Types of batteries
Batteries on your boat handle two basic kinds of tasks, starting an engine and running electrical loads like lights, electronics and accessories for longer time periods. To choose a battery, first determine the battery’s application and then choose from one of the three battery chemistries: Flooded, Gel or AGM.
Starting Batteries: Starting batteries, which crank the starter of your boat’s engine, are the sprinters of your electrical system. They deliver between 75 and 400 amperes for 5-15 seconds, and then are recharged in short order by your engine’s alternator. Like all lead-acid batteries, they are constructed with alternating layers of negative and positive plates with insulation between them. Starting batteries have thinner and more numerous plates, providing extra surface area to generate high amperage bursts of current. The two drawbacks of this construction are that the plates are relatively fragile in high-impact environments, and that starting batteries do not tolerate deep discharges, which reduce their operating lifespan.
Deep Cycle Batteries: Your boat's House battery bank uses Deep Cycle batteries, the marathon runners of the storage system. They power the electrical loads on your boat when no charge source (shore power charger, engine alternator, wind generator or solar panel) is available. Consider them a kind of savings account into which energy is deposited or withdrawn.
   

Compared to starting batteries, which deliver high bursts of energy for short periods, deep cycle batteries recover fully after being heavily discharged over longer periods because their design features thicker plates with a high content of antimony. Overnight, their use might deplete 50-70% of the battery capacity, depending on the house loads of the boat. When the batteries are recharged, energy is re-deposited into the bank, and the process, or cycle, starts over.  

Dual-Purpose Batteries: We generally advise that you choose either a deep cycle or starting battery for best performance and battery life, but dual-purpose batteries work well in some applications. With large thick plates containing more antimony than starting batteries and an active lead paste chemistry, dual-purpose batteries are a good compromise, tolerating deep discharges that would ruin a typical starting battery. Since they have lower storage capacity than comparably sized deep cycles, we recommend them for the following applications:
   

• Runabouts or other small powerboats using a single battery for both starting and running loads with the engine turned off.
• Sailboats with two identical batteries used interchangeably for starting and house electrical loads
• Boats with one battery bank that does double duty for house applications and engine starting. Dual-purpose batteries will last longer and give more reliable service than a starting battery, for about $20 more per battery.

What to look for
Starting functions: The amount of power available for cranking a starter is measured several ways:
CCA vs. MCA: The two common power measurements are CCA (Cold Cranking Amps, the number of amps a battery can deliver for 30 seconds at 0°F while maintaining its voltage above 7.2 volts) and MCA (Marine Cranking Amps, similar but measured at 32°F instead of 0°F). The reason that MCA are 20-25% higher than the CCA is because batteries work better at higher temperatures.
Reserve Minutes indicate how long a battery can sustain a load of 25 amps before it drops to 10.5 volts. A battery rated at 150 minutes can operate a 25A load for 2 1/2 hours. Starting batteries aren’t used to handle loads for long periods, so reserve minutes are less critical.
Size: Engine size, type, and ambient temperature determine what size cranking battery you need. High cranking power (and a larger battery) is required for cold temperatures, diesel engines, or large and high compression gas engines. If a Group 24, 550 CCA battery worked well for five years, we’d recommend replacing it with a similar model. If, however, it cranked too slowly, or failed after a season or two, we’d suggest that you look for a battery with a higher CCA or MCA rating.
Deep Cycle Functions
   

Battery Capacity measurements are commonly expressed in Amp-hours (Ah) and Reserve Minutes. Amp-hours measure the total amount of energy that a battery can deliver for 20 hours at a constant rate of discharge, before the voltage drops to 10.5 volts. This means that a 200Ah battery can run a 10A load for 20 hours. The reserve minute rating is the number of minutes that a battery can run a 25A load until dropping to 10.5V, just like with starting batteries. A Group 27 deep cycle battery with a rating of 180 reserve minutes will run a 25A load for three hours. House loads range from 5A to 25A or more. Ah is generally the more relevant measurement for house banks.

Longevity
   

Battery manufacturers measure longevity by discharging full batteries until their voltage drops to 10.5 volts. The batteries are recharged under controlled conditions, and the process is repeated until the battery fails to hold half of its rated capacity. This measurement, called cycle life, shows how many discharge cycles a battery provides over its lifespan. This ability to cycle repeatedly is what differentiates deep cycle batteries from starting batteries, which can’t withstand more than a few deep discharges before they begin to fail. If nothing else, cycle life provides a baseline for comparing one battery to another.

