What You Need to Know About Boat Batteries

Usually, by the end of October, many of the lakes have “turned over” for the winter. Morning steam rising from the water indicates that the heat energy stored in the lake from the long hours of summer daylight is beginning to return once again to the atmosphere as the water begins its annual slumber and ultimate renewal. 

There is perhaps still a little time left for one final outing, but most – if not all – of you have taken your boats out of the water and gotten them cleaned out, “winterized” and ready for next spring’s trek back to your favorite fishing spots.  You take very good care of your boats, motors, poles, reels and other gear inside the boat.  But have you really taken the time to look after one of the most important pieces of equipment that you have on board – your batteries?

Trolling batteries

Most of us take our boat batteries for granted, in much the same manner as we do our car batteries. I used to do that. In fact, I had a car that sat in the warmth of the garage all winter. I thought that an occasional blast from a charger would be enough to do the trick. Boy, was I wrong. And the result was a new battery about every other year – until, after a lecture from my mechanic, I got a little bit smarter about batteries.

A battery can lose – depending on how and where it is stored – up to 30% of its charge per month – just sitting around the house or garage! And there are all kinds of things that affect battery charge and loss thereof (like temperature, humidity, state of discharge, age of battery, etc). Most of us never consider any of them. Not unlike young children, batteries require loving care: call it maintenance.

Getting the right battery for the intended task is only part of the story – keeping that battery healthy with the proper charging and maintenance will ultimately ensure that you will have the necessary power when you call for it!Which type of battery you choose is based on your needs (deep cycle vs. starting), the capacity and lifespan you are looking for and your budget requirements.

Although I have found that the subject of batteries and battery maintenance to be a multi-faceted subject that encompasses many different topics, I am going to limit my discussion to a few of the most important “basics”. I want to briefly discuss:

Battery Types

There are two basic types of 12-volt batteries:

Cranking or Starting Batteries

These are designed specifically to start your main engine. They are made with thinner and more numerous lead plates inside, allowing for more surface area and thereby providing the quick and massive amounts of energy required for tough starting jobs. While the motor is running, the alternator inside will easily and quickly replenish the used energy.

If your boat is powered by a newer model outboard with sophisticated computers, pumps and sensors, you definitely want to make certain you have enough starting power. It's a good idea to check your owner's manual for the recommended MCA (Marine Cranking Amps: a measurement of the number of amps a battery can deliver at 0 ° F for 30 seconds and not drop below 7.2 volts) rating before shopping for a battery; always choose one with a rating equal to or greater than the recommended value.

Deep-Cycle Batteries

These are designed to power on-board electrical accessories such as trolling motors, fish-finders, GPS, radios and the like. In general, these batteries use energy at a much slower rate and often don't get re-charged until the end of the day. This deeper and more strenuous discharge is hard on a battery and requires a different design type; the result is a battery with fewer but much thicker lead plates that will withstand the deep cycling. Deep-cycle batteries can withstand the rigors of several hundred discharge/recharge cycles, while cranking batteries cannot.

It's important to understand that, because of their design differences, substituting one battery type for another is not a good idea. Use a cranking battery to power a trolling motor will cause the battery to overheat and fail. Besides leaving you without power in a moment of need, purchasing a new battery will definitely be in your future. Substituting a deep-cycle battery for a cranking battery will likely not provide the power needed to start your outboard, possibly leaving you stranded a long way from your dock. As it turns out, the design strengths of each battery type also are their weaknesses in opposite applications.

Having said that, dual-purpose batteries that can perform both these functions -- to some extent -- also are available. Keep in mind however, they will not supply the same starting power as a true cranking battery, nor will they provide the same number of discharge/recharge cycles as a dedicated deep-cycle battery.

On board battery charger


As if all that doesn't make things difficult enough, batteries can be further described according to the configuration of the electrolyte inside the battery... leading us to four categories that all have their own advantages and disadvantages.

Wet-Cell or Flooded-Cell Batteries

Generally the most popular and the type most all of us are familiar with, they have a number of cells inside that contain a liquid mixture of sulfuric acid and distilled water. They have the advantages of a somewhat lesser price, a high number of discharge/recharge cycles (if properly maintained and taken care of), are less likely to be damaged by overcharging and are a bit lighter in weight than comparable AGM or Gel batteries.

