Does ocassionally draining and then recharging an auto battery increase the lifespan?
No. Battery lasts longest at a full charge.
Discharging and recharging a lead acid motor vehicle battery will reduce its life unlike some rechargable batteries used in laptop computers, cell phones etc. which can benefit from this deep cycling.
No, It will ‘kill it quick’.
You’ll have a completely useless battery after about seven cycles. As many as ten or as few as three are possible, depending on the age of the battery, but every cycle damages it significantly.
I agree, I killed a fairly new (less than one year old) battery a few weeks ago by letting it go completely dead several times (I was traveling and my alternator failed). Auto batteries do not like to be “cycled.”
Draining and recharging works best on NiCad batteries and is somewhat beneficial on NiMH and Lithium Ion batteries but is detrimental to Lead Acid batteries. Your car battery is a lead acid battery and is designed to stay charged. It will not improve its performance by being cycled. You will quickly destroy it by draining it.
If you need to discharge it from time to time, then you need a special deep discharge battery. Otherwise deep discharges will damage a car battery quickly.