Choose the right battery chemistry
Marine batteries are available in three chemical types: flooded, Gel and AGM (Absorbed Glass Mat). Which type you choose is based on your needs (deep cycle vs. starting), the capacity and lifespan you are looking for, and your budget.
Flooded batteries, unlike other types, use a reservoir of liquid sulfuric acid to act as a pathway between positive and negative plates, which produce hydrogen and oxygen when the battery is being charged. Vented wet cell batteries allow the gases to escape into the atmosphere, unlike gel and AGM batteries, which recombine the gases and re-introduce them to the system. Vented hydrogen is an explosive gas, so battery boxes and compartments must be vented to let the gas escape safely outside the boat. Flooded deep-cycle batteries require maintenance-periodic inspection and topping-off with distilled water.
Flooded batteries handle overcharging better than gel and AGM batteries, because of this hydrogen venting and because they are not sealed like the other types. They self-discharge at a higher rate (6 to 7% per month) and thus require off-season charging. Wet cells must be installed in an upright position and do not tolerate high amounts of vibration. Their initial cost is lower than similarly sized AGM or gel batteries. Properly charged and maintained, our premium wet cell deep-cycle batteries are capable of between a few hundred and over a thousand discharge cycles, which can translate to many years of dependable service.
Gel Batteries: The SVR design nearly eliminates gassing, so they are safer to install around people and sensitive electronics (but gel and AGM batteries still need to be vented). Gel batteries are manufactured to very high standards of quality and consistency, since it is not possible to add water or gain access to the interior. The "gel" is a combination of sulfuric acid, fumed silica, pure water and phosphoric acid. After mixing during manufacturing to a thin liquid form, it is sucked into each cell by vacuum pressure up to six times, eliminating voids and air pockets on the plates that would cause dead spots and reduce performance. Once it is in place, the gel becomes quite viscous, which prevents leaks if the battery is inverted or the case is damaged.
Charging causes a small amount of hydrogen and oxygen to be generated at the plates, like a flooded battery, but the pressure inside the cells combines the gasses to create water (so they are called "recombinant" batteries). This keeps the battery from drying out due to charging, but it also requires that the vessel’s charging system be very carefully regulated to prevent high voltage over-charging.
AGM batteries: Sealed Valve-Regulated AGM (Absorbed Glass Mat) batteries feature fine, highly porous microfiber glass separators compressed tightly between the battery’s positive and negative plates, which are saturated with just enough acid electrolyte to activate the battery. During charging, precision pressure valves allow oxygen produced on the positive plate to migrate to the negative plate and recombine with the hydrogen, producing water. In addition to providing equal saturation across the entire surface of the battery’s positive and negative plates, the fibers in the dense glass mats embed themselves into the plates’ surface like reinforcing rods in concrete, providing more plate support and better shock and vibration protection than in conventional batteries.
High-density AGM batteries have lower internal resistance, allowing greater starting power and charge acceptance, and quicker recharging than other types of deep cycle batteries. High acceptance means that AGM batteries can accept the highest charging current, up to 40% of the amp hour capacity of the battery, compared to about 25% for the other two battery chemistries. Long life, a low 3% self-discharge rate and outstanding performance make AGM batteries excellent dual-purpose batteries for boaters who require quick starting power and reliable deep cycle ability.
Battery tips for best performance
No matter what kind of battery chemistry you choose, follow these recommendations to get the best performance:
   

• Stay with one battery chemistry (flooded, gel, or AGM.) Each battery type requires specific charging voltages. Mixing battery types can result in under- or over-charging. This may mean replacing all batteries on board at the same time.
• Never mix old batteries with new ones in the same bank. While it seems like this would increase your overall capacity, old batteries tend to pull down the new ones to their deteriorated level.
• Regulate charge voltages based on battery temperature and acceptance (manually or with sensing) to maximize battery life and reduce charge time. Ensure that your charging system is capable of delivering sufficient amperage to charge battery banks efficiently. This generally means an alternator with 25% to 40% as many amperes as the capacity of your entire battery bank.
• Keep batteries clean, cool and dry.
• Check terminal connectors regularly to avoid loss of conductivity.
• Add distilled water to flooded lead acid batteries when needed. Keep them charged. Leaving them in a discharged state for any length of time will damage them and lower their capacity.
• Clean corrosion with a paste of baking soda and water.

   

 

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   Need more juice?
If your boat came from the factory with only one battery, or with two batteries of inadequate size, consider installing a second battery bank. Boats often need two banks to have a second "reserve" bank to be sure the engine will start, or to have a large "house" bank so you can run bigger DC loads for longer time periods. If you are installing a reserve starting battery, you can usually ensure enough amps of cranking power by duplicating the type and size of your original battery. To install a "house" bank, you should probably invest in a quality deep cycle battery.
Marine Batteries
Starting batteries are similar to automotive batteries. They can supply lots of current for a short period of time, but like to be recharged almost immediately and are not tolerant of deep discharges. They are appropriate as the single battery for ski boats, runabouts, personal watercraft, and other boats with minimal DC loads where the engine is always running.
Deep–cycle batteries can endure repeated deep discharging and recharging without damage. They are appropriate for sailboats, fishing boats, and all other boats with larger DC power requirements. Any battery used to power onboard DC loads (lighting, trolling motors, inverters, etc.) should be a quality deep–cycle battery.
 
Safety Precautions
 

   

• Batteries contain a tremendous, and potentially dangerous, amount of stored energy. Whenever you are working on your electrical system, remove the positive battery cables from the battery terminals to eliminate the chance of a short circuit and/or possible electrocution.
• The American Boat and Yacht Council (ABYC) publishes safety standards for circuit protection, wire sizes, and other related issues in electrical systems. Where appropriate, we have provided information on circuit breakers and wire sizes which are in accordance with these guidelines. The complete text of these standards is available in reprints from the ABYC.
• Since the connections made in the battery circuits can conduct hundreds of amps, it is imperative that you have low resistance connections. This means having clean metal–to–metal connections, the right size terminals which are properly crimped and waterproofed, and secure mechanical fasteners.
• For outboard engine applications we recommend replacing wing nuts on the tops of the battery with nylock nuts or hex nuts and lock washers that are tightened to at least 10 foot pounds of torque. For inboard engine applications you should use clamp–on battery terminalsconnected to the correct gauge of battery cable.
• Be sure to wear safety glasses when working on batteries and have a water hose or other available method of flushing battery acid out of the eyes and off the face. Be careful of using un–shielded droplights or any open sources of heat or flame around the batteries. Be very careful with uninsulated tools that might short across the battery terminals.