JFP you and I have different definitions of "Maintainer." To me a maintainer turns off or floats when the battery has reached full charge.
Trickle charger just pushes steady state voltage/amperage. OP device indicates "Direct plug-in CHARGER" "Output 12v 500mA" "Class 2 Battery Charger"
"Both float and trickle chargers are devices that slowly emit low-voltage charges to your car battery, restoring the battery to a fully charged status over several hours. Subtle differences between the two devices determine which charger is best for you.
Float chargers are equipped with electronics that allow the device to cycle on and off. This is beneficial if you are going to have the charger connected to the battery for a long period. The device automatically turns off when the battery is fully charged and turns on again when the battery's charge lowers.
Trickle chargers work similarly to float chargers. You connect the trickle charger to the battery terminal and a slow, steady stream of low-voltage electricity recharges the battery. Trickle chargers, however, do not come with the on/off feature of the float charger. Electricity continues to flow to your battery until you remove the trickle charger.
A trickle charger is most effective when you are available to turn it off after the battery has charged. Overcharging your battery can cause permanent damage. If you are not available to monitor the charging process, or if your vehicle is in storage for a long period, a float charge that automatically turns itself off and on is your best option. However, if cost is a concern for you, trickle chargers are significantly less expensive."
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What is the Difference Between a Float Charger & a Trickle Charger? | eHow