I suggest looking at "Batteries Plus". Their products are not the lowest cost, but I have had good service from their products both for automotive batteries and rechargeables for cameras.

I live in a cold climate but my vehicles may also see very hot weather. I have 2 vehicles: one a 4 wheel drive for winter service and a sedan for the remainder of the year. The sedan gets stored in my garage when hte first road salt hits the streets and does not come out again until salting quits. Its typcial storeage time is from Dec. 10 until April 30. During storage time, the engine may be started and run 3 to 4 times for about 45 minutes. This not often enough for best practice. It should be run at laeat once a month. The battery can be a little low on charge. It must be kept charged enough to prevent freezing.

During the summer months, my winter driver does not get driven much. It, too, gets started and run if it has sat for more than one month. It is stored in a closed garage that gets hot inside.

Depending on the purpose, either vehicle may be taken on a trip of 1000 to 3000 miles. My summer driver is preferred since highway mileage for these vehicles are 30 miles/gal for the summer car and 20 max for the 4 wheel drive.

My batteries last on the average of 5 years. Some would go longer, but gets changed if I detect reduced cranking ability or the battery is 4 yrs old and I am preparing for trip where increased reliability is a must. (Believe me, you do not want to be 200 miles from the nearest service with a dead battery with no available phone, no available cell phone, CB, or other communication.)

For your case, your batteries are failing early. Even for the cheap models, I'd expect much better service. You say that your charging system is ok and the vehicle gets driven every 2 to 3 weeks. Your battery should last several years. I suspect that the battery is not getting enough charge and/or something is draining the battery. Some of those 6 mo. old batteries that were thrown away may have been serviceable and only needed a good charging.

In modern cars, there are systems that place a small drain on the battery. One is the "keep alive" current supplied to various memories, the second is the security system, and a third is the remote key fob system. However, a charged battery should be able to supply these systems for several months. Look for excessive loads such as a glove box lamp, trunk lamp, or underhood lamp that remains lighted all the time. In some cars, the interior "dome" lamp is controlled by a solid state switch. A leak may occur here if the switch does not fully turn off.

If your battery is not getting fully charged, a trickle charger could help. A simplier solution might be to drive the vehicle enough to charge the battery.

For a number of modern cars, the ignition system will not work if the cranking voltage falls to 9 volts.

I go by the results in Consumer Reports. I normally change them after 5 yrs. regardles of how good they seem. My last sports car battery (summer use only) ws changed after 9 years because it seemed weak. I don't think I've ever changed one at 5 years because it was bad. I have 3 vehicles that are stored for 6 months (while I'm off in Florida). The batteries for these are connected to a "Battery Tender"...a self regulating trickle charger.
When I do replace a battery I don't go by the charts. I measure for the largest battery that will fit in the space and get that size. Most batteries, in the code, have an "N" or "S" for North or South. Yes, they are made differently for different climates.

my GM car uses group 78? got a delco unit on sale for $40. 6yr model. 700cca. think price is awesome. about 1/2 of normal. hope it lasts awhile. i like seeing delco parts under the hood.

Got my last one at Costco. A few yrs. ago Consumer Reports said the best battery on the market was the one sold at Walmart...believe it or not. The drawback is that they have limited sizes. I think last year the Sears DieHard was one of the best rated along with Interstate.