When selecting or finding service for your boat's batteries, there are many factors to consider; but it’s hard to consider those factors if you don’t understand all the acronyms and types of batteries out there. Unsurprisingly, there are different types of marine batteries and they all serve different purposes depending on how you intend to use your boat. Without further ado, let’s talk about marine batteries!

Battery Language

There are some terms used in “battery language.” When selecting a marine battery, you need to know these terms and their acronyms to help with purchasing decisions and maintenance.

  • Marine Cranking Amps (MCA) - number of amps a battery can deliver at 32 degrees Fahrenheit for 30 seconds while maintaining a voltage of 1.2 volts per cell
  • Cold Cranking Amps (CCA) - number of amps a battery can deliver at 0 degrees Fahrenheit for 30 seconds while maintaining a voltage of 1.2 volts per cell
  • Ampere Hour Rating (Reference Rating) – number of amps a battery can deliver for a 20-hour period; the higher the rating, the more power the battery can deliver in the time period
  • Battery Capacity (Ah) – amp hours (Ah) is the total amount of energy the battery can produce for 20 hours at a constant rate of discharge before voltage drops to 10.5 volts
  • Reserve Capacity (RC) – minutes a battery will deliver 25 amperes at 80 degrees Fahrenheit; length of time the battery would operate boat accessories should the alternator or generator fail

To determine the amperage draw of your boat, you have to add up the amp draw of each component, like the bilge pump, navigation lights, refrigerator, anchor windlass, radar, VHR radio, depth finder, inverters and so on. If there is a trolling motor, the amps for that are added too. Multiply the total amps for accessories and multiply by 20 to get a good estimate of your vessel’s amp hour battery requirement for a 20-hour period. Then add 20-percent to the total to accommodate factors like increased usage or decreased capacity due to battery age.

The 20-hour rate is a general industry factor, and it’s often expressed as a C rating with C standing for capacity. The C rating has a number like C-100 which is a value at a 100 hour rate. The basic rating for comparison purposes is C-20 (20-hour rate). A 12-volt battery with 100 Ah at a 20-hour rate means the battery should be able to produce 5 amps of electrical current for 20 hours before reaching a minimum voltage level of 10.5 volts.

Starting, Deep Charge or Both: Job Assignments for the Marine Battery

There are three main types of batteries – one for starting, one for managing a deep charge for steady energy production and one that can handle both jobs. The type of battery chosen depends on what you expect the battery to handle. If you only need a battery that starts an outboard motor or engine but not power electronics, then you can choose a cranking battery. If your boat is loaded with electronics, which any boat with an engine is today, you need a battery that has an enduring charge.

Starting or cranking battery –The starting battery has one main job assignment – quick start the engine. However, it can also power equipment like a bilge pump, lights and some electronics. It will have high cranking power and can be recharged quickly by the engine’s alternator. Its basic features include:

  • High energy burst of 75-400 amps
  • Energy bursts last 5-15 seconds
  • Energy is easily depleted, so recharged by engine’s alternator
  • Made of alternating negative and positive charged plates with insulation in between the plates
  • Power coming from the plate surfaces to start the engine so plates can be thinner
  • More fragile than other marine battery types because of thinner plates
  • Battery life is shortened when regularly used for long periods of time, like trolling, because it’s not designed for such use

The power level needed depends on the cranking needs of your motor or engine. The starting battery has its purpose, but realistically, the deep cycle battery is more practical.

Deep cycle battery – The deep cycle battery is designed to handle a continuous energy draw over a long period of time and to recover quickly.

  • Discharges slowly over a long period of time
  • Designed to provide continuous operating time to run trolling motors and electronics
  • Designed with heavier thicker plates with fiberglass reinforcement, special material for producing power and heavy duty separators so able to manage several hundred charging and discharging cycles
  • Power comes from deep within the plates
  • Lower MCA rating and higher reserve capacity/amp hour rating compared to starting batteries and dual purpose batteries
  • Doesn’t substitute for a starting battery because it doesn’t have the high energy burst needed to start a motor

Dual purpose battery – The dual purpose battery has enough power to serve as a starting battery and a deep cycle battery.

