Lowering amperage a good thing?

[Olle]

When you charge, the battery warms up. The BMS has a cooling target (somewhere 40 - 50°C) which the battery may reach and sit at during your charing session. After charging is done, the battery will approach the ambient and cabin temperatures.

As long as you are not parked in the sun or a hot place, the ambient temp will most likely be less the the battery cooling target.
Since battery degradation is accelerated by temperature * time, one could argue that you should charge with the highest current available to shorten the time.

 

[Feathermerchan]

The energy that is warming your battery comes mostly from resistance to electricity flow in the battery cells. The formula for energy from resistance is W=I²R where W is energy in Watts, I is current and R is resistance. So doubling the charge current quadruples the energy that creates heat. That energy not only heats the cells but it also is energy lost which cannot be recovered for moving the car. In addition, increasing the current reduces the charge time so the cooling system has less time to remove that heat energy.
So considering that there are constant operating loads while charging which would be minimized by speeding up the charge, and there are the energy losses heating the calls which increase as the square of the charge current, a simulation would have to be devised to calculate an optimal charge current.

 

[Olle]

The formula for energy W=I²R where W is energy in Watts, I is current and R is resistance. So doubling the charge current quadruples the energy that creates heat.

Energy is measured in Joules, not Watts, but I understand what optimization you're looking for.

OP was looking to optimize for longevity.
I hear my battery cooling running every time I charge, no matter if I set it at 25 A or 40 A, which indicates that the cooling target has been reached. If that is the case and if the battery heat cannot overpower the cooling, the temperature becomes a constant. The remaining variable for battery longevity is time, which means that highest available charging power is optimal.

If the car is parked in a cold enough place where the battery cooling isn't always on, I think your optimization makes a lot of sense.

 

[jerry33]

Energy is measured in Joules, not Watts, but I understand what optimization you're looking for.

OP was looking to optimize for longevity.
I hear my battery cooling running every time I charge, no matter if I set it at 25 A or 40 A, which indicates that the cooling target has been reached. If that is the case and if the battery heat cannot overpower the cooling, the temperature becomes a constant. The remaining variable for battery longevity is time, which means that highest available charging power is optimal.

If the car is parked in a cold enough place where the battery cooling isn't always on, I think your optimization makes a lot of sense.

There are so many other factors for battery life, that any home charging variation in battery life will be just noise. Having driven Teslas for nine years, battery life really isn't a concern--just purchase the largest battery, charge to 90% daily, and you'll be fine. At seven years and 130K miles, the 2013 S85 was down about 8%. This was actually better than what was estimated at the time (11%).

 

[Olle]

There are so many other factors for battery life, that any home charging variation in battery life will be just noise. Having driven Teslas for nine years, battery life really isn't a concern--just purchase the largest battery, charge to 90% daily, and you'll be fine. At seven years and 130K miles, the 2013 S85 was down about 8%. This was actually better than what was estimated at the time (11%).

No, home charging is not noise if your home is where you charge most of the time.

Having taken a degree course in BMS and read Jeff Dahn's publications I can almost certainly tell you that time and temperatures are big factors for a Tesla pack. Sure there are many other factors outside of home charging.
Congrats on only losing 8%, my S has lost 12%. I don't worry about it either, but that's beside the point of OP's question.

 

[UncleCreepy]

No, home charging is not noise if your home is where you charge most of the time.

The point is that home charging regardless of amperage has little to no impact on longevity whereas letting the car sit at high or extremely low SoC and/or in high temperatures does have an impact. As does supercharging. Amongst all those things affecting longevity, home charging is indeed just noise as it barely contributes to battery degradation. I believe that was @jerry33's point.

