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If you drive the car regularly for a reasonable amount of time (that is, "drive it" doesn't mean 3 minutes to the corner store!) then the alternator will properly charge the battery and there will be no material depth-of-discharge. Starting batteries DO NOT like any depth-of-discharge AT ALL; they will be severely damaged by even *ONE* such excursion below ~50% of capacity, so if you like to sit with your car not running and blast the stereo.....
All storage batteries self-discharge at a small, but real rate. There is also parasitic drain on all car batteries in the modern era since they all have electronics that are powered up all the time (e.g. for the push-button fob on the key.) The drain is not large, but it's there and over a period of weeks or months it matters. Thus, if you don't drive it regularly then a tender is a very good idea.
Your typical car battery has ~80AH of energy in it. If there is a 100ma draw for that parasitic load then you have ~800 hours of "idle time" before it's dead. But typical starting service only draws a couple of percent of the available power out of the battery, and impact on service life starts for a starting battery begins at about 20% depth-of-discharge, so in REALITY if you regularly sit for 160 hours between uses (a week), and those uses don't have enough runtime to replace the ~16 AH you consumed... oops! This is also why short trips are nasty as there isn't enough charge time to replace the consumed energy and thus the battery's state of charge continually decreases over time until it starts accumulating serious damage -- and shortly thereafter dies.
In addition it's pretty easy to consume 10 amps or more of power sitting in ACC mode. Do that for an hour and a half (e.g. listening to the stereo) and you're into the "damage" range, ESPECIALLY if you don't quickly run the vehicle for enough time to replace all the energy you consumed.
If your usage pattern fits something like this then a charger while idle is a real good idea. For those who don't fit such a usage pattern, however, it's unlikely you'll see any material difference in its use. In addition while many batteries are claimed to be "maintenance free" with the exception of AGMs or Gels they're NOT -- and if the water level in any of the cells gets low that will immediately and permanently damage the battery too, so you NEED to check that -- especially if you have a maintenance charger on the battery since if it's even slightly out of calibration water consumption will be excessive.
Most "early death" battery problems in cars, save for the rare factory defective battery, are due to depth-of-discharge abuse -- specifically, sitting in ACC mode (and especially if you have an aftermarket, large amp that draws lots of power!) Don't do it. If you must do it buy an AGM battery (they're expensive) which can tolerate that sort of abuse better, but expect shorter service life anyway, and make damn sure after such an episode you either charge it externally or drive the car long enough to fully recharge, which will take AT LEAST a half-hour of operation. SOME modern alternators have at least two-stage charging regulators (older ones, and many newer ones don't) and if you have a voltmeter on the dash (my Suburban does) you can tell when it switches out of bulk mode as the voltage will drop about a half-volt. From there you need AT LEAST 30 minutes of additional driving for the absorption charge phase to reach reasonably close to "full."
PS: Never put anything but distilled water in a flooded battery. Any minerals or contaminants in the make-up water will react with the sulfuric acid and destroy the cell(s) in question.
Last edited by tickerguy; 12-18-2018 at 12:21 PM.