The disadvantages are that most have vented, interior-accessible designs, requiring regular inspection and making certain the cells are topped off with distilled water. Venting also releases hydrogen gas, meaning the battery compartment must be well ventilated. Other drawbacks include possible spilling of corrosive battery acid, a higher rate of self-discharge (6% to 7% per month) and the fact that wet-cell batteries are more fragile in high-vibration environments such as boats.

AGM Batteries

AGM is short for Absorbed Glass Mat, and these batteries feature a dense filling of very fine absorbent fiberglass matting that is saturated with acid/electrolyte and packed tightly between the battery's plates. The design allows oxygen to recombine with hydrogen gas, thus replenishing the battery's water content and alleviating the need for refilling. Advantages include being truly maintenance free (except for external cleaning).

They are sealed (which doesn't allow for gases or acid to leak), can be installed at any angle, are shock and vibration resistant, have a relatively low self-discharge rate -- ~ 3% per month) and can even withstand being immersed in water (I guess that's in the event your boat gets swamped?). The biggest disadvantages are that AGM batteries are more expensive, are heavier and are quite sensitive to charging currents and voltages; they are somewhat easy to overcharge, which can ultimately ruin the battery.

Gel Batteries

Gel batteries too, are much like wet-cell batteries in that they are filled with liquid electrolyte. The difference is that the electrolyte is gelled with silicates before the battery is sealed. Like AGM batteries, they use the special technology that eliminates the need for adding water. Advantages? They are maintenance free, sealed, low-temperature tolerant, shock/vibration resistant and have long cycle life. Their most notable advantage is resistance to over-discharge that can damage other battery types.

Gels have an internal self-discharge rate less than 1 percent per month, so they can be stored for long periods without being recharged. And because they aren't prone to develop life-shortening plate sulfation when left uncharged, they are a good choice for boaters who often forget to recharge batteries promptly after use. The bad news is that these batteries are often almost twice as expensive as comparable flooded-cell batteries and are also fickle about charging; they require special chargers.

Lithium-ion Batteries

Lithium-ion batteries are among the emerging “super battery” technologies. They have a high energy density and are excellent for deep cycle applications. Compared to flooded batteries, lithium batteries deliver a savings of up to 70 percent in volume and weight and can offer three times as many charging cycles. They handle incredible amounts of current and therefore can be recharged faster than any other type. Currently in use onboard high-performance offshore racing sailboats and others whose owners demand extreme weight savings and cutting-edge performance, Lithium-ion may be the future... Be prepared and willing to pay premium prices.

Things that will destroy a battery

There are essentially two things that will quickly and easily destroy your battery: either 1) undercharging it or 2) overcharging it. The majority of both deep cycle and starting batteries are simply containers for a number of lead-plates, filled with sulfuric acid. Undercharging them will ultimately cause lead sulfate to accumulate on the plates; this will eventually destroy the battery because the normal chemical reaction will be unable to continue. Overcharging the battery will accelerate the natural corrosion of the plates due to excess electrons being literally boiled out of the electrolyte. Ultimately, the fluid boils away and the plates are exposed to the air, which ruins them.


Have you ever had a battery that seems to work well but “dies” much more quickly than expected? The problem may be a battery that is heavily sulfated – often the result of only light use (being discharged by only 15% or so). The sulfuric acid has become concentrated on the battery’s bottom and sulfate crystals have begun to form. And even though a multi-meter will indicate that all is well in terms of voltage, the capacity of the battery has become severely reduced. Sure, the battery will work just fine, but it will die much sooner than normal.

There are certain chargers that have a setting that will temporarily boost the charging voltage for a brief period of time, causing a mixing of the electrolyte, and dissolving the crystals. The process is called equalization and can be done – depending on the manufacturer – about once a year. But beware: this should only be done on wet cell batteries; this same process can ruin other types. And you will need to make sure that you disconnect the batteries from the electrical system to prevent the higher voltage from damaging other equipment.


