Best Practice Charging

For some, charging the battery is almost a religion. I charge my Model 3 to 80% and always have the charger plugged in when I'm at home.

In the owner's manual Tesla states:

Info from a Tesla Owners Event, at which one of the presentations was on service and proper battery charging and maintenance. At this session, the two Tesla gentlemen acknowledged that where charging and proper battery maintenance there was a lot of misinformation on social media, and that this was not helped by materials on Tesla's site and even from Elon.  He said that his information comes straight from the engineering team.  

 

The major points he discussed were:

1. Despite what you might have heard, the best thing to do for your battery on a regular basis is to charge up to 80% and not go below 20%.   If you have to dip below 20% on occasion, that's OK, but charge up ASAP.   If you are going on a trip, charge up as much as needed, but don't let the battery sit above 80% for long.  Definitely don't let it sit above 95% for long at all - drive immediately when charging is complete.

2. Set percentage instead of range on the display.   The range number is NOT set to EPA figures for the given battery percentage, but is calculated off the estimated charge of the pack AND your driving habits (See the discussion on the BMS below).   Basically, a degradation of that number most likely does NOT indicate a battery problem.   If you get in touch with service, they will use a sophisticated diagnostic to check the state of the pack and will most likely not see any major faults (as more than one person on FB has complained - "they aren't taking me seriously/they are hiding something/they don't want to fix it").   

3. The BMS - battery management system.  First, a little background is required: 

You don't have one battery.   Depending on the model you have, you can have upwards of 4000 individual battery cells all wired together in 3 or 4 "bricks", which combine to make one battery pack.   The cells discharge at varying rates, from 4.2v (100%) down.  However, when the car charges, it charges until the FIRST one of those thousands of cells reaches the charge limit percentage REGARDLESS of the charge level of the other cells.    So if some batteries are 4.1v, 3.9v, etc, they will NOT get a full charge, and this will impact range. 

If you are familiar with lithium ion battery packs, you may be familiar with the term "balance charging" - which is a way to ensure that the individual cells have the same voltage on them.  

 

These cells do NOT like to be overcharged and can catch fire - which is why the BMS shuts down when the first cell reaches capacity.   So obviously you "lose range" because you cannot charge the other cells to capacity.   Repeated short drives (and short recharges) don't give the BMS the data it needs to calculate battery health - both gentlemen stressed repeatedly that the algorithm used for the "miles remaining" figure gets more and more inaccurate over time, and that allowing the BMS to balance the cells "solves" most cases of "lost range". One gentleman stated that it's like filling a glass of water to a point and asking how much water is in the glass. You can't truly tell until you pour it into a measuring cup and measure it. It's the same with the BMS - if you use 10%, add 10%, use 5%, add 5%, etc the BMS is using the amount of current added during the charge cycle to decide what the capacity of the battery is. As mentioned in #3, you may not be adding a true 10% of capacity because one cell may not have discharged 10% - maybe it only discharged 9%, and therefore really only 9% is added in before charging stops. Those little discrepancies add up over time, and are rectified by balancing the cells:

a. Drive the car on a long trip - down to 20% is optimal.

b. Charge up to 90% and leave the charger connected overnight.   The BMS will discharge the cells until they are all at the same voltage and then recharge them so that they are all the same voltage at a 90% overall state of charge.    Then your remaining miles should be back to spec or thereabouts.   The BMS will not balance the cells unless the battery is charged to at least 90%.

4.  For optimal battery health, let the car sit for an hour before charging and for an hour after charging.   They suggested using the scheduled charge feature for that.  For some people like me who don't commute to work or have a set driving schedule, this won't work as well since the scheduling turns on the climate control for departure time and therefore wastes power if you aren't going anywhere.  

5.  To get a MUCH better estimate of your remaining miles, use the Energy app, set to Average at 30 miles.   The algorithm there uses your driving style (average Wh/mi over the specified number of miles) and the percentage of battery remaining to perform its calculation.   The algorithm on the main display is NOT as accurate according to them.  They stressed that several times during both their presentation and the Q&A session.  

6.  Charging via a 110v circuit (for those of us who have it) is more efficient on the Model 3 than the Model S or X due to the improved charging circuit and battery technology. 

(However, in my personal testing, it's about 6x slower than 240v/32a 14-50 charging, so I can only imagine how slow it is on an S or X....)

7. Supercharging is NOT good for the battery when done regularly.  If you are relying on it for much/most of your charging, you WILL experience faster degradation of the pack.  (NOTE: This *MAY* only apply to Model S/X and NOT the 3 per other information.  The Tesla folks did not differentiate between models, so I can't say if this is true or not.)

8. ABC - Always Be Charging.   The battery pack does much better when it is charged up after each use. 

9. They are trying to get updated materials on the web site that reflect this information.   Unfortunately, as you may have guessed, there are several higher priorities that the company is focusing on, so it will take time.