How to Select a Marine Battery
By Tom Burden, Last updated 4/7/2021
Types of Batteries
Batteries on boats handle two basic kinds of tasks, which are 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 four battery chemistries: flooded, gel, AGM or Lithium.
This Group 24 marine starting battery is not just a car battery. It uses lead-calcium chemistry, has a no-spill design, and can be tilted to a 45° angle.
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 Consider them a kind of savings account into which energy is deposited or withdrawn.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 percent 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. Generally, deep cycle batteries should be sized to store three to four times the expected amount of energy to be used between recharge cycles.
Group 24 Dual-Purpose Battery
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.
Marine batteries are available in four chemical types for different applications: flooded, gel, AGM (Absorbed Glass Mat) and Lithium. Which type you choose is based on your needs (deep cycle vs. starting), the capacity and lifespan you are looking for and your budget.
Six volt “Traction Batteries” are heavily over-built for industrial applications. This GC2 deep cycle battery (Model 15020340) is rated at 230 amp-hours and weighs 64lb. You'll need a pair connected in series for 12 volts.
Thin Plate Pure Lead (TPPL) and Lithium (Li NMC) Batteries
New technologies will require new ways of thinking about your boat's battery system. NorthStar TPPL batteries are among the most advanced AGM batteries in the world. They're made from 99.99% pure lead that's rolled (not cast) into thin plates. Combining the pure lead's low electrical resistance and greater surface area from more and thinner lead plates, they can accept much higher charging amperage than typical AGM batteries. Not only do they charge incredibly quickly, but they're capable of 400 charge/discharge cycles with an amazing 80% depth of discharge.
These batteries add intriguing options for long distance cruisers. Instead of installing a 1000Ah battery bank and running your diesel for six hours, you can install a 500Ah TPPL bank and charge more frequently, but faster. Or you can forego a genset and install one or more very high-output alternators—turning your propulsion engine into a high-output charging machine. Your diesel will run for short periods but with a high load (which it wants).
Lithium/nickel/cobalt/manganese batteries, like those used in Torqeedo electric outboards, weigh less than lead acid batteries, can be discharged 800 times to 100% depth of discharge, and can be recharged in a little over an hour. They excel as battery banks in electric boats, as well as for other high-performance battery tasks.
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Flooded batteries, unlike other types, use a reservoir of liquid sulfuric acid, and produce hydrogen and oxygen when the battery is being charged. Vented wet cells allow the gases to escape into the atmosphere, unlike gel and AGM batteries, which recombine the gases and re-introduce them to the system. Hydrogen is an explosive gas, so battery boxes and compartments must be vented to let the gas escape safely.
Flooded batteries require periodic inspection and the cells must be topped-off with distilled water when levels get low. Since flooded batteries are not sealed and allow excess hydrogen to escape, they handle overcharging better than gel and AGM batteries. They self-discharge at a higher rate (6 to 7 percent per month) and thus require off-season charging. Wet cells must be installed in an upright position and don't tolerate high amounts of vibration. Their initial cost is lower than similarly sized AGM or gel batteries, and MUCH lower than the new type of lithium batteries. Properly charged and maintained, wet cell deep-cycle batteries are capable of between a few hundred and over a thousand charging cycles.
Sealed, valve-regulated (SVR) gelled-electrolyte batteries offer advantages over regular flooded batteries. They self-discharge at only three percent per month, handle the highest number of lifetime charging cycles, are maintenance free, spill proof, submersible and leakproof. A pressure release valve keeps their internal pressure at a slightly positive level, but they can release excess pressure if needed. 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, because they're sealed, are manufactured to very high-quality standards. They need carefully regulated smart charging to prevent damage.
More boaters are switching to this type for a performance improvement over flooded 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, up to 45 percent of the battery's total capacity, and quicker recharging than other types of deep cycle batteries. Long life, a low three percent self-discharge rate and outstanding performance make AGM batteries excellent dual-purpose batteries for boaters who require the fastest recharging, quick starting power and reliable deep cycle ability.
Northstar's Thin Plate Pure Lead AGM 400 sealed battery offers great resistance to vibration, high cranking amps and is capable of cycling 900 times at 50% of their capacity.
What Battery Ratings 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 (at 80°F). 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. The first sizing criteria is to meet the minimum CCA (if any) stated by the engine or boat manufacturer. 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. Amp hours is generally the more relevant measurement for house banks.
Longevity: Battery manufacturers measure longevity by discharging full batteries at a temperature of 80°F 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.
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 an output that is 25% to 40% 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.