Go to www.vdcelectronics.com
- they make a product called BatteryMinder that you leave hooked up to your battery all the time while in storage. It maintains the battery and uses about 10 cents of electricity a day to do so and also reverses sulphation - which usually builds up and destroys a battery if it is not regularly charged while in storage. I sprang for one a few months ago and am extremely pleased with it. You can hook up up to 4 batteries at once - and forget them for months! One battery I hooked up was 7 years old and not working very well - It now performs like new. They offer a 100% money back guarantee on the BatteryMinder if you don't like it along with a 5year no hassle warranty. I have found that it will not help a battery that has been frozen. Since my Mercedes is not driven 7 months of the year - it is great not having to worry about the battery!!!