Install a battery tender in a location that minimizes the length of the leads between the unit and battery, but avoid placing it over the battery. The location should provide adequate ventilation along with protection from moisture. A battery tender helps to keep batteries at full charge on recreational vehicles, motorcycles and boats, where they often discharge due to a lack of use.
Minimizing the length of the lead and following the manufacturer's directions regarding wire gauge for the application ensures the tender keeps the battery charged and ready for use on demand. Installing it directly above the battery subjects it to the caustic fumes of the battery acid and leads to corrosion and early failure of the unit.
Most tenders require protection from the elements but also need some space to dissipate the heat they produce. When installing one on a boat or RV you store outside, choose a location under cover that keeps the tender dry but is not so enclosed that it causes it to overheat. With a vehicle that is garaged, there is less worry about the elements, but the unit still requires adequate ventilation.
When installing the battery tender, ensure the alternating current lines are not in locations where they suffer from abrasion. While it is important to protect all wiring from insulation wear, the AC lines are more likely to cause a problem if there is a short due to wear of the insulation.