QUESTION: I'm hoping to learn what it takes to keep a car battery charged during storage. After purchasing a new truck, 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 good, or should it be a plug-in charger?
--Dave Conroy
ANSWER: This is a great question. In my day job as an automotive technology instructor, I face a similar issue, times 40. Some training vehicles in our fleet see regular use, while others sit for weeks, or even months at a time. Keeping the batteries charged is important, as a battery rapidly deteriorates if it loses charge (goes dead), even for a short period of time, and the cost of replacement batteries eats up funds that can be better used elsewhere.
There are two factors to contend with: self-discharge and vehicle parasitic drain. A typical automotive battery, with a clean upper surface between the posts, will maintain adequate charge for perhaps 9-12 months when stored. The vehicle it's installed in will drain the battery additionally, to maintain system memories such as engine computer RAM, clock and radio settings, and memories for a sometimes surprising number of additional vehicle modules. This is called parasitic drain, and is typically around 30-60 milliamps -- about one-tenth of the current needed to illuminate a tail light.
Disconnecting the battery may be an acceptable choice for an older vehicle, but with a newer car or truck you may find it inconvenient to reestablish the lost settings and memories. The battery of a vehicle stored outdoors can be maintained with a solar charger, if it can be parked so that the solar panel sees 6 hours to 8 hours of direct sunlight daily. A common plug-in, 2-watt panel, usually around $25, should be adequate if your vehicle has minimal parasitic drain and there's lots of sun.
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A more difficult application may require a 5 watt panel, at around $75, plus a charge controller at perhaps $35. The charge controller is recommended, as the larger panel could, under favorable light conditions, overcharge the battery, causing fluid loss. In either case, be sure your lighter socket is active with the ignition switch off. Otherwise, the solar panel leads will need to be attached directly to the battery. Also, I've found the solar panels we use prefer optimal aiming (propped up), and a clean windshield.
A plug-in charger may be a better choice if your parking location lacks adequate sun. Look for a smart charger rather than a trickle charger, to avoid overcharging. A popular choice is the Battery Tender Plus, at about $50. This 1.25-amp charger intelligently switches between three modes: charging, conditioning, then maintaining the battery for indefinite time periods. It can be mounted in the engine bay and left connected to the battery during vehicle use. Another great choice is the Schumacker SE-1-12S 1.5-amp automatic battery charger at about the same price. I've used this charger in some unfavorable outdoor applications and have found it to be tough as nails. Both of these chargers have modest maximum output. Should you someday encounter a deeply discharged battery, it might take as long as a day or two of charger operation before the battery is significantly recharged.
If it will be a few months or longer between gasoline fillings, it would be a good idea also to treat your gasoline with an application of STA-BIL or a similar product. Fuel stabilizers help minimize fuel oxidation and the gum and varnish that forms as the fuel becomes stale.
