Marine Battery types, terminology and properties - by Peter Kennedy Yacht Services

Deep cycle These are normally used for house batteries on boats and are intended for prolonged discharges. They have heavier plates to avoid damaging buckling that is caused by deep discharging, but because of the heavier plates thay have less surface charge than starting batteries and less cranking ability for any given size. The capacity of deep cycle batteries is measured in Amp Hours. Other qualities of deep cycle batteries such as the weight and strength of the plates and thus the life expectancy are not reflected in the amp hour rating.

Starting Starting or cranking batteries have plates which maximize the surface area and thus provide the maximum cranking ability for any given size. They are not designed to be deeply discharged as this will cause the plates to buckle and fail prematurely. Starting batteries are measured in CCA for cold cranking amps, or occasionally in MCA for marine cranking amps.

Dual Purpose These batteries combine the best (or worst) features of the other types. They are normally found in small runabouts and are not really suitable for larger vessels.

Gell cell Gell cell batteries were developed for applications where acid spill were prohibited. The electrolyte is in the form of a gell which is sealed inside the case. They are maintenance free, can accept high recharge rates and are less subject to damage if left in a discharged state. They can be deep cycled fewer times than a heavy duty wet cell battery but despite this and their high cost they are justifiably popular with boaters. Gell cell batteries need to be charged at a lower voltage than wet cell batteries so care must be taken that the charging equipment can be set or adjusted to suit, otherwise premature failure will result. An example of a Gell cell battery is Prevailer.

Wet cell Deep cycle wet cell batteries are the work horses of most boating installations. Premium models such as the Rolls battery are less common due to their high price but the Trojan golf cart battery is very common and provides the best value for money of any of the battery types. Many other types are available, its hard to choose a type but usually cost and weight are a factor. You get what you pay for, and heavier is better.

Sealed wet cell These are normally found as automotive batteries which can be used for starting but are not really suitable for deep cycle applications.

AGM Absorbed Glass Mat batteries are the newest type commonly in use. They have been used by the US navy for years but have been relatively untested in the recreational boating market. They are spill proof, maintenance free, have a low self discharge rate and are relatively expensive. Except for the Optima battery shown above I have relatively little experience of their long term use.

I normally like to put house batteries in one large bank and set up the switching in such a way that the house supply can only come from this bank. I then set up a smaller starting battery with provision for starting the engine from either bank in an emergency.
This is not the way production boats normally come, but it has considerable advantages. It is simple, it provides protection from accidental discharges for the starting battery, and it can prolong the life of the house bank by having a large bank that is less likely to be heavily discharged.

My preferred battery layout drawing. Negative wires omitted for clarity.

CCA Cold cranking amps. Maximum number of amps a battery can supply for 30 seconds at a temperature of 0 degrees F, before dropping to 7.2 volts. This is a more severe test than MCA and results in a lower number.

MCA Marine cranking amps. This is similar but is measured at 32 degrees and so results in a higher number for the same battery.

Amp Hours This is normally measured at a 20 hour rate. The amp hour rating is the number of amps a battery will supply for 20 hours at 80 degrees F before dropping to 10.5 volts, multiplied by 20 hours. ie If a battery can supply 5 amps for 20 hours then it is called a 100 Amp Hour battery.

Reserve Capacity The number of minutes a battery will supply a specified constant current (normally 25 Amps) at 80 degrees F before dropping to 10.5 volts. This measure is more appropriate for an automotive application and is intended to tell you how long you can keep driving with a dead alternator.

Want to learn more about batteries? Charlie Wing's book gives an excellent breakdown of the various types and the economics of their use.
Also try Battery FAQ's on this website. List of Battery Manufacturer websites.