Batteries fall into three rough categories: automotive starting batteries, deep cycle batteries, and dual purpose. Marine starting batteries fall into the dual purpose category.
For lead acid (aka wet cell) batteries, the electrical capacity is a function of the weight of lead in the battery. A heavier battery has more capacity. For a given amount of lead, there are three ways to use that capacity: Automotive start batteries have many thin plates. This gives more surface area to the lead, and gives higher cranking amps. It also reduces "reserve capacity" which in an automotive battery is how many minutes the battery will produce 25 amps of current - that is the average current draw of a car running with the alternator not working. Thin plates make the battery prone to early failure when subjected to shock and vibration. But a car is a relatively shock free environment, and an automotive battery seldom gets discharged below 90% charge, so low amp hour capacity and lower durability are not a problem. A deep cycle battery has fewer but thicker plates. It is designed to use the electrical capacity of the lead to maximize the amp hours of current produced - which is the product of the current delivered times the number of hours for which the battery will deliver that current. Deep cycle batteries are designed to be discharged to 50% for several hundred cycles. The thicker plates make them much more durable. The have less surface area, so produce lower cranking amps. A dual purpose (marine start) battery is a compromise. Fewer plates than a starting battery but more than a deep cycle. Greater durability than an automotive battery and more cranking amps than a deep cycle. Cranking amps in a marine environment are different than in your car. My 4 cylinder 36hp Westerbeke draws 170 amps to start. Seldom is it started in near zero conditions, and when it is started in the cold, the glow plugs assist easy starting. A car engine will often need 250-350 amps in normal condition and is often started in sub zero weather. So 700-800 CCA makes some sense for a car battery but is not needed in a boat battery. The setup on my 38 is 4 deep cycle group 27s with total of 460AH for the house bank plus a group 29 marine start batterie for the engine (which is overkill but cost effective) set up as bank 2. The area under my quarter berth is large enough for all of the batteries and wiring, plus battery tools, distilled water bottles, and some other miscellaneous stuff. I did need to put a second access panel into the top of the locker under the quarter berth in order to give access to the top of the enlarged house bank. BTW, my deep cycle batteries were purchased in 2006, and the start batter was replaced in 2011, if that gives you an idea of the durability of these types of batteries. Rick Brass Former product manager for batteries for a construction machinery manufacturer and former design engineer and aftermarket marketing mam Anger for a forklift manufacturer. Sent from my iPad > On Sep 28, 2014, at 10:46, Indigo via CnC-List <cnc-list@cnc-list.com> wrote: > > Looks like I need new batteries! I have lived for 12 years with two > batteries - one used for house and the other kept in reserve via off-1-all -2 > switch. > > Would like to have a dedicated starting battery in addition to two deep-cells > but do not see how I can fit the three in the locker under the quarter berth > - and am reluctant to give up any other locker space. - so it looks like I > will stick to the two. > > I googling marine batteries I see that there are dual purpose ones available. > Anyone know if this is just a marketing "hoax". In particular Batteries Plus > have a Group 27 dual purpose Duracell battery for $95 - seems like a steal - > or am I going to be disappointed with how long I can expect it to last (I > presume all group. 27 give out the same amp/hours - or do I have to watch out > there? > > -- > Jonathan > Indigo C&C 35III > SOUTHPORT CT > _______________________________________________ > This List is provided by the C&C Photo Album > > Email address: > CnC-List@cnc-list.com > To change your list preferences, including unsubscribing -- go bottom of page > at: > http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com > _______________________________________________ This List is provided by the C&C Photo Album Email address: CnC-List@cnc-list.com To change your list preferences, including unsubscribing -- go bottom of page at: http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com
