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I beg to disagree. I can assure you that deep cycle batteries DO NOT mean that their life is extended by deep levels of discharging. Check with any manufacturers site to verify this. Trojan, etc... Deeply discharging those batts will cut their life span in half or more. I think this old wives tale got started in the early days of NiCad battery manufacture. Early NiCads had to be deeply discharged in order to avoid the memory syndrome. Whether the lead acid batteries are flooded or starved electrolyte, starter or deep discharge, maximum life expectancy will always be achieved with shallow discharging and frequent charging. I have been doing lead acid battery research into recovery and restoration techniques for over a year. The longest life expectancy achieved from la batts will be if it is kept cool, with clean tops (acid spill on the tops aggravate self discharging), at float voltage (trickle charged), desulfation pulsed (full time), and given a equalization charge (15V for a 12V battery) periodically (follow your manufacturers recommendations). The following is an extract from a manual I had written for a custom built desulfator with aeration controls.
Lead Acid Battery Cycle:
When lead acid batteries discharge, some of the sulfuric acid in the electrolyte is converted to lead sulfate crystals and begin to coat the plates. These crystals interfere with and reduce the capacity of the battery. During recharging operations, sulfate crystals are converted back into sulfuric acid. Most, but not all of the sulfates will be reconverted. These stubborn patches of sulfates will continue to grow and will harden to a class III bond. If unchecked they will grow to the point of battery failure. Sulfation is the failure mode of 84% of lead acid batteries that are discarded yearly. Sulfation on the plates are in three classes in bonding strength to the plates or grades of difficulty to remove. The first grade is removed by recharging. The second grade rapidly dissolves with pulsing. The third or hardest grade (long term sulfation deposits) SLOWLY dissolve with pulsing, as the chemical bonds are really tough to break. Equalization charges, while it accelerates the consumption of the positive plate(s) serve two purposes. First it is a crude and inefficient method of desulfation. Secondly, it creates gassing in the battery. This results in the stirring of electrolyte. Without this stirring, electrolyte can stratify (layer) with the acid settling to the bottom of the case. This condition will cause the bottoms of the plates to be rapidly consumed and severely reduce capacity.
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