 

[Feathermerchan]

Olle - You are correct. Energy in Joules or Watthours, power in Watts. So if the cooling target is reached on a home charge (I've noticed it too) bringing on the pumps and maybe fan, and the charge current is split 46 ways, then these cells must have a pretty high resistance. I think I can measure it with my 'Scan my Tesla' app. I can see battery voltage while sitting still (very little load) and then see it under load and make some calculations.
I fly model airplanes as a hobby. We use the pouch style (lighter) Lithium Polymer cells. When they first came out, you could not discharge them faster than 4C (15 min) so every flight was 15 minutes. Todays batteries we can discharge in less than 5 minutes (12C) without damage. We can also charge at 2 - 5C. Much faster than we can charge the Tesla batteries. The reason is lower internal resistance. They can supply more current and power without overheating.
If we could discharge the Model 3 battery in 5 minutes that would be 75 kWh ÷ 1/12 hr = 900 kW or 700 ÷ 0.746 = 1,206 HP
We would need some big tires and motors.
Back of the napkin, if you drive 210 miles in 3 hours and use up about 90% of the battery (I did that once) that's 67.5 kWh over 3 hours or about 22.5 kW. So driving 70mph on my model 3 uses about 20 kW. It's also about 321 Wh/mile. Might be a little high but these are round estimates.

 

[Wattsisname]

It's unlikely that you're helping you battery much since the difference from 30A to 48A is a rounding error compared to the stress of driving, regenerative braking and supercharging. BUT... you are marginally making charging sliiiightly more efficient with lower line losses and reducing the thermal cycling of your EVSE. I usually charge at ~20A for that reason.

Right! Would it follow therefore that the "High Amperage Charger Upgrade" That an advertised 2019 S75D offers, is not of any great benefit?

 

[beenmb]

I am charging a 2018 Model 3 Performance primarily using a mobile connector plugged in to a regular wall outlet, in a covered garage, probably 250-300 nights a year. (Only power option available in the building) and set to a max of 80%. The car automatically sets the charging at 12A. If I leave it there after a few hours the plug on the mobile connector gets very warm. If I manually set the charging at 8A no heat at all. Getting the 50 miles range x 300 = 15,000 miles a year occasionally supplementing with Supercharging works for us. Sound ok for the wiring and car? Thanks.

 

[EdSt]

There are so many other factors for battery life, that any home charging variation in battery life will be just noise. Having driven Teslas for nine years, battery life really isn't a concern--just purchase the largest battery, charge to 90% daily, and you'll be fine. At seven years and 130K miles, the 2013 S85 was down about 8%. This was actually better than what was estimated at the time (11%).

Do you think there’s any difference between charging the battery to 80, 90, or 95% every couple of days?

 

[jerry33]

Do you think there’s any difference between charging the battery to 80, 90, or 95% every couple of days?

Between 80% and 90%, not enough to worry about. 95%, if you drive as soon as the charge is finished, not much of an issue but don't leave the car at anything over 90%. I'd only charge to 95% regularly if I really ended the day at or below 20%. (And since I have a certain amount of OCD, I'd try to pick up a few miles at lunchtime, not only to avoid the 95% charge, but to account for any unforeseen). However, assuming you are only doing this on trips--then it's only a small part of the car's total charging over its lifetime. (e.g. Ten times in a ten day trip, two to four times a year). My recommendation is to go with what Tesla says: Daily, up to 90%. Trips, up to 100%. Avoid letting the car sit at above 90% and below 20%. Note that Wh/mi is terrible until the charge level drops to 90%.

 

[!igs]

As others, have already pointed out, home charging, even on a 60A circuit is very slow compared to Supercharging:

The maximum you can get is 48A sustained or 11.5 kW. For Model 3 that will result in a charging rate of 44 miles of range per hour of charging.
Compare that to a Supercharger. With V3 you can now get: 250 kW, a 1000 miles per hour charging rate!

So, go ahead and charge at 48A and don’t worry about the battery!

Supercharging degrades the battery so that is a poor comparison to make.

 

[Lmsb]

I have seen multiple instances of under-torqued high amperage connectors causing fires or nearly so. We had an electrical distribution box catch fire internally because the manufacturers quality control missed an under-torqued terminal (~100 A). After my Mother’s 50A wall receptacle and plug melted in the wall (probably due to insufficient tightening of the receptacle terminals) while charging her model S, I spoke to an electrician who said the car charging circuits are the worst for this. The thermal cycling associated with long charge times at close to rated current can loosen connectors and loose connections can get hot - further stressing the connector. A/C breakers and circuits are upsized for higher motor start current, while car chargers don’t need this. Electric ranges typically don’t stay on for hours at a time at max current. Backing off a little on current could reduce this risk.

 

[UncleCreepy]

I spoke to an electrician who said the car charging circuits are the worst for this. The thermal cycling associated with long charge times at close to rated current can loosen connectors and loose connections can get hot - further stressing the connector.

