Scheduled Charging at lower amps

Scubastevo80

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#1
I started charging my car last night at home - scheduled to begin at 1am to avoid any excess power drain on our system when running the air or heat in the daytime. I only drive about 40-50 miles round-trip for work on a daily basis. Instead of charging at the full 32 amps (limited by the mobile connector), I lowered it to about 20 amps to reduce the constant electricity draw.

Does anyone else do this and is there any benefit? Instead of charging at 25-30mi/hr, I thought it might be beneficial to charge at a slower rate over a longer period of time (i.e. 4hrs instead of about 2hrs). Thoughts?
 

kort677

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#2
as long as the car gets charged to a level that you need in the time allotted there is no problem continuing what you are doing. as for wear and tear I really can't answer with facts but it stands to reason that while charging at a lower rate might be less efficient overall it isn't harmful
 

garsh

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#3
Does anyone else do this and is there any benefit?
There's no appreciable benefit. It'll produce less waste heat, but will do so over a longer period of time, resulting in slightly more wasted total energy. Even at 48 amps, charging current isn't high enough to affect battery lifetime:

https://www.tesla.com/blog/bit-about-batteries
Two outtakes from this article:
  • Avoiding very high charge rates. Charging faster than about C/2 (two hour charge) can reduce the cell's life.
  • Limiting our charge rate is less of a compromise, since the wire size and availability of very high current outlets limit us much more than the batteries do at this point.
This was written in 2006, during development of the Roadster. So even back in the Roadster days, it sounds like Tesla was of the opinion that AC charging even at some of the higher-available currents was still seen as pretty "low" to the Tesla battery pack. They recommended staying under a two-hour charge. The Roadster had a 53kWh battery, and according to wikipedia:
...using a 240 V charger on a 90 A circuit breaker... a complete recharge from empty would require just under 4 hours.
So this seems to support the theory that AC charging at even the highest-possible 48 amp rate isn't going to hurt the battery noticeably compared to charging at a lower current.
 

JML

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#4
I did that for a few weeks. I was originally charging on a 20 amp 110 circuit. Normally, the maximum continuous draw for that circuit would be 16 amps, but I charged at 15 amps, because there was .5 amp of other stuff connected to that circuit. The car didn't care. I've since installed a dedicated 30 amp 220 circuit, so the car charges at 24 amps.

It sounds like you have a 50 amp circuit for charging, in which case the charging outlet should be the only device on the circuit, and it should be able to handle 32 amps without a problem. If that's not the case, and you have multiple items on that 50 amp circuit, then you're doing something weird. The maximum continuous rating for a 50 amp circuit is 40 amps, so 32 is well below what the wires are capable of. One reason I can think to lower your charging amperage is if the capacity for your whole house is too low. If, for example you have a 100 amps coming in from the utility, and have 32 going to the car, 20+ to a dryer you might run overnight, a bunch of household stuff, etc., and it's just too much for your feed

Another reason to lower your charging amperage is if you pay based on peak use. This is a rate system where you pay a low rate, but then there is a surcharge for the maximum use during the billing period. Generally the maximum use is only counts during high demand hours, so charging overnight shouldn't effect it, but if you're on a plan like that, you'll have to check with your utility.
 

Scubastevo80

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#5
Thanks all - I think I'll keep it at the max 32 and just schedule the charging for later in the middle of the night. Now that we're approaching colder weather season in the northeast, how do you all think about charging to pre-condition the car/battery? Just keep it plugged in until you're ready to leave in the morning?
 

JML

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#6
Thanks all - I think I'll keep it at the max 32 and just schedule the charging for later in the middle of the night. Now that we're approaching colder weather season in the northeast, how do you all think about charging to pre-condition the car/battery? Just keep it plugged in until you're ready to leave in the morning?
This is what I'm doing, but I'm not sure it actually matters. I'm using Tasker for Android and the Tesla Tasker plugin to adjust my charge level. Maybe this can be done with Teslafi, or other services. At 8:55pm my phone tells the car to set it's maximum charge to 75%. At 9pm, when my low electric rates start, the car itself begins to charge. At 6:45am my phone sets the max charge to 80%, and at 7am tells the car to start charging. The car will finish charging a few minutes before I'm ready to leave for work.

The reasons I don't think this really matters: Due to poorly insulated duct work running through the garage, and all the waste heat from charging in the first place, the garage is about 58F, even though the outside temperature is 33F. The cars interior temperature is 59F, so the battery is probably close to that. At that temperature and 80% charge I don't get full regen, but it's still plenty for one pedal driving, and after a few miles full regen is available.

When overnight temperatures are in the 10s and 20s it might help.