I use a battery Tender on my Acura every day. I drive it 1.4 miles each way to work so the alternator has no chance of recovering the battery from the start, especially with a seat heater and rear defogger going.
A cars alternator does a very poor job of properly charging the battery. It's usually nowhere near a full charge even after a several hour trip down the freeway. The Tender took almost 3 days to turn green the first time I hooked it up. Now it's usually green in 15-20 mins, sometimes longer in the winter. The engine turns over noticeably faster when starting. It also helps preserve starter life.
I have the Battery Minder on the BMW and GN. It seems to be superior (temperature compensated and it's a desulphating charger which usually means a lifetime battery) than the Tender.
The Acura has a battery in the front so I installed the pigtail that came with it to plug it in at the grill. The other two cars have the battery in the trunk. The BMW has a huge dry cell. The GN has a tiny motorcycle sized dry cell (it might make 720hp at the crank but it's still just a low compression V6 to the starter). The GN charges in no time. The BMW can take hours each day. The BMW tries to only run the alternator when the energy is free such as coasting and stopping. The alternator is usually off or at full power with very little in between.
Something to think about if you make lots of short trips, newer cars don't run the alternator all the time. It's on a need to run basis. If my Acuras battery is topped off, and the alternator does not have to run for the whole 1.4 miles, that's about 3.5hp less consumed. When you figure it takes only 30hp to maintain 45mph or 5hp to idle the engine at a light, it can show up as improved mpg but it is subtle and hard to measure without great records and consistency.
The BMW has the start stop feature and I notice with the battery being charged all night, the engine shuts off almost every time I stop and it will stay off much longer at say, a long drive through. This definitely saves fuel.
I went way off topic with my rambling but the Battery Tender Jr is nice and portable and works well but lacks some of the features of the next model up and I believe temperature compensation is one of them.
The Battery Minder includes desulphating which is a huge feature IMO. None will overcharge the battery even after months. Once fully topped off it continues to monitor and charge very slowly when needed. At its lowest, the rate is lower than my iPad charger.
These are not for charging a dead battery. They will but prepare to leave it for a week. The desulphating charger can bring an old battery that won't take a charge back to life. Used as a maintainence charger it never gives the chance for the crap to form on the plates in the first place and it's likely the battery will last the lifetime of the car in my experience.