Do you need a Battery Maintainer?  I did.

Until a few years ago, simply turning off the engine and removing the switch key disconnected the battery from the circuitry so there was no drain on the battery as your vehicle sat idle for hours or days or even longer. Batteries seldom lost their power if the vehicle was used some.  (If the drain on a vehicle battery is little to none, a battery that gets no charge at all will eventually lose its power by self-discharge and need recharging).

Unlike older vehicles, today's vehicles have computers and other electrical components that require the battery to remain connected to the vehicle circuitry full time thus requiring a small but constant use of battery power, even if the vehicle is not being driven. If you drive your vehicle often and at highway speeds, the alternator will keep a sufficient charge going into the battery. But if most of your driving is city stop and go driving or if your vehicle is not driven often, your battery may not receive enough current from the alternator to keep it fully charged.  

Since my wife and I are both retired our two vehicles get little use. We need both vehicles at times so It's important that both of them are always ready to run. That fact requires both batteries to be fully charged at all times, which is not a simple matter. My 'low battery' problem was solved by buying two battery maintainers in January of 2011. I've had no 'low battery' problems since then.
About two years ago I  bought a Solar Charger which soon stopped working. It was replaced with another Solar Charger which also stopped working. It has an open circuit which I cannot access without destroying the sealed frame. So I gave up on Solar Chargers. I do know that some people use them and get good service but I'm not one of those fortunate ones.

After searching for a solution I found the AC connected Battery Maintainer removes all doubt and the cost was no more than for the Solar Charger. There are many Battery Maintainers on the market. I found that in our small town, the Schumacher was available at several stores. I've used regular Schumacher Battery Chargers for years so I trust the brand. I bought two maintainers, one of each model, about $25.00 each.

Either of these Maintainers will work for a 12 Volt vehicle battery but if you have a mixture of 6 volt and 12 volt batteries, you will need the model that selects the correct  6 or 12 volt application. I bought one of each model and added them to our cars in January 2011.   
The maintainers have several built-in protections including LED's to indicate whether your battery is being charged, is fully charged, or is disconnected from the AC power. They also cannot discharge your battery if disconnected from the AC power so they can remain connected to your battery as a permanent installation.
Schumacher 12 Volt Model
SE-1-12S
Schumacher 6 Volt plus 12 Volt Model
SEM-1562A
If you have a vehicle such as an ATV, Motorcycle, Motor Home, Boat, Farm Implement, a vehicle that is not operated often, a vehicle that is used mostly for city stop and go driving or some other application that uses an automotive type, 6 or 12 Volt battery, you might want to invest in a battery maintainer.
The next page is how I installed my maintainers.
WARNING: This site is not an invitation to mess around with your battery. Batteries are dangerous. A few years ago, I was standing a few feet from a car whose battery exploded when a lady simply turned on her ignition key. It was not a pretty sight. Not only is the battery explosive, it is filled with acid which can cause severe burns to your body and destroy your eyes. That little square box that does wonders for you can turn instantly into your worst enemy if conditions are right to cause an explosion. Wearing safety glasses is recommended. Search the Internet for Battery Explosions and you will see what I mean. 
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NOTE: There is a big difference in a regular Battery Charger and a Battery Maintainer. A Maintainer will keep a battery charged and can be connected full time without overcharging. The usual Battery Charger is for intermittent use at times when your battery is so weak that it must be recharged in a short time, then disconnected from the charger to keep it from overcharging. A good Maintainer can not overcharge.

"The only thing I might add is: Make sure your connections are made to clean, bare metal,
and consider using a dab of silicone caulk over the ground connection to cut down on corrosion.

After four years of installing communication devices and equipment on vehicles, I've found
bad ground connections are a sure spoiler, and a bit of silicone on them after making them secure is cheap insurance against future failure."

If you do install the Maintainer, here is some advice from a man who knows.
Thanks to Pete (from near Madison, Wisconsin) for the following:
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