12 Volt Batteries

Have you ever wondered what all those terms and acronyms refer to when shopping for a new boat or RV battery?  Terms like Deep-Cycle,  CCA, MCA, Ah and Gel don’t mean a thing to many of us.  But, those terms do indeed have meaning, and they can have an effect on your pocketbook, and/or the lifespan of your new battery.

Generally speaking, there are three distinct types of batteries in use on today’s boats and Rvs;  Flooded Acid, Gel’d Acid and AGM (Absorbed Glass Mat).  And, (still speaking generally) these three types of batteries are divided into two categories:  Deep Cycle, and Starting.

A battery is a device which stores energy; stores it; not manufactures it, but stores.  It’s much the same as collecting rain water in a barrel for later use.  Batteries store this energy in the form of electricity, and are able to do so with a chemical reaction, using sulfuric acid as an electrolyte with lead plates.  The grids of lead plates (sometimes a lead alloy) are suspended in a grid within the battery case, whereby they are surrounded with the electrolyte.

The type of electrolyte will differentiate the three “types” of batteries, referred to earlier:

1) FLOODED ACID  batteries are the conventional “add-water” types we have grown up with in the family automobile.  We all remember opening the small plastic caps and checking the water (acid) level,  to ensure each cell was full to the indicator ring.  Most recently, batteries have been available without the little caps, and are now called “Maintenance Free”.

2) Then came GEL ACID batteries.  Gel batteries offer ease of maintenance, as the water (acid) inside has been thickened with silica gel, which turns it to a jelly. The jelly won’t  evaporate or spill; even if the outer battery case is cracked open.  These batteries quickly became popular in wheel chairs, Rvs and snowmobiles as the gel offered support to the fragile lead plates in areas of impact and vibration. Additionally, the inability to spill acid as in the flooded acid types, made them safer for travel.  A disadvantage of the gel battery is it cannot be “fast-charged”.  Therefore the battery charging equipment may have to be adjusted to the battery manufacturer’s specs. To avoid damaging the battery.

3) The newest lead acid type of battery is known as the AGM (Absorbed Glass Mat) type.  This technology offers all the advantages of the Gel battery, yet it does all things better, having no disadvantages when compared to the flooded acid types, other than cost.   (2 -to- 3 times that of the flooded acid)

A battery will self-discharge, due to internal resistance; even when not in use.  Flooded acid batteries will discharge at the rate of  approximately 1% each day.  Gel and AGM batteries (sometimes referred to as VRLAs*) will discharge at around 1% -to- 3% per month. * Valve-Regulated Lead Acid

Batteries also lose (or convert) some of the energy put into them into heat, while being recharged.  While a flooded acid battery will convert 15% – 20% into heat loss, a gel battery will lose only 10% – 16% to heat and the AGM will only lose around 4%.  Thus, your charging system can be smaller, or simply recharge faster.

“Starting” and “Deep Cycle”

Starting batteries are constructed to produce large amounts of current for short periods of time, for high amperage needs, such as starters and anchor windlass’.  Deep cycle batteries on the other hand, are designed to produce a smaller flow of current, over an extended period of time.

The DOD (depth of discharge) is a measurement of how much energy is removed from a fully charged battery.  Several factors will determine battery life.  A battery cycle is defined as bringing a battery from full charge, to DOD and then back to full charge.   A battery which is not routinely depleted to a deep DOD (say 50% DOD) will out-live one that was depleted 80% each cycle.  Other factors play into battery life as well, including ambient temperatures.  While operating in warmer temperatures provide a battery with more capacity, it also shortens the lifespan.

An interesting note:   A fully charged flooded acid battery will not freeze in winter temperatures; a dead battery will freeze.  Also, the VRLA batteries do not have to worry about freezing temperatures.

This brings out another advantage of the VRLA batteries:  They do no emit gases or water, during recharging.  They are termed “recombinant” – which means the hydrogen and oxygen re-combine inside the battery, with no outside emissions.  These batteries are suitable for installation areas where no ventilation is available, such as bilges, which are nice and cool.

So you ask:  What part of the construction makes it a “Deep Cycle versus Starting?”

It’s the lead plates.  A Starting battery will have many thin lead plates that appear sponge-like, creating a lot of surface area.  A  Deep cycle battery will have much thicker, solid, smoother surfaced (not sponge) plates inside.   A deep cycle battery is capable of  up to 80% discharge, although a better cycle would be 50% DOD on a continuous basis.  A starting battery is designed for a cycle of 2% -to- 5% DOD, and if taken to deep cycles on a routine basis, the thin sponge-like plates will quickly deteriorate.

The larger (sponge-like) surface area of the starting battery provides added area for chemical reaction, creating high current flow for starting.  While the thicker, solid plates of the deep cycle battery cannot produce the rapid, high current, they are built for the “long-run”, and are more durable.

Group Size is a description of the physical size of the battery case, and terminals.  It does not necessarily refer to the electrical capacity.

Terms used to describe Starting Batteries:

CCA and MCA refer to “Cold Cranking Amps” and “Marine Cranking Amps”  Each is a rating of current flow ability of the battery.  MCA is the same as CA “Cranking Amps”.    The difference is, CCA is measured at 0 degrees Fahrenheit, whereby MCA and CA are measured at 32 degrees Fahrenheit.

Deep Cycle batteries are described in terms of Amp Hours (Ah):

The ability to produce 1 amp of current for one hour = 1 Ah.  Or, 2 amps for ½ hour is 1Ah; 20 amps for 1 hr = 20Ah, and so on……..


When shopping for a battery, a good rule of thumb is the weight of the battery.  The heavier the battery, the more lead in the plates.  Lead plates deteriorate over time.  Therefore, more lead can relate to a longer battery life.

Keep your batteries charged, and cool.

Flooded Acid versus AGM  (Gel is less favorable when compared to AGM for similar money):  Flooded acid is 1/3 -to- ½ the cost of AGM or Gel.  Flooded acid is a “good fit” for the money, if ventilation and spills are not an issue.  But if the application allows, then “Flooded Acid” batteries are still widely used, as they can be more cost effective.

It is a myth:  Sitting a battery on a concrete floor will discharge it.

The deeper the cycles (DOD) of discharge;  The fewer cycles in the lifespan.

Don’t over-charge ( or “quick-charge”) gel batteries; it can cause permanent damage, by creating bubbles (voids) in the gel.

Can battery banks mix flooded cells batteries with gels and AGMs ?

Some say it’s okay.  But I don’t recommend it.  In fact, I’ll go one step further and recommend that all batteries within a bank are the same size, type and age.  The reason for this is to maintain equal charge/discharge rates among the partners.  Internal differences, create unwanted current flow between the individual batteries.  A battery’s electrical characteristics change with age.  Therefore, it is advantageous to use batteries of the same size, type and age within a single bank.  IE:  Don’t install a brand new replacement, into a bank of older batteries, if it can be avoided.

Ron Smith

Principal Surveyor For:

Tr-State Marine Services, Inc.

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