My main car is a Volt which will cool the batteries/inverter while plugged in but not charging. Obviously that's the ideal (besides Tesla, cools while parked using battery power). Now the question is how big of a difference does not having that make?
The battery heats up the most when:
* driving/regening - as long as you don't shut off the car in the middle of a cooling cycle, should be fine.
* (quick) charging - not a problem
* parking on a hot blacktop. - This is the biggest issue IMO.
Assuming you're arriving at work before the pavement has heated up you should be fine parking outdoors without plugging in even if it's hot out. The with the thermal mass of the battery it takes some time for it to heat up.
I really wish there was a temperature gauge in the 500e.