Group 31 johnson controls Wal Mart battery. 110 amp hours @20-hours rated
One salvaged transformer type wall wart. Unregulated 18-volts open circuit.
800-ma rated. Connected to battery. Maintainted battery at 13.62 volts. 4/5 amp.
This will give you some idea of how little amperage is needed to maintain a disconnected battery.
Older higher antimony batteries may cosume more float amperage due to negative plate antimony migration. But these batteries would be noticeable consumers of distilled water during everday use.
Connect the battery tender to fully charged, 100% fully charged batteries. The voltage of the battery tender should never sag less than it's nameplate voltage rating or there is a problem and you need to find out what it is. Be sure to note if the tender is temperarure compensated and if it is, float voltage values will adjust up or down for very cold or very warm days.
The most common error I see is when folks connect a voltage float device to batteries that are not 100% charged.