Earlier this year, I joined a fairly large segment of the population in purchasing a Tesla Model Y. It’s currently the best selling car in the world by some accounts … so it’s certainly not unique. But it’s new to me.

I have a lot of thoughts about this car. Spoiler alert: I’m thoroughly enjoying it. But we’ll focus on one question for now … and it’s an obvious one. When you get the car, you want to know “What’s the best way to charge this thing?”

Tesla’s answer (dumbed down for consumers)

Tesla guides you to their answer in a couple of ways.

First, in most Tesla’s, the car displays some indicators on the charging screen about Daily vs Trip charging limits. The implication is that you should keep the charge limit somewhere in the Daily charging limit range most of the time. which is usually anywhere from 50% to 90% battery state of charge. The Trip charging limit range goes from 90% to 100% (and sometimes 80% to 100%). The implication of that one is that you should only charge that much when you really expect to need the extra miles of range … such as a long trip.

Second, in certain Tesla’s with LFP batteries, they avoid displaying the “Daily” vs “Trip” charging range indicator, and instead simply show a 50% to 100% charging limit range. With this different charging screen, they’re giving people the OK to charge to 100% most of the time.

The real answer (for nerds)

OK, sounds good. But is what Tesla guides the average customer to do really the best way to charge?

The answer to that depends on a few factors. As with many things, the binary fallacy is something to avoid here … there is no exact single good way to charge your car. Charging best practices vary on your particular situation … and mostly on your driving patterns.

Calendar Aging

One main takeaway you should have is that for most drivers, the amount of miles you drive is not the dominant factor in battery capacity loss over time. Instead, it’s calendar aging … aka, the inevitable passage of time.

And, as far as calendar aging goes, it turns out that is that charging to and letting your car sit at around 50% is significantly better for battery health. This is true even of LFP batteries.

The following graphic illustrates this quite clearly.

Ref: Calendar Aging of Lithium-Ion Batteries

But if this is true for LFP battery cars, why does Tesla have a different charging screen for those cars? The answer is most likely because hiding the distinction from consumers encourages simple and mostly acceptable charging behavior.

Tesla doesn’t want customers to get scared off by the idea of having to micromanage their battery health. The truth is that there are all kinds of vectors along which you can keep your Tesla’s battery healthy. LFP batteries behave differently in three major ways from other Li-Ion batteries.

• LFP are less energy dense. That’s not great.
• LFP batteries age about half as quickly. This is good!
• LFP batteries have a particularly flat voltage curve. This makes calibrating them more difficult than other Li-Ion chemistries.

So given all of the above, the reason Tesla took away the daily/trip indicators is that:

1. They need you to charge the car to 100% every so often to calibrate battery level estimates (due to the flat voltage curve)
2. They decided that the improved durability of the LFP battery chemistry can handle the additional stress on the battery well enough.

But make no mistake … you can still extend the lifetime of your LFP battery quite a bit by leaving it at 50% or lower as much as possible while parked.

Depth of discharge

The other dominant factor is referred to as depth of discharge (DoD). It turns out that the battery literally physically expands and contracts when charged to full and discharged to empty. This expansion causes microscopic cracking and degradation within the battery.

Basically, charging to 100% greatly decreases the battery lifetime. If you do it repeatedly, it will become the dominant aging factor above and beyond standard calendar aging.

Summary

All in all, a better way to charge a Tesla for extended capacity over time is as follows.

General best practices for maintaining maximum battery capacity

• Always plug the car in at your home location(s)
• Use scheduled charging to reach your desired state of charge right when you expect to leave. This keeps the battery at a lower state of charge overall.
• Set the charge limit to the lowest amount possible that meets your daily commuting needs and whatever buffer you deem psychologically necessary to feel comfortable. In Tesla’s, this can be as low as 50%.
• If you have a longer day trip planned, raise the charge limit so that you’ll have whatever range you need by the time you leave.
• Level 2 charging is better than Level 3 “supercharging” for your battery. So keep your supercharging stops to a minimum … use them when you need to top off right away, or when you are road tripping.
• Keep your car parked in shaded or cool areas whenever possible. Heat ages the battery faster. If you park in a hotbox garage, consider installing a fan to vent the trapped heat.
• Avoid charging above 80% unless absolutely necessary. I wouldn’t even do it on trips since most of the time you can operate in the 80 to 20 percent range to get to the next supercharger. But if you need to go to 100% to get somewhere with a margin of safety, do it.
• When letting your car sit overnight or for longer periods of time, try to do it at an SoC of 55% or lower.
• Unfortunately, I have to recommend avoiding the Scheduled Departure feature as I’ve found it to be unreliable. Use Scheduled Charging instead. Hopefully this gets fixed.
• I don’t recommend buying any particular model of Tesla if you regularly need to charge above 80%. You’ll find yourself aging the battery, which will require you to charge the battery even more, and so on … which will accelerate a kind of “doom loop” in the aging process.

Personal interpretation

If you want to know what this looks like for me …

I do keep the car plugged in every time I park at home.

I’ve simply gotten used to leaving the charge limit at 50% and setting a scheduled daily departure time of around 8 AM. This usually means the car will start to top off around 2AM to 4AM. I’ve found that for most of our weekend errands, we can be out for most of the day in a 30-45 mile radius around us, and I’ll still get back with 20% battery.

For longer day trips, I will set the charge limit higher … typically enough to reach the destination at 50% SoC … and then draining below that to make our way back.

For long distance trips, I just charge to around 70 to 80 percent. In our experience so far, charging to 100% hasn’t ever actually removed any stops for us on our road trips. Someone needs to take a bio break anyway, or things are just spaced in a way where the extra charge never really helps us.

So this works plenty well for us. We only drive on the weekends right now, so calendar aging will definitely be the dominant factor in our Tesla’s battery capacity loss.

So far, I can’t see any range loss at all on the car after 6 months, so that’s nice. Over time, I hope to see the benefits of this approach with a still relatively high range after a few years!