longevity?

I read that cycles of charge is what determines the SoC but I am unclear if the depth of the cycle makes a difference. Is fewer, deeper discharges (I never go out of 20%-90% range) worse that more often, shallow discharges?

There are two main drivers of capacity loss in lithium batteries:

1. Calendar-life aging - this is driven by the average state of charge and temperature of the batteries
2. Cycle-life aging - this is driven by the depth of cycles and temperature while cycling

This tells you 3 things to do to maximize battery life:

1. Keep the batteries cool. The car will heat the battery up as necessary if too cold.
2. Keep the average state of charge low. This means only charge the battery as high as you need to, and charge right before you need to use the car to avoid letting it sit at higher states of charge.
3. Keep the charge cycles shallow - recharge enough to give you enough range until your next charge.

If you only use 4 kW / day (about 5-6%), ideally you might charge to 15%, then recharge back to 15% every day. You'd have nice shallow cycles and the average state of charge would be very low. Some people will argue that such low SOC is bad for the battery, but it's not actual studies show it's better (search for posts from @AAKEE for references).

Of course, the charge limit slider only goes down to 50%, so instead, you could take one of two strategies:

1. Charge to 50% and recharge daily.
2. Charge to 50% and recharge when you need or want to.

I would opt for the latter - because that lowers your average state of charge and because the car spends so much time sitting around, I believe calendar-life aging ends up being a bigger factor than cycle-life aging. Also, because I'm lazy and would prefer to not plug in my car every day.

One other thing to keep in mind is that it can help the BMS keep track of actual capacity of the pack by occasionally cycling through near the full capacity of the pack. This won't help actual capacity of the cells, but will make you feel better about it. So every month or two, charge to 90% and don't recharge until you're down to 10%. The BMS needs to see the car asleep for a few hours to register the data, so make sure sentry is off.

 

[MontyFloyd]

Its conclusion is that going from 75-25% offers you the best overall longevity. While 75-65% offers you the most number of cycles,

I do not follow what you mean by % numbers.
Is it the range of the battery charge state?

 

[Empiro]

I would tack on a bit of a caveat -- if it's very cold or very warm, it might be better to keep it plugged in, and pre-condition before leaving. If you're plugged in, then the car can use the wall power for heating or cooling. The car is very smart about how it can heat the batteries when you drive without preconditioning, but the net result is that the power you're using to do the warming is coming from the battery (leading to greater wear), rather than from the wall.

 

[Empiro]

I do not follow what you mean by % numbers.
Is it the range of the battery charge state?

Yes. Basically it's saying that letting the battery run down from 75% to 25% and then charging it is better than charging it whenever it reaches 65%. Even though you get more "cycles" out of charging it at 65%, you're effectively charging the car 5x as often compared to letting it run down.

 

[MontyFloyd]

because that lowers your average state of charge and because the car spends so much time sitting around, I believe calendar-life aging ends up being a bigger factor than cycle-life aging.

In short, the consensus is the less of Li battery total capacity is used, the longer it will last.
Seems like Li batteries are like machines, more you use, faster they ware.

Compared to:
Lead Acid that want to be fully charged all time (but not over charged)
Ni-Cd that need to be full cycles

 

[Empiro]

Yep, I think the biggest determining factor that you can control is State of Charge. Keep it reasonably low if you can (but not too low in case of an unexpected errand or traffic jam).

When it comes to temperature, the Tesla is good about keeping the battery from being too hot, so you don't need to worry about it. Cold is actually good for the battery's longevity, even though very low temperatures temporarily prevent you from using the full capacity.

While the link I gave shows that it's best to let the battery run down a bit between charges, I don't think it actually matters all that much to EV batteries. They're designed to allow for frequent charging and discharging because of regenerative braking. Therefore, I'd definitely just keep the car plugged in every day, but keep a lowish charging limit.

 

[Boza]

Good point about the regen. So, it seems that frequent shallow charging is better than infrequent deeper charging (given 20%-60% and L2 charging)

 

[MontyFloyd]

Good point about the regen. So, it seems that frequent shallow charging is better than infrequent deeper charging (given 20%-60% and L2 charging)

Regen is not something that can be controlled (very situational)
Charging is fully under our control.

All because there is Regen does not mean it is ideal for battery.