I presume Porsche will design their battery pack and cabling to take advantage of the 800V. 15 minutes to 80% charge on a 400 mile pack would be AWESOME. Basically rivals ICE cars. Tesla needs some competition, it will be good for Tesla loyalists.
As others have pointed out... the 800 volt thing is a red herring. The #1 issue is the battery chemistry. Maybe VW/Porsche and their battery partners can deliver. Maybe not. As it stands right now, Tesla's cell chemistry has specific energy of somewhere around 270 Wh/kg. Tesla allows us to discharge at around 6C, and charge at around 1.5C. With that, we expect degradation around 12% after 3,000 charge cycles with 60% depth of discharge. I think Tesla pushes these boundaries as hard as anyone, and I think their priorities are, 1) cell degradation 2) specific energy 3) cost 4) discharge c-rate 5) charge c-rate.
You can play around with these specs, but if you change one, you affect another. Now, all the major automakers with any real sort of mass production of battery packs is choosing to go down the route of NMC chemistry. It can tolerate a higher discharge and charge c-rates, but has lower specific energy. And we aren't quite sure about cell degradation.
For Porsche to have a competitive sports vehicle, not just against Tesla but benchmarked against their own internal metrics for what a sports car should be, would need much lighter batteries. So their priority list probably looks like:
1) specific energy 2) discharge c-rate 3) charge c-rate 4) cell degradation 5) cost
Specific energy directly affects the weight of the car, driving the overall design of basically everything. This is why LiFePO4 chemistry made in great quantities in China doesn't end up going anywhere. It's too heavy. For Porsche, the vehicle must be light in order to have any of the sports car qualities they really want versus the GT sports car feel. Increasing specific energy likely lowers the charge c-rate for a given cell degradation. If they choose to forgo liquid cell thermal management, they save weight, but then shedding heat during high charge c-rate becomes a big problem.
If Porsche does manage to ship a high c-rate battery pack, I would be very interested to know the cell degradation characteristics. Maybe they think the solid electrolyte chemistries will be available by then. But 2019 is very, very close for new cell chemistries to make it into production cars. Since they have no real Tesla competitor shipping, they feel free to tout vaporware... there is no cell chemistry that is production that can fulfill their requirements. We'll see if anything in the labs makes it in time. But this 800 volt thing is misleading because it doesn't change the charging c-rate. It does help put more energy across a fixed amperage cable... ie. the 350 A CCS connector. But at 400 volts x 350 amps, you have to be able to tolerate a 140 kW charge before you bother going to 800 volts. Technically, you can do 500 volts x 350 amps at 175 kW before tripping across a whole series of standards as you get near 600 volts. Going above 2 C would be very interesting... we'll see what they sacrificed to get there.
We need a few more data points from Idaho National Labs AVT battery testing:
Library - Alphabetical | Advanced Vehicle Testing Activity
They are likely going to lose funding as part of the DoE cut down by the Trump administration, right as we, as consumers, really need to have independent testing of the batteries in our cars. They are the only ones I know of that does long term cycle testing and then pulls the battery pack off the car and does bench testing to see much more the real effects on the pack versus what the BMS is hiding from us.
