Josh;
What you say is correct. Here is some amplification: For a given battery chemistry, the capacity of the battery is proportional to the weight of material in it. For a lead-acid battery that means more lead, more capacity. Basic Amp Hour ratings for deep cycle and marine batteries are published using a 20 amp discharge rate because that is an industry standard. Similarly Reserve Capacity ratings for automotive batteries are always for 25 amp currents. Some manufacturers publish capacity number in addition to the standard 20 amp rate. As far as Amp Hour capacity goes, the lower the current draw, the longer the battery will deliver that current. But it would not be a straight line on a graph of current vs. time. So a battery delivering 5 amps (typical for one of our boats) may well deliver 5 amps for significantly longer than the published AH rating divided by 5. There really is no relationship between Amp Hour capacity and cranking amps. For a given weight of lead, the more surface area in the plates (more and thinner, plus some refinements for conductor length and shape) the higher the cranking amps. The thicker and fewer the plate, the lower the cranking amps and the longer the battery will deliver lower current flow (Reserve Capacity). But a given weight of lead can only produce a given amount of free electrons, so the tradeoff is between cranking amps and reserve, not Amp Hour, capacity. An automotive starting battery has more thin plates, and is less suitable for the environment of a boat with vibration and deeper discharge cycles. Conversely, a deep cycle battery has fewer thick plate to deliver lower current for a long time. A marine start battery is sort of a compromise between the two, in an attempt to provide a happy medium between the two, and really is most desirable for inboard V8 applications in smaller powerboats. Remember that we don't usually try to start our boats at below freezing temperatures, so high CCA isn't generally needed. For that reason, the standard for marine cranking amps is determined at a higher temperature than CCA and the two ratings are not comparable. For the engines in our boats, delivering 200 amps or so to the starter is well within the capability of just about any deep cycle or marine starting battery, and will result in more durability and battery life and in longer hours on the hook between engine use. If your need lighter weight or smaller batteries for racing or day sailing, go with a smaller marine start battery like a group 24. Rick Brass From: CnC-List [mailto:[email protected]] On Behalf Of Josh Muckley via CnC-List Sent: Wednesday, February 25, 2015 9:49 PM To: C&C List; Edd Schillay Subject: Re: Stus-List Starting Battery I checked my Yanmar manual and it looks like it calls for a 100 Ahr battery for my 3 cyl 35hp diesel. So now you ask yourself, how do I compare CCA to Ahrs. The short answer is, you don't. CCA is a bit arbitrary but to achieve higher CCA in the same size you have to have more and thinner plates. This makes them more fragile and decreases their ability to be deaply discharged as well as survive vibration and uncovering. If you seek higher CCA exclusively then you will find a battery which is less suited for a marine environment. AHrs is a less arbitrary spec but still a small bit of fuzzy math since C/20 = 100 and C/10 = 80 and C/1 = 60 could all be Ahr ratings for the same battery. Most manufacturers provide their rating based on C/20. Ultimately, the best batteries (regarless of all other ratings) are the ones which are heaviest. These will have the most lead and subsequently the strongest plates. They will usually also be at the top end of the Ahr ratings but not the highest CCA (if those ratings are even mutually present). Disregard CA and RC ratings. A sufficiently large deep cycle battery is better than a smaller, lighter, starting battery. The weight is only an accurate comparison if the chemistry is the same. Josh
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