Cranking amps don't really matter in your case because we are not cranking over an engine, just spinning a little trolling motor.
The bigger a battery is, the more lead and the more acid is going to be inside of it. The more lead and acid, the more power, both in cranking and reserve.
What you need to look at is not the cranking amps, but the reserve capacity. And that will vary size by size. The group size is not the capacity of the battery, or its power, but just its physical size in height, width and depth.
When you go to the stores to look at the batteries, don't even look at cranking amps, because it just does not apply to you. (we are not cranking over an engine) Look at reserve capacity. Reservice capacity is how long a battery can supply current (amps) before the voltage drops. Most batteries are rated at 25 amps. So the battery I gave you above had a 200 minute reserve capacity. At a full 25 amp draw, the battery should last 200 minutes before it needs to be recharged. Compare batteries by size and reserve capacity.
All batteries, both starting type and deep cycle types will list both cranking amps, and reserve capacity. A starting type makes good cranking amps, but has little reserve. A deep cycle will not deliver as much cranking amps, but it will have a lot in reserve. They are opposites.
I am copying and pasting this from a battery FAQ that I keep. But here are more or less textbook definitions between the battery types.
- 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. Automotive batteries will generally fail after 30-150 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. This gives less surface area, thus less "instant" power like starting batteries need. Although these an be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge.
- Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The golf car battery is quite popular for small systems and RV's. The problem is that "golf car" refers to a size of battery (commonly called GC-2, or T-105), not the type or construction - so the quality and construction of a golf car battery can vary considerably - ranging from the cheap off brand with thin plates up the true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.
- Marine batteries are usually a "hybrid", and fall between the starting and deep-cycle batteries, though a few (Rolls-Surrette and Concorde, for example) are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid. Starting batteries are usually rated at "CCA", or cold cranking amps, or "MCA", Marine cranking amps - the same as "CA". Any battery with the capacity shown in CA or MCA may or may not be a true deep-cycle battery. It is sometimes hard to tell, as the term deep cycle is often overused. CA and MCA ratings are at 32 degrees F, while CCA is at zero degree F. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open - not much of an option.
Let me know if you still have questions about this.
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