Upgrading batteries

1 Rachet Type Tie Down Strap
Keeping batteries in place
A ratchet-type tie-down strap keeps these batteries in place.

For a very long time, high-capacity battery technology remained pretty simple. Wet cell lead-acid batteries were pretty much the only choice boaters had. Over the last 10 to 15 years, battery technology has advanced and more choices of battery design and type have developed. All batteries age, and at some point it becomes time to replace them. This is the time many consider upgrading to one of the newer types of batteries. This may seem a simple task at first, but there is more to consider when changing or upgrading batteries than just the batteries themselves. Batteries are part of a system that must work together to be truly efficient.

The battery system
When thinking about batteries for your boat, you must also consider how the batteries are to be used and recharged. Cabling and connecting the batteries to the boat electrical system, as well as how the batteries are mounted and secured, can also impact battery performance. How a battery is recharged has a large effect on the performance and life of a battery. Not all batteries charge at the same rate or voltage. Incorrectly charging a battery can greatly affect performance and shorten its life.

There will often be several charging sources on a boat, each utilizing a different technology. A typical cruising boat will have a battery charger used when connected to shore power or generator. Otherwise, charging is done via an alternator run off the propulsion engine. Solar and wind chargers will also likely be used. Each of these charge sources must be correctly configured to work with the type of battery installed.

How the batteries are installed and wired also affects performance. Batteries installed  poorly can suffer from excess heat, making them less efficient and possibly shortening their life. Poor cabling and connections also can reduce performance and life.

How the batteries are used will also influence selecting the right battery type. Engine-starting batteries, for example, are going to be different from a house bank because they each are required to release their energy differently. The starter battery will need to provide bursts of high energy for brief periods of time, while a house battery will release a little energy over a longer period of time. Each of these requires a different internal structure of the battery. If you do a lot of anchoring, the batteries will be subject to deeper cycling than a boat that goes from one marina to another. Where you plan to cruise and the availability of replacement batteries can also affect the choice of battery type.

All these factors should be considered when selecting replacement batteries. It is beyond the scope of this article to fully explain all the types of batteries, along with the pros and cons of each, but it is possible to help understand some of what is available, as well as to grasp that the battery alone is not the only thing to consider.  See sidebar for more on battery types.


Charging is key to efