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Slow charging on NEMA 14-50: any advantages?

Would there be any benefits to charging slow (lowering the Amps in the car) on a NEMA 14-50 capable of drawing 32 Amps? My commute is very predictable every day and charging my M3 LR AWD takes about 2-3 hours to reach 80% SOC.

I tried 2 other approaches while charging with NEMA 14-50 at 32 Amps:
(a) start charging as soon as I get home and leave it plugged in till next day morning and
(b) schedule the start time 3 hours before I leave next day morning.

Approach (b) has the advantage of leaving the car at 80% SOC for a short period of time daily. I do not have an EVSE so I am not sure what the most efficient way of charging is. Since I will be repeating this every day for at least next 5-8 years, I am wondering if there is a better charging habit I should follow.
 
There aren't many benefits to slow charging outside of trying to time a heated battery right before you leave in the morning (when it gets colder).

Longer charging times are less efficient because you're keeping the car awake longer, which uses energy. There isn't a benefit to battery health when you're talking about 32 amps. Keep in mind, regen alone will send over 70 kW into the battery every time you lift the accelerator. 32amps at 240v is ~7 kW, so even at max charge on your NEMA 14-50, it's almost a trickle charge in context of the battery :)
 

srs5694

Active Member
Jan 15, 2019
1,622
2,181
Woonsocket, RI
First, you do have an EVSE. That's the generic name for what the Tesla Mobile Connector and Wall Connector are; Tesla just doesn't use that term much, if at all, in their marketing and documentation. The term "EVSE" is more commonly used to apply to J1772 EVSEs that come with non-Tesla vehicles or that people buy from third parties; but Tesla's devices are definitely EVSEs, too.

The only advantages I know of for Level 2 charging at a slower rate (say, 16A vs. 32A) would involve rare conditions -- say, if your house is running close to its amperage limit and charging at 32A might pull it over that limit. Those charging rates are pretty low compared to Supercharging, and so are unlikely to affect the batteries in any notably different way. There's also a monetary advantage of charging at 32A provided by the Mobile Connector vs. paying to buy a Wall Connector and charging at 48A in an LR Model 3.

Charging up just before you leave might have some slight advantages, though, particularly if you were to charge to closer to 100%, since leaving a battery at 100% for an extended period can be detrimental to its long-term health. At 80%, it's not a big deal.

Some people have time-of-use (ToU) electric plans, which charge different rates at different times of day -- typically lower at night than in the day. If you have such a plan, then it would make sense to charge when electric rates are low. This might mean charging as fast as possible, and setting the timer to charge only after the lower rate kicks in. (Note that even a 16A or lower charge rate might be adequate to charge your daily use during low ToU rates.)
 
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There aren't many benefits to slow charging outside of trying to time a heated battery right before you leave in the morning (when it gets colder).

Longer charging times are less efficient because you're keeping the car awake longer, which uses energy. There isn't a benefit to battery health when you're talking about 32 amps. Keep in mind, regen alone will send over 70 kW into the battery every time you lift the accelerator. 32amps at 240v is ~7 kW, so even at max charge on your NEMA 14-50, it's almost a trickle charge in context of the battery :)

Thank you. I will go with (b) schedule the start time 3 hours before I leave next day morning.
 
First, you do have an EVSE. That's the generic name for what the Tesla Mobile Connector and Wall Connector are; Tesla just doesn't use that term much, if at all, in their marketing and documentation. The term "EVSE" is more commonly used to apply to J1772 EVSEs that come with non-Tesla vehicles or that people buy from third parties; but Tesla's devices are definitely EVSEs, too.

The only advantages I know of for Level 2 charging at a slower rate (say, 16A vs. 32A) would involve rare conditions -- say, if your house is running close to its amperage limit and charging at 32A might pull it over that limit. Those charging rates are pretty low compared to Supercharging, and so are unlikely to affect the batteries in any notably different way. There's also a monetary advantage of charging at 32A provided by the Mobile Connector vs. paying to buy a Wall Connector and charging at 48A in an LR Model 3.

Charging up just before you leave might have some slight advantages, though, particularly if you were to charge to closer to 100%, since leaving a battery at 100% for an extended period can be detrimental to its long-term health. At 80%, it's not a big deal.

Some people have time-of-use (ToU) electric plans, which charge different rates at different times of day -- typically lower at night than in the day. If you have such a plan, then it would make sense to charge when electric rates are low. This might mean charging as fast as possible, and setting the timer to charge only after the lower rate kicks in. (Note that even a 16A or lower charge rate might be adequate to charge your daily use during low ToU rates.)

Thank you for the explanation.

I have an electrician check if I am near the max Amps of the house with everything running. It is not a problem in my case.

The price difference between ToU and standard plan for my situation is very small. I don't see opting for ToU in the near future. With that said, I now understand the advantage of scheduling better.

I will go with (b) schedule the start time 3 hours before I leave next day morning.
 
This is an interesting question / discussion and even though I have read multiple posts on charging and therefore try to subscribe to the “plug it in and forget about it / don’t overthink it” school of thought, I have had a similar question.
I recently bought myself underground parking at work. The building right across the street from where I work recently installed about 20 Tesla wall chargers which charge anywhere between 32A and 48A so I used that as some sort of “cost mitigation” strategy and bit the bullet.
My drive to work is pretty short so even with some driving around in the evening (kids to hockey etc), leaving the car on sentry mode etc, if I leave the office in the evening with 83-84%, the next morning, I will typically roll into the parking lot at about 68-69%.
I plug in every day and charge to about 83-84% but by 10 am (more or less), the car has reached the set SOC and then sits there for the balance of the day.
(Public service announcement - there are more chargers on any given day than parked Teslas, so no worries about being plugged in and denying someone a charge who may really need it).

I also thought that perhaps there would be some advantage to reducing the charge rate, etc so that it doesn’t finish up so quickly, but it is pretty apparent that there is no such advantage.

In the winter however, I will time it, to either start later, or top up before I leave in the evening.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
8,773
11,447
Boise, ID
There is certainly no advantage as far as the battery. Any kind of home charging rates are all really slow from the battery's perspective.

The only thing I would say as a maybe is about the charging cable or equipment people are using. On their cables, Tesla seems to build them to just barely meet what their maximum current rating is, and so they sometimes run kind of hot at their maximum current. Over the years I've seen some reports of problems and failures with the wall connectors running at 80A, or the 1st generation UMC running at 40A, and I predict we'll be seeing some issues with the newer 2nd generation UMC running at 32A.

And it's not just about instant failure kinds of things. When things heat up during use and then get cold when not being used, that temperature cycling gets hard on materials. Connections and solder joints in electronics get stressed and weakened. This kind of frequent and extreme cycling between hot/cold is what can accumulate wear and eventually kill electronics stuff. So if you run a few amps below the maximum rating, the cable will stay noticeably cooler and not heat up quite so much and probably will extend the life of the equipment some. And if you're talking about the difference between the car finishing charging in 4 hours or 5 hours while you're sleeping, that really makes no difference to you anyway.

So I have been running my old 1st generation UMC at low 30's amps, when it is capable of 40 to keep it a bit cooler and lasting a very long time. And that has been over 5 years now on my original cable.
 

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