Optimal Charge Type for Battery Longevity

[STxTesla]

I was wondering what the best charge, (rate, amperage, etc.) would be for the Tesla Model S batteries so that you maximize longevity, and minimize degradation.
Expert opinions would be greatly appreciated.

[Lloyd]

This was done by Tom Sax for the Roadster which has a smaller battery and different battery format and it was found to be 40 amps at the sweet spot. Similiar testing will need to be done on the model S batteries to find the same spot for each battery format. My guess is that 40 amps will work well for each of the battery formats.

[brianman]

Lloyd, I can look at the chart and make a guess but you can probably answer definitively.

Is the recommendation (for Roadster) more like (30,40] or more like [40,50) as the ideal charging range. In other words "at most 40" or "at least 40"?

[strider]

There are 2 different things at work here. For battery longevity, it doesn't matter. The car will not allow the batteries to be charged too quickly based on their current state of charge, temperature, etc. So you can charge at any rate you want (AC, not supercharging) and the batteries will be fine.

Tomsax's work that Lloyd is referencing is related to charging efficiency. This is where the maximum amount of power from the wall goes into the battery. Above or below this point a slightly higher percentage of incoming power is being used as overhead to manage and cool the batteries.

[Lloyd]

There are 2 different things at work here. For battery longevity, it doesn't matter. The car will not allow the batteries to be charged too quickly based on their current state of charge, temperature, etc. So you can charge at any rate you want (AC, not supercharging) and the batteries will be fine.

Tomsax's work that Lloyd is referencing is related to charging efficiency. This is where the maximum amount of power from the wall goes into the battery. Above or below this point a slightly higher percentage of incoming power is being used as overhead to manage and cool the batteries.

Tom should comment as it is his work. but my assumption is that the most efficient (least heat wasted) is also the best for the batteries, and allowing proper ballancing.

[strider]

Tom should comment as it is his work. but my assumption is that the most efficient (least heat wasted) is also the best for the batteries, and allowing proper ballancing.

His blog post is here: Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog
There was no effort to measure battery temps that I can discern and so we have no data at this point to make that assumption. There is every reason to believe that any and all excess heat due to higher charge rates can be dispatched via the Roadster's battery cooling systems and therefore has no effect on battery life.

As for balancing, as the batteries near full charge the rates automatically ramp down to take care of the balancing.

I will concede that there is an emotional (gut) feeling that charging at lower rates are "nicer" to the batteries but there is simply no evidence. It's not like your laptop or phone that gets hot as you're charging it - the car carries this heat away.

[hcsharp]

As Strider said, Tom was trying to determine the most efficient charge rate, not the rate that resulted in longest battery life. He determined that 40A and above had the same efficiency. In theory, the slower you charge, the longer your battery will last due to heat from internal resistance. That said, 80A is still very slow for a 60 or 85kwh battery, and will never generate heat anywhere near as fast as the cooling system can handle it.

But there's another factor that supports slower charge rates. The charger has to use higher voltages across the cell to maintain higher charge rates, although it never lets the voltage go above about 4.2v. That means that higher rates will result in longer times at higher or maximum voltage for the cells, which is known to shorten their lives. But once again, we're not talking about a big difference between 40A and 80A for an 85kwh pack. Nor would I worry about 70A on a 53kwh pack. These rates are still pretty slow compared to what these cells can handle.