If you decide to leave them in your vehicle, I would just make sure they are not in direct sunlight and keep them in the glove box or center console. if you know your vehicle is going to get warm, I would also make sure they are not in there for an Over extended amount of time. But being that they are lithium ion batteries, you want to make sure that they don't get too warm, because there are operating temperatures for lithium ion batteries.
Direct from Apples website:
"It’s especially important to avoid exposing your device to ambient temperatures higher than 95° F (35° C), which can permanently damage battery capacity. "
"Even storing a battery in a hot environment can damage it irreversibly. When using your device in a very cold environment, you may notice a decrease in battery life, but this condition is temporary. Once the battery’s temperature returns to its normal operating range, its performance will return to normal as well."
