Starting Batteries vs. Deep Cycle
Batteries are constructed of a series of positive and negative plates. In starting batteries the plates are thinner and there are more of them. This allows the batteries to produce high, brief amp bursts and crank your engine starter.
You will want a deep cycle battery to operate your electric trolling motor. Deep cycles store up energy to gradually power electronic devices. The deep cycle has thicker plates internally, and it can be used for starting in a pinch but they do not like to repeatedly dump the big amps in one quick chunk that a starting battery (with thinner plates) is designed to do.
· Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Starting batteries will generally fail after 30 or so deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).
· Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores that specialize in automotive batteries. Deep cycle batteries are commonly used to run electric trolling motors.