600mA would indeed drain a battery in about a week. Other options might be to buy a solar charger and leave it plugged in during storage.
This is an inexpensive 100Watt charger that I have been thinking of getting:
http://www.amazon.com/Renogy-Foldabl...eywords=renogy
Of course, you would have to find a way to secure it or it would quickly "walk off".
You probably don't have to go with such a big charger if you get good sunlight during the winter. But due to the sun's angle and the yukky grey skies we typically get in the winter time in my area, 100W would not be overkill.
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