Assuming you're using the correct wires, the thermal cycling is well within the design limits. Long cycles are generally less stressful. What stresses the connection isn't the connection being cold or hot, it's the change of temperature. EVs as such are actually less problematic than other appliances that don't run at a constant amperage but keep turning on and off instead.

Take a look at your main breaker box. You'll see a couple of connectors and chances are they have never been retorqued in decades. They still don't get hot or cause other problems because someone did their job right and they used the right hardware.

Lowering the amperage can't be the solution, especially not if a fire can occur at any amperage you can reasonably use to charge your car. The key to safe charging is to use proper hardware, tools and workmanship.

 

[nwdiver]

, I spoke to an electrician who said the car charging circuits are the worst for this.

Yep; Really no other load that regularly stays at the circuit capacity for over 4 hours straight. Regardless of how it’s designed and installed more current is still more heat is still more thermal expansion is still another opportunity for something to come loose.

My S is designed to go from 0-60 in 4s… doesn’t mean that 0-60 in 10s isn’t gonna put less stress on the car. If you only need to charge at 20A why charge at 40?

 

[TSLA Pilot]

Assuming you're using the correct wires, the thermal cycling is well within the design limits. Long cycles are generally less stressful. What stresses the connection isn't the connection being cold or hot, it's the change of temperature. EVs as such are actually less problematic than other appliances that don't run at a constant amperage but keep turning on and off instead.

Take a look at your main breaker box. You'll see a couple of connectors and chances are they have never been retorqued in decades. They still don't get hot or cause other problems because someone did their job right and they used the right hardware.

Lowering the amperage can't be the solution, especially not if a fire can occur at any amperage you can reasonably use to charge your car. The key to safe charging is to use proper hardware, tools and workmanship.

Yes, in your perfect world there's nothing to worry about . . . .

In the REAL WORLD, if there's no need to charge a max amperage, then WHY DO IT?

As I type our S is charging at the slowest possible charge rate of 5 amps/240 volts, less than we're getting from our solar panels at the moment--now that is a perfect world

 

[Lali77]

I charge my 2023 model Y on a 3rd Gen 48amp Tesla wall connector at 15amp, because slow and low is definitely beneficial in the long term but mostly because it allows me to allocate more electricity to other appliances in my house since I’m on only 150amp whole house system.

 

[ewoodrick]

I charge my 2023 model Y on a 3rd Gen 48amp Tesla wall connector at 15amp, because slow and low is definitely beneficial in the long term but mostly because it allows me to allocate more electricity to other appliances in my house since I’m on only 150amp whole house system.

There is VERY little difference when charging at 15A or 45A on a battery pack with thousands of cells.

 

[UncleCreepy]

I agree it doesn't make a difference, although it's not so much about the number of cells. Assuming @Lali77 has a 75 kWh battery we're talking about 0.05 C at 15 Amps vs 0.15 C at 48 Amps. That's virtually nothing. A C-rating of less than 1 is usually considered slow charging. Even DC charging on a Chademo charger is slow.

If that really made a difference the battery would be toast after supercharging once at 250 kW (3.3 C).

I always charge at the highest power available which is 11.5 kW at home. With more than 100,000 km on the clock I still have about 94% battery capacity left (based on the guess-o-meter in the car). It really doesn't matter in terms of longevity. Other loads in the house may or may not be a factor, although with a 150 Amp service you can still burn almost 29 kW continuously. That's quite a lot, even while charging at 11.5 kW you'd still have 17.3 kw left.

 

[douglaspfortin]

Running lower amps through a higher amp circuit will create less heat loss from the wire to the environment. The car battery will also lose less heat.

As far as what the battery can handle, as long as you don't charge beyond Level 2 specs the battery is designed to handle it with no additional degradation to the battery elements, up to a point (see below). The only benefit is slightly more cents in your pocket, granted you keep the whole charging time-frame within off-peak electric company hours.

Last thing is the battery degrades more if you charge beyond about 20% (i.e. charging from 20% up to 80% as opposed to charging from 40% up to 60%). So, if you can squeeze from 20% up to 80% using lowest amps possible within the off-peak hours this will keep money in your pocket and result in the lowest degradation of the car battery.