The battery maintainer type trickle chargers don't need to be switched on and off with a timer. They have a "float" function. Shumacher was mentioned. I have one. I also have several from HF for under $10 each and they work just as well. They do not go too high in voltage where they would "boil off" electrolyte.
A plain Jane diode and XFMR can overcharge as most 12 volt transformers (typically designed to put out 12.6 volts under a large fraction of their design current) puts out more voltage as the current drops such as when the battery is charged. Even with 12.6 volts RMS output the peak voltage in the waveform of typical 12.6 volt power trasformer is about 17.8 volts. This will, over time, boil off your electrolyte.
The cheap HF maintainers (and their more expensive bretheren) will hold the batt at no more than about 14.2 VDC at most and that is a very good float voltage.
Again, no timer required. These things are quite low current draw, like on the order of a small fraction of an amp (maybe 1/10th.) A single or dual timer approach will help prevent overcharging by a more robust charger but you can buy a maintainer at HF for less than many timers and certainly two.
These maintainers are not intended to charge a discharged batt, just keep a fairly well charged batt at or near full charge. I have two batts for my electric fence, one on duty and one on a maintaineer for a couple months waiting its turn. I also use maintainers on all our vehicles some of the time (except Prius and tractor)
I got over 5 years of excellent service from the OEM batt on the Kubota, never a hard to start or wouldn't crank well situation till it finally it suddenly got a bit lazy and got replaced. Changing batts, even yearly, will not guarantee against batt failure but will waste $.
For the marginal reduction in the chance of a disrupted schedule I'd get for changing batts every two years, I'd just as soon suffer a small hassle every 5-7 years and have the extra $ in my pocket. If you have a load tester, you could test the batt every so many months and replace it when the performance fell down (before cold weather starting performance would be seriously compromised) and reduce the chance you'd be inconvenienced by batt failure while simultaneously using all the good out of the batt and not tossing out a lot of unused performance on a gegular schedule.
Load testers are available at HF and auto parts stores and catalogs. A good tester (at least a 100 amp model) costs less than a battery and would eventually pay for itself in saving you from throwing out good batteries. It would likely save you the inconvenience of an unsuspected incipient batt failure. No matter what testing or replacement philosophy you follow, batts can just fail for no easily known reason so no philosophy will prevent all potential inconveniences of a dead batt.
Humor... sort of... When in the USAF and not rolling in $ but statioined in Minot ND where batts were a concern, especially in winter, some of us built simple batt chargers. One simple circuit is a 120VAC extension cord wiht the female end cut off. You wire a lamp socket in series with a full wave bridge (a single dioide will work OK too and give 1/2 the charge rate) and put some BIG aligator clips on the ends of the wires. You have to remember to attach the clips to the batt before you plug in as there is 120volts available if you touch anything. The higher the wattage of light bulb you put in the lamp socket the more charging current you get. To a fair approximation you get about an amp per 100 watts. If you use a reflector flood the heat of the light helps keep the batt warm. This is not a highly sophisticated setup. It is not well requlated and if left connected for long periods (several days) can overcharge the batt. We typically used it overnight.
Pat