  • Has thick large plates
  • Can tolerate deep discharges
  • Lower storage capacity than a deep cycle battery
  • Higher cranking amperage, i.e. can start a 350-hp engine
  • Provides low amp draw service, i.e. minimum of 7 hours of continuous 10 amp 12-volt draw
  • Particularly good for uses like small powerboats, boats with one battery bank used for engine starting and accessories, and sailboats with two batteries used interchangeably

All in a Name: Types of Batteries

There are several types of batteries. The terminology gets a little difficult because batteries are like mini-chemical plants.

Flooded Batteries – During charging, the flooded battery uses a reservoir of liquid sulfuric acid to produce oxygen and hydrogen. The gases escape through vented wet cells in compartments in the battery box. The venting is crucial to safety because hydrogen is an explosive gas. The wet cells must be topped off with distilled water when the water level gets low.

The fact the flooded battery is not sealed means it can handle overcharging better than AGM and gel batteries. The drawback to this battery is that it will require off-season charging because it self-discharges at a higher rate than other batteries (approximately 6-7 percent per month). Proper installation is important because the wet cells must be in an upright position in order to properly vent. They also don’t handle high amounts of vibration well.

(Note: Though most flooded batteries are not sealed, there are sealed flooded batteries available in the market. In that case, you cannot access the cell compartments. The manufacturer has designed the battery so that it maintains the right amount of acid during the battery warranty period.)

Gel Batteries – Sealed, valve regulated (SVR) gelled-electrolyte batteries self-discharge up to 3-percent per month which is half of the rate of flooded batteries. The gel batteries offer the greatest number of charging cycles during the battery life. A pressure release valve manages the internal pressure, ensuring it doesn’t get too high. They resist over discharging. In fact, there is a 1-percent self-discharge rate per month at 68-degrees Fahrenheit.

Excessive discharging will shorten battery life. Since the batteries are sealed, they are maintenance free (MF), leak proof and spill proof. They are also submersible. Another big advantage is that the SVR design means gassing is minimized, so they can be installed around sensitive electronic equipment and people.

AGM Batteries – Sealed, valve-regulated (SVR) AGM batteries are made with fine microfiber glass separators that are highly porous. The separators are compressed tightly between the negative and positive plates. AGM is the acronym for “absorbed glass mat.” The fibers in the dense glass mats are embedded into the surfaces of the plates which provides reinforcement, and vibration and shock protection. The plates are saturated with acid electrolytes for battery activation.

The highly porous microfiber glass separators work by absorbing the electrolytes which in turn lowers internal resistance and boosts capacity which increases efficiency. While charging, pressure valves allow the oxygen produced on the positive plate to migrate with the negative plate. There it mixes with the hydrogen and produces water. This enables equal saturation across the whole surface of the plates. The batteries can be recharged faster because of the lower internal resistance. Once again, the SVR design eliminates acid leakage and gas emissions. They are maintenance free.

Thin Plate Pure Lead Batteries (TPPL) – These batteries are a variant of AGM batteries. They have plates stamped out of a pure lead roll with a low internal resistance. The densely packed ultra-thin plate grids reduce the time it takes for current to flow into and out of plate areas. The heating effect is reduced as a result.

The TPPL batteries have higher discharge and recharge rates compared to the conventional AGM batteries, largely due to the thin plates maximizing surface area. This can extend the life of the battery. However, it also means the battery will need to charge more frequently. A higher recharge rate means it takes less time to get a full charge. This type of battery has a high cycling capability. This is also one of the more expensive batteries because it’s newer technology.

Lithium-ion (Li NMC) – This is another advanced battery technology. It uses lithium ions as its electrochemistry. The battery can be 100-percent discharged but no more than 80-percent is recommended. It has 30-percent more usable amp-hours compared to other battery types. By adhering to the 80-percent discharge, you can get more than 2,000 cycles.

The Li NMC weights about half as much as lead acid batteries and approximately one-third less than AGM batteries. These batteries recharge at a fast rate (a little over an hour), and the self-discharge rate is lower than the rate of lead-acid batteries. It is a more expensive battery too.

Next steps

Now that you’re an expert on all different types of boat batteries, see our article on battery selection and maintenance to apply what you’ve learned.