If you don't drive your car very much or drive your car less than 7500 miles a year, wouldn't it be a reasonable suggestion to use a battery charger? The reason I ask is because today I just checked my parent's car with a "smart" battery charger and their battery voltage was only 11.9V which apparently means it's nearly fully discharged with 11.8V being fully discharged battery (not under load) and 10.5v being fully discharged battery when under load. I'm now charging the batteries but to see this, it would explain why I had to replace the battery in their cars only after 6 years despite them being garage kept.

 

Yes a trickle charger is something you want to use. A car will drain a good healthy battery in about 30 days or so. All modern day cars have at least some type of draw from the PCM and its enough to drain the battery over time.

 

I have another car that gets 15K miles a year but when I put my charger on it, it says it has been discharged to 75/100. This would seem to indicate that even if you do drive your car a lot, they still overtime get discharged and that the alternator is at best good at maintaining charge but not actually for charging.

 

Depends on the type of battery and the type of loads placed on the battery. But yes batteries are wear items and will eventually die. A good brand name battery usually only lasts about 4 years or so in todays cars.

 

yeh no doubt no doubt