You should charge battery every month, like clockwork.
Otherwise, battery will lose its charge, and then it will sulfate. Nothing you can do except: 1) Quick charge it to break up sulfating, makes battery useful, but, shortens its life considerably, 2) exchange battery with a new and start over with a better maintainance program.
It doesn't take much every month, maybe a couple hours of charge to keep it fresh. Buy a DVM, Harbor Freight sells a cheap one for $10, and use to monitor battery charge. Anytime charging voltage gets over 16 volts, time for a new battery.
Another method of checking charge is to buy a battery hydrometer, one of those with the floats is affordable enough. If you happen upon a professional one, this one being temperature compensated, the closer to a Spec Gravity reading of 1.260, the better. SG of 1.260 means 100% charged.
To answer your second post, you're not allowing the battery to charge up enough. Usually, from a dead battery, slow charging, less than 10 amps, it will take about 8-10 hours for a good charge.
Another suggestion, is to buy two of the same battery model and swap out with another vehicle every couple weeks, this way both get exercised.