What is a safe charge rate or voltage setting for a battery used outdoors?

No fixed voltage setting or current will work with certainty for applications with wide temperature fluctuations if a temperature-sensing charger is unavailable. A temperature-sensing, voltage-regulated charger must be used. Anything else will damage any battery and cause premature failure! It may be possible to limit this potential by using an ambient temperature sensing charger and assuming that the battery temperature is similar to the surrounding temperature. NOTE, however, that the battery's temperature will fluctuate during discharge and recharge, which will eventually damage the battery and cause premature failure if not controlled. If the recommended charger is unavailable, limiting the charge current and extending the charge time will reduce the chance of damaging the battery.