View Single Post
      12-31-2012, 06:07 PM   #103
Lieutenant Colonel

Drives: 2009 Crimson 328i
Join Date: Oct 2012
Location: Los Angeles

iTrader: (0)

Originally Posted by shah269 View Post
NICE photo shop but the laws of physics are a fucking bitch!
You do know what happens if you let a LiPo out charge right?
And for one of those are looking at...oh i don't know maybe three weeks?
For one, 'out charging' isn't a thing, so I'll A. assume you mean 'over charge' and B. not take you too seriously.

We're talking about an 85 kWh battery, and 1C is totally reasonable for lipos. Conservative, even - the problem charging an 85kWh lipo at 1C is that you're talking about 85kW, which is hard for a normal mains system to provide. We'll assume they've overcome that issue, since they get to design the mains system. But in general, 500V/200A is a crapload, but a perfectly valid way to provide 100kW. It's totally doable.

Over charging: you're thinking 90kW, 85kWh battery, I think. 85kW (approx. 90kW) into an 85kWh battery for 30m is a half charge, no over charge here. And for that matter, there are losses associated with charging batteries, 30% for a fast charge. Which, again, isn't what's going on here. 1C into a lipo isn't, by and large, fast, we're just talking about a battery vastly larger than the charging capacity of residential supplies. So this will actually be a relatively efficient charge. Still, at least 10-20% inefficiency, likely more. So I guess we can only assume based on the 150mi statement that they're talking about charge delivered (realized in the battery) rather than shipped (power out of the supercharger)

Anyway, charging works fine, so 150mi range:
It takes something between 20 and 30 horsepower to maintain 60 or 70mph on a compact sports sedan with Cd .30. I'm punting hard on the numbers here, because honestly I don't remember the details of the calculation, but this is coming from my Aero roommate and I calculating horsepower required to maintain 70mph (I believe) on my 328i. Keep in mind the Model S has the lowest Cd of any production car today, .24 stated. So this is conservative. Let's compromise, and say 25 horsepower to keep the S running at 60mph. That's probably fairly conservative, even, I'd imagine the S requires less power to maintain 60.

So 25hp * 750W/hp = 18.75kW. At 60mph, that's 2.5h to get 150mi. 2.5*18.75 = 46.875 kWh to go 150 mi.

If we recall from earlier, 90kW * .5h is 45kWh. So that's right on target.

TL;DR: given some reasonable assumptions, the math works out that a supercharger, as quoted, can add 150mi range to a model S in 30m. Please, leave math to the engineers.

EDIT: Researching what a bunch of EV guys have to say on the matter of power required for X speed, that calculation above is QUITE conservative. Maybe 30% so, so it could well be that 90kW is power delivered, and the charging inefficiency makes up for the difference to get down to 150mi range.

Last edited by alexwhittemore; 12-31-2012 at 06:13 PM.