Can Heavy Rain Affect Your Car Battery?

Heavy rains often cause traffic to slow to a crawl, even on interstate highways. Poor visibility and traction may force immediate, and possibly erroneous, driver reactions that impede forward progress. The engine revolutions needed for full charging system function are not achieved when inching along for extended periods. Although modern automotive charging systems are more capable than ever, no alternator produces maximum power at idle. Emergency vehicles are fitted with beefed-up alternators and engine speed controls to maintain good charging energy while powering all the elaborate lighting needed on such vehicles. Passenger vehicles lack this advantage and battery reserves are tapped heavily at low speeds.

As the alternator struggles to keep energy flowing to the ignition system and fuel pump at low speeds, further battery reserves are stolen by the accessories used in inclement weather. The ventilation settings that clear the front glass employ electric blower motors to distribute the air, which in most cases is dried somewhat by the air conditioner. The rear glass might have an electric grid that dries the exterior and interior surfaces. The windshield wiper motor is also electric, while the headlights would be on in such instances. Other electrical equipment that could further rob the battery reserves can include fog lights, seat heaters, and repeated brake applications that illuminate a host of light bulbs to warn neighboring motorists.

Just as car tires lose traction when wet, the same is true of the rubber belts that drive alternators. The squeal that normally accompanies belt slip may not occur when the pulleys and belts are wet. A slight imperfection in the belt drive system that escapes notice might have a minimal effect on sunny days, but cooling and lubricating water can aggravate the situation. Increased loads on the alternator make it harder to turn over and it is possible for the belt to glide silently through the alternator pulley when good traction is most acutely required.

Colder temperatures that can coincide with rainy days might be blamed for some battery weakness, as cold does affect the specific gravity of the electrolyte contained in the battery case. Poorly sealed engine compartments could allow water to collect on the top of the battery and form a wet short circuit across the battery posts. A slight power drain occurs in this instance and will continue as long as the water remains in place. Side-post batteries are not subject to such ills; however, both top and side-post designs can be fouled by corrosion. Cables and connections that are kept clean and dry work best, and these considerations have lead to alternative battery placements. The battery is mounted under the rear seat bench, in some car models, and this particular design has gained in popularity recently because of its success in keeping the battery out of the elements.