I have had considerable experience with 12 volt automotive batteries in vehicles and equipment that sometimes sit idle for long periods prior to use. I personally prefer solving problems through prevention rather than fixing it afterwards. There is an engineering basis for my choice. The lead acid storage battery will remain healthier if maintained in a charged condition and not allowed to self discharge or be run down by various loads.
In my case I have some pretty sophisticated and extensive alarm systems which even on a diesel truck with parallel 100+ amp hour batteries will run the batts down in a relatively small number of weeks especially in cold weather with low batt efficiency.
There are battery maintainers about the size of the plug-in-the-wall power supply for cordless phones (some are larger) that plug into a standard wall outlet and attach to the batery or convenient connecton point. These are designed to prevent long term self discharge or discharge from alarms, computers, whatever... Some of these are designed to mount near the battery and remain connected all the time. In use, you connect an extension cord to the unit's short power cord when you park the vehicle and disconnect before you drive away.
In self defense, I mounted one of mine so that if I forget to unplug, it will unplug itself harlessly as I drive away. On another vehicle (street legal VW dune buggy) that gets used infrequently I installed a maintainer of a different design. It plugs into the wall like the afore mentioned power supply. I have a quick disconnect plug and socket (Radio Shack for a buck or so) to use to hook up the cord from the maintainer to the car. I have leads permanently attached to the battery cables in the car. I located the plug and socket where it was convenient to get to but not in the way and it too will "auto-disconect" if forgotten and the vehicle is driven away.
These maintainers are not designed nor capable of effectively charging a deeply discharged battery. They are to keep a fairly well charged battery at full charge for an extended period of time without damage to the battery. Most common automotive type battery chargers will damage a battery if connected for extended periods of time.
These battery maintainers are commonly available where automotive accessories are sold such as your FLAPS (Friendly Local Auto Parts Store), JC Whitney catalog or web site, farm/ranch supply stores, etc. I have found them for less than $10 and as much as about $25. I have not noticed any problems with the cheap ones or the more expensive ones. I have used them on small batteries (chipper shredder) all the way up to parallel diesel truck starting batteries with excellant results and nary a problem. They are specifically designed for long term connection and do not "boil" out your electrolyte like many regular charges can, if left connected for extended peripods.
I even use one to keep my moms emergency lighting system battery (deep cycle 100 amp hour maint free) fully charged. It stays connected year round.
I'm not knocking the portable jumpstart battery thingy. They are extremely handy! They will probably work for what you need but they just aren't my prefered approach for the stated reasons above. I have both. If you don't have a screw driver, you can drive a screw with a hammer but I recommend the right tool.

Pat
