Adjusting charge time by adjusting amperage.

Adjusting charge time by adjusting amperage.

I've been experimenting with different charging strategies. This past week when I set the car up for overnight charge, I've been calculating the charge needed then lowering the amperage from the 40 max to whatever number is necessary to have the car complete charging right before I leave the next morning. (Please note that at least this time of year, I live in a cold location.) Advantages I've noticed: I leave with a full standard charge with no vamp loss. The battery and the cabin are warm, even with no preheating, presumably from heating of the battery for charging. No limit on regen when I leave. No massive energy use spike as the battery warms when I leave.

My question is, what are the downsides to low amp charging? Overall more energy use? Detriment to the battery in some way? The energy use question is a complex one. You have longer battery heating, but you don't have that big spike in the morning. You also are regenning from the start, which I normally would not be able to do in cold weather. Also, is there generally a preference or benefit for faster charging at 40 amps versus slower charging at lower amps? Finally, do the answers to these questions change with changes in ambient temperature?

PaceyWhitter | 1 May 2013

Depending on how low you set your charge there may be effeciency issues with charging at a lower amperage as well as the longer battery conditining that you mentioned.

Since you are doing the calculations every night, why don't you just use the timer to set the car to start charging full at the appropriate time such that it finishes right before you leave. That should reduce your electricity use.

mdtaylor69 | 1 May 2013

I have read of people doing this on TMC as recommended by Tesla Service, and lower amperage charging has an added benefit of better maintenance recharging of the 12V battery.

PaceyWhitter | 1 May 2013

I knew I read about this somewhere, just found it:

There is a lot of data there (about the Roadster), but the main answer to this question:

As you can see from the table, there's not much variation in charging efficiency when charging at or above 240V at 32A, but energy use rises noticeably at lower power levels.

Mark K | 1 May 2013

This is a clever tactic.

You needn't worry about charging too slow. Slower charging is always gentler on the battery, though the effect in this case is negligible because TM manages the cells so carefully.

The efficiency is optimum above the 32A level due to process overhead, but the effect is not large.

32 or above seems like the sweet spot.

The reason your strategy works is that the charge completes at the time you want to leave. That means the car is warmed up and has a full charge, with zero vampire loss.

The warmup comes without further cost because it's the result of the ~90% efficiency of charging. That is, about 10% is lost to heat, which in this case has the beneficial result of preheating your pack for driving.

People who are not in colder climates don't really need this though.

TM should let everyone do your trick by adding an app feature: "Ready Time".

Instead of saying when you want the charge to start, you'd say when you want the car ready. (You could still constrain the starting time by entering a "best rates" period).

The algorithm could easily figure out the best charge path to save money, energy, and your time.

Totally doable, and very cool to have.

Robert22 | 1 May 2013

If I have a short commute, say 20 miles, and charge every day, are you suggesting the optimal scenario given the above is to schedule charging for the early am hours at 32A so the car tops off at the same time I leave for work? This would be in contrast to 15 amp or less charging starting earlier in the evening. I was not aware 32 amps provides optimal efficiency. It would be helpful to have this information verified for MS.

Brian H | 1 May 2013

The difference between optimal and lower is not worth fussing about. You'd need instrumentation to detect the difference. Don't sweat the small stuff.

Mark K | 1 May 2013

Yep. As to the 32 amp number, that's an inference from roadster experience, but something analogous likely exists with MS. TM needs to confirm where that is for the S.

The idea is you order room service the night before, and your breakfast is piping hot, right on time.

An algorithm considers time, temperature, utility rates, etc, and figures out the right charge plan to optimally deliver what you need.

All without you giving it a second thought.

It could even infer your habits, like a Nest thermostat, if you don't set anything.

This is a natural evolution of the car's intelligence.

I think this will simply show up one day in your auto-updates.

noel.smyth | 2 May 2013

this strategy worked well for me this past winter. With the warmer weather and no off peak rate advantage, I just charge it when I arrive home in the evenings now. I will likely use the scheduling software next winter and yes, I suggested the same 'ready time' function in a thread earlier - and I do hope we will see something like that in a future update.

DouglasR | 2 May 2013

@Robert22 - With a 20 mile commute, does it really matter whether you start your day with 240 rated miles rather than 230?

lazyav8tr | 2 May 2013

The Leaf has has ready-time/end-time charging timer since day one.
It surprses me that Tesla hasn't made this relatively simple update yet.

Brian H | 2 May 2013

Hasn't the Roadster had it for some time?

DonS | 2 May 2013

If you have a 100' run, charging at low current can save about as much as running a lamp all night because loss through the wire is I-squared * R. Exact savings depend on wire gauge, length and currents.

Robert22 | 3 May 2013


Actually, it may. That commute is frequently punctuated by errands that can easily drop me substantially below 200 miles. There's been prior discussion about what constitutes a charge cycle and its effect on battery longevity and performance. Some of the simplifying assumptions made in those threads may not be completely accurate when applied to the MS batteries which may be more unique than I first appreciated. There are still unknowns about the battery that make definitive calculations difficult. The definition of charge cycle for these batteries needs to be better defined.

Brian H | 3 May 2013

It's an industry term: a cycle is one full charge and discharge, or the equivalent sum of partial charges and discharges.

mal42north | 4 May 2013

The efficiency measurements were for a roadster with a 70A charger. I would expect most of the charge losses to be in the AC to DC converter, which usually show good efficiency above 50% load. That's consistent with the 32A number. For the Model S with a single 40A charger the number might well be 20A. I'd guess that's true with two chargers as well, assuming that the setup is to fully utilize the first charger before adding current from the second.