Batteries have come a long way since the 1960’s. Back then, most – if not all – wet cell batteries were dry to start with. In the storeroom of the gas station at which I worked, there was a large box with a plastic bag filled with electrolyte – sulfuric acid. I found out it was acid the hard way; in filling a battery, I once managed to splash the electrolyte all over my pants. I didn’t realize the power of the stuff until my work pants got washed. They looked as if I had been shot by a machine gun: one leg was nothing but holes! Now there is an incredible variety of batteries that are available for every imaginable use: marine starting & trolling, auto, agricultural, industrial).

They all have at least one thing in common: they need to be maintained to some degree – even the “maintenance free” types. They will lose their electrolyte during normal use and need to be checked; all need to be recharged. Most of us still use the wet cell types for marine applications, although there are essentially three distinct types of lead acid batteries (see "Categories" above) manufactured for marine applications, and any one type can be designed and built for either starting or deep cycle applications.

As I mentioned, the gelled acid and AGM types are essentially maintenance free since they are sealed. But because of this, be very careful when recharging them; "smart charger" technology is required or damage can easily result. They are also more expensive but do have their advantages. The most important thing for the flooded acid variety is to keep them full. Top them off with distilled water only whenever possible, as minerals in tap water can contaminate the electrolyte.

Keep the terminal clamps clean and free of corrosion; coat them with anti-corrosion spray or even petroleum jelly. Check connections and keep them tight; watch for frayed wires and replace them. And for a few bucks spent at your local auto parts store, a Battery Hydrometer – used to check the specific gravity (concentration of acid) is a good investment. It is a great way to determine if one of the cells is bad. If the difference in specific gravity is 30 points or more… it’s time to replace your battery!

Checkup and Storage

Whether or not your boat is stored for the season in a warm garage or out in the cold, your best bet is to remove all batteries and bring them inside. A fully charged battery with a perfect electrolyte level can probably withstand temperatures down to zero degrees without freezing. But the colder it gets, the more easily a battery can discharge, and therefore the more easily it can then freeze at higher temps. If even one of the cells freezes, the battery is shot!

Fully charge them about once a month over the winter and they will be ready when you are in the spring. Try to keep them off of concrete floors if possible and cover the terminals to help prevent discharge. The last thing you want is a dead battery on the launch ramp on opening day, or a dead trolling motor battery. No matter what kind of battery chemistry you choose, follow these recommendations to get the best performance and longest life from your batteries:

  • Stay with one battery chemistry (flooded, gel or AGM). Each battery type requires specific charging voltages and currents. Mixing battery types can result in under- or over-charging, which may mean replacing all batteries on board at the same time.
  • Never mix old batteries with new ones in the same bank. Old batteries tend to pull down the new ones to their deteriorated level.
  • Regulate charge voltages based on battery temperature and acceptance (either manually or with smart-sensing devices) to maximize battery life and reduce charge time. Ensure that your charging system is capable of delivering sufficient amperage to charge the battery banks efficiently.
  • Keep batteries clean, cool and dry.
  • Check terminal connectors regularly to avoid loss of conductivity.
  • Check fluid levels and add distilled water to flooded lead acid batteries when needed. Keep batteries charged; leaving them in a discharged state for any length of time will damage them and lower their capacity; it also reduces lifetime.
  • Clean corrosion with a paste of baking soda and water.

With summer now officially here, I hope that most of you have either fished the opener somewhere or are, like me, chomping at the bit to head to the lakes once again. Make sure to check your batteries before you head out. And when next October rolls around, you will perhaps remember a few things from this article and make sure to take proper care of those very important pieces of equipment: your batteries. For information on charging and chargers, see my article on Charging Your Boat Batteries.

See you On the Lake!

R. Karl

I recommend and use the Dual Pro 15 Amp/Bank Professional Series 2 Bank Charger in my 16' Lund - Mr. Pike Anniversary Edition!

More by R. Karl

Note: Due to the amount of traffic on this page and the number of e-mails on boat batteries, I've decided to post some of the questions and answers on a new Q&A page.  I hope you find this information helpful!

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