UPDATE: Turns out the warning message seems to be tied to the ambient temperature and not the battery temperature, so in hindsight I'm pretty sure I had nothing to worry about. See this post by michael on myfocuselectric.com (thanks michael!). Equipped with an OBD scanner, he observed that in one case the battery temperature was in the low-80F range when the warning message popped up.
The user manuals for each of our electrics - a Focus Electric, a Volt, and a RAV4 EV - have something to say about parking in the hot sun - which is to plug in, if you can, while parked outdoors in hot weather. But none give any hint at what we should do if the opportunity to plug in just wasn't there.
Last summer I figured out a mitigation approach for parking outside on hot days that works quite well on the Focus - that is to keep the battery at less than full charge - at something like 80% or so - and remote start the car before the battery pack gets too warm. I first realized this might be an issue when, in the late evening following a hot 105-degree summer day, I noticed that the Focus Electric, while sitting on our driveway, was sending me email hourly since about 3pm telling me to plug it in. But instead of plugging in, I remotely started the car, then let it time out and shut down. That stopped the hourly email warnings; the theory is that the thermal management system (TMS) would kick in and bring the battery temp down to a level that was low enough to prevent the rest of the day's heat from elevating the battery temperature up to an alarming level. On subsequent 100+ degree days that summer, I kept the state of battery's state of charge at 50% to 80% and remotely started the Focus Electric at about 1pm-3pm, which ran the TMS for about ten minutes. I received no email alerts from the car on those days, so the scheme apparently worked.
The best practice, of course, is to keep the EV plugged in while parked in the driveway on days like this, but sometimes I forget to do that, and now I have a backup plan. Which did come in handy yesterday, since my Volt was sitting in the driveway, fully charged. So at about 3:00pm I jumped onto myvolt.com to remotely start the Volt an let it run for ten minutes, just for good measure. Probably really didn't need to do so, since the ambient was only 93F at home, not at 101F, and the Volt has a lot more buffer built into its battery management system than the Focus Electric - it only uses about 10.8kWh of the 16kWh battery's capacity (~65%), where the FFE seems to use a larger percentage - about 19.8 kWh of its 23kWh capacity (~85%), with most of that buffer allocated to prevent full discharge. Unfortunately the Volt doesn't seem to have the same warning capabilities as the FFE, so I'm not sure if I'll ever be able to know when the Volt thinks it's too hot outside. Fortunately, a member of gm-volt.com wrote up a great article on this subject - quite impressive, with graphs and real data - which leads me to believe that the remote start scheme will work just fine for the Volt.
As for the RAV4 EV - I wasn't worried about that one this time, since my wife was out and about throughout the day, so its TMS was engaged a fair amount. And the RAV4 EV's "full" charge utilizes only 80% of its battery capacity, much less than the purported 90% of the Focus Electric's capacity utilization. I did remotely start it earlier in the week though, when the temps first started approaching 100F, just to be on the safe side.
So, long story short (yeah, too late like usual), it's best to keep your EV plugged in while parked in the hot sun, just like the manual says. If you can't do that, my advice is to try to keep the state of charge (SOC) under 80% (50% is optimum), and remotely start your EV and let it run for about ten minutes, before the battery temp is heat-soaked to an alarming level, which should be something less than 86F (read this article if you want to know why 86F and less than a 100% SOC is important). Finally, if you park in a spot that's been soaking up solar rays all morning, your battery's temperature could possibly reach an alarming level faster than if your EV was parked there all night and shading the ground below it all day, depending on how well the bottom of battery (or batteries, in the FFE's case) is insulated.
There are, of course, other religious beliefs that various early adopters follow to keep their batteries healthy well beyond the 8-year or 10-year / 100,000 mile warranty and to prevent degradation that's not covered as a warranty defect by the manufacturer. On one end, some folks do nothing because they believe that the TMS (if equipped) and battery management system are good enough, or they simply don't give a damn because they have a 36-month lease and in the end it's not their problem. On the other end, there are those that advise others, typically know-it-all evangelists that have yet to purchase or lease an EV, that will tell folks to move to another state or just forget about driving an EV in Southern California, Arizona, or any other arid climate. I say ignore the extremists, for there is a happy medium.