If your electrical system is wired and operating correctly, (and you use your airplane more than once every 6 months) there should be no need for those "always left on" chargers. When your electrical system is off, there should be NO drain on the battery, except for possibly a clock which must stay on all the time, and generally these clocks only draw microamps in standby.
If your battery is being flattened between flights, either a) your battery is stuffed, or b) there is something constantly "on" in your system. From my observations with airplanes, the most common "always on" culprit is a miswired regulator.
There is a control wire that activates the regulator and I've seen a few cases where this activation wire is permanently wired to the battery, rather than through the master switch. This means that the regulator is working and trying to regulate the power system all the time. On the Rotax supplied regulators, this is the "C" input. Other regulators may have different designations for this input.