For an expensive rechargeable bicycle light that is bright enough for daytime use, does it detract from charged state if left in a hot garage in the summer? In other words is it better to bring in the house, especially if you customarily charge them in the house.
While this is more a question of battery science in general, it should be stressed that most batteries don't like heat and should be stored accordingly.
Also worth mentioning that all batteries lose some charge over time, even when disconnected and properly stored. The rate of self-discharge depends on the battery type and since you mentioned the equipment being expensive, I'd bet on it having a li-ion battery. Frequently charging them to full is a bad idea because they will suffer damage if kept at full charge for long, plus they self-discharge rather slowly. Aim for 70% charge every few months instead.