Battery Tender

Under The Hood: How to maintain unused car battery

18.06.10

Cast doubt upon: I'm hoping to learn what it takes to keep a car battery charged during storage. After purchasing a new odds, my old one will be kept for occasional use and will be given to my son when he reaches driving age. Are the solar chargers you see any orderly, or should it be a plug-in charger?

-Dave Conroy

Retort: This is a great question. In my day job as an automotive technology scholastic, I face a similar issue, times 40. Some training vehicles in our task force see regular use, while others sit for weeks, or even months at a time. Keeping the batteries charged is substantial, as a battery rapidly deteriorates if it loses charge (goes insensible), even for a short period of time, and the cost of replacement batteries eats up funds that can be preferably used elsewhere.

There are two factors to contend with: self-report and vehicle parasitic drain. A typical automotive battery, with a unspoiled upper surface between the posts, will maintain fair to middling charge for perhaps 9-12 months when stored. The mechanism it's installed in will drain the battery additionally, to maintain system memories such as motor computer RAM, clock and radio settings, and memories for a sometimes surprising bevy of additional vehicle modules. This is called parasitic withdraw, and is typically around 30-60 milliamps - about one-tenth of the in the air needed to illuminate a tail light.