Benefits to Charging Slower?

[DaveVa]

I have a NEMA 14-50 and am getting the HPWC. My Model S currently charges at a full 40 amps and 29-30 rated miles/hour. Generally, I plug in at night and, while the Model S could charge for 10+ hours, it unsually tops off in a couple of hours and shuts off. Tonight I'm doing a Max Range charge so I limited the amperage to 20 Amps so it spends less time at 100% in the wee hours of the morning (I know the couple of hours is probably insignificant).

However for everyday charging is it any better for the batteries to charge 6 hours at 20 amps rather than 3 hours at 40 amps? Less heat and load? Or is it better for the batteries, within the Model S limits, to charge at the highest current available and spend less time charging?

 

[William3]

I have almost the same question. I've got the HPWC on order. If I'm not in a hurry, should I set the charge rate to something less than 80 amps? Some people have mentioned that it is safe to use the SuperChargers as often as you'd like (as long as you're just doing "standard rage" charges), so wouldn't that imply that it is also safe to do 80 amp charging as often as you want (daily)?

 

[Doug_G]

You can charge at 80A with no ill effects whatsoever. However it might be more efficient to charge at lower current. This is certainly true for the Roadster, where the optimum efficiency is in the 30-40A range.

 

[dsm363]

Tom looked at this a few years ago
Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog

at least on the Roadster this is what he found. Very useful information. If you have the time as well, might be easier on the grid too to charge at 40A at night (if they ever activate timed charging).

 

[William3]

Is Tom getting a Model S?
Is anyone planning on testing the charging efficiencies of the various charge speeds?

 

[ChadS]

No, Tom is not getting a Model S (he and Cathy are already a 3-EV family and don't need a car that big).

I have a Model S, but don't have the tools (or likely patience) to do it, especially with my wife not wanting me to "mess" with her car.

I think these forums are just chock full of people willing to grab the baton, though...

 

[vfx]

... Some people have mentioned that it is safe to use the SuperChargers as often as you'd like (as long as you're just doing "standard rage" charges), so wouldn't that imply that it is also safe to do 80 amp charging as often as you want (daily)?

The Superchargers are DC power direct into the battery.

 

[brianman]

No, Tom is not getting a Model S (he and Cathy are already a 3-EV family and don't need a car that big).

I have a Model S, but don't have the tools (or likely patience) to do it, especially with my wife not wanting me to "mess" with her car.

I think these forums are just chock full of people willing to grab the baton, though...

I might be willing to have Tom apply the same methodology to my Model S at some point. If he's interested in putting in the work.

 

[DrComputer]

I have charged my Roadster for the past 3+ years at 70A with zero battery or PEM issues. There are many other people in our SoCalGreenSpeed group that have had battery sheets, or entire batteries, replaced and/or have had their PEM replaced. Most of them charge at 40A or below. Although it might be a bit anecdotal, I think it puts far less strain on the car to charge for a shorter time at a higher amperage. Once again, this is just by observation and speaking with other owners. I am using my same 70A Roadster HPC to charge my S at 70A each time I charge.

 

[strider]

There are no battery health reasons to charge at a lower rate - the car will manage the temps of the chargers, PEM, and batteries to make sure they stay within spec regardless of charge rate. If things heat up the charge rate will drop automatically. Now, as others have said, it's more efficient (at least with the Roadster but the principles remain with Model S) to charge at 32A-40A as at higher or lower rates more of the current from the wall is going to charging overhead (cooling) than into the batteries - although this is a very small amount. Also, it is easier on the power grid to charge at a lower rate for longer periods. Imagine if everyone on your block got a Model S and HPC and then at midnight everyone kicked on 80A charging for 2 hours. It would be easier on the grid for everyone to charge at 40A for 4 hours - less of a spike and longer draw period for the grid to adjust to the load.

 

[aviators99]

The people at the Dania Beach, FL retail center always tell owners to charge at the lower amperage--to the point of saying that when you charge, you should go to the charging screen and reduce the amperage even below 40 if you have the time, and that it's "better for the battery". I'm not sure what to think of this.

 

[Cottonwood]

The people at the Dania Beach, FL retail center always tell owners to charge at the lower amperage--to the point of saying that when you charge, you should go to the charging screen and reduce the amperage even below 40 if you have the time, and that it's "better for the battery". I'm not sure what to think of this.

Remember the charge rate is all relative to the battery capacity. The normal way to refer to this is as C/H, C for capacity and H hours. For a Roadster with a 53 kWh battery, 40 Amps at 240 V (or about 9.6kW) is roughly C/5.5 hr. For the 85 kWh Model S and max supercharger (80kW), this is about C/1 hr. A Model S on a 240V, 80A charge (19.2kW) is about C/4.4 hr. Given that Tesla has said the supercharging causes no battery degradation, that the Model S is very smart at reducing the charge rate to protect the battery, and that the Model S has newer battery chemistry than the R, I can't imagine 80A, 19.2kW, C/4.4 charging causing any problems to a 85 kWh pack. Even with a 40 kWh pack, I would trust the Model S charge controller to throttle charging as needed.

The only reason that I see to charge slower is to reduce resistance losses in the wires to the car (can be a few %), and being nice to the grid.

 

[brianman]

Imagine if everyone on your block got a Model S and HPC and then at midnight everyone kicked on 80A charging for 2 hours.

Wait, I'll need a few minutes to finish that...

 

[tezco]

One disavantage to charging more slowly is that the fixed costs (power needed to run coolant pumps, computer power etc) begin to add up, resulting in decreased efficiency. My Leaf takes three times as long to charge when the charging current is halved.

 

[Jason S]

I strongly suspect that charging at 12A is very inefficient. Charging at 60A is my guess for the most efficient way to juice the Model S that has twin chargers. Otherwise go at 40A.

Completely guessing. Until we have good tracking like Tom did, we won't know.

 

[WarpedOne]

One disavantage to charging more slowly is that the fixed costs (power needed to run coolant pumps, computer power etc) begin to add up, resulting in decreased efficiency.

Cooling pumps only run when battery temperature is above desired level. If charging is 'slow' enough, battery won't need active cooling (or preheating). Computer power only starts to add up if charging is really slow, measured in weeks.

 

[FlasherZ]

Cooling pumps only run when battery temperature is above desired level. If charging is 'slow' enough, battery won't need active cooling (or preheating). Computer power only starts to add up if charging is really slow, measured in weeks.

Source?

When my model S is charging, you can hear the pulsing of pumps to circulate the battery coolant to keep the entire pack at the same temperature. It doesn't seem to be "on demand", but rather just a desire to keep the entire pack at a consistent temperature.

 

[dhrivnak]

On my Roadster the collant pump runs all the time during charging; I guess to ensure there are no hot spots.

 

[Babylonfive]

Source?

When my model S is charging, you can hear the pulsing of pumps to circulate the battery coolant to keep the entire pack at the same temperature. It doesn't seem to be "on demand", but rather just a desire to keep the entire pack at a consistent temperature.

I'm surprised at that, FlasherZ. My initial thought was same as WarpedOne, that the pumps would run only when measured temperature showed a requirement.
I suppose that having the pumps already pumping would avoid any delay time in the control loop, from detecting a threshold to engaging, coming up to pressure, circulating and then finally cooling. Still, if I was the TM designer I'd set the pump to a much slower (and efficient) rate if the charging rate was lower, and I'm guessing from your reaction that you don't think that happens...

 

[FlasherZ]

Still, if I was the TM designer I'd set the pump to a much slower (and efficient) rate if the charging rate was lower, and I'm guessing from your reaction that you don't think that happens...

It may very well be at a (s)lower rate, and undoubtedly the cooling/heating circuits may not be operating if the pack temp is within spec, but there is value to circulating the coolant fluid to keep the entire pack at a consistent temperature. Not doing so would allow the potential for some hot spots. It certainly does not do absolutely nothing.