A variety of battery chargers can be found at any automobile parts store in Northern Virginia. Most are designed to give your battery a quick charge so you can get your car started that day. For long term car storage you need a charger that produces a much slower charge at low amps. This type of charger is known as a battery tender or trickle charger. Although they all put out low amps, not all trickle chargers are the same. Most good quality trickle chargers will stop sending current to the battery when the battery indicates it is fully charged. Some less expensive brands will continue to charge at a low rate and eventually damage the battery. Even good trickle chargers will vary from one brand to another. Most only have the ability to keep a battery properly charged if it was properly charged to begin with.
Some better quality chargers will actually take an under charged battery and bring it up to its optimum power level then keep it there without over charging. This is the level of trickle charger that you will need if you want to save your battery during long term car storage. Still one last innovation in chargers is the ability of a very limited few that actually are able to take a slightly worn, damaged battery and over time re-energize the lead plates inside the battery and bring the battery back to its original power output.
The trickle chargers we use are purchased at a BMW car dealer and have that capability. We now find it very rare to have a battery fail. With the average cost of a battery of 80.00 to 120.00 and more expensive batteries in the price range of 160.00 to 240.00 the quality chargers have been a good investment. Another advantage to the use of a full time trickle charger on all of our stored cars is once we put the charger in place when the car first arrives there is far less risk of scratching a fender during the periodic use of a quick charge type of battery charger to run the cars each month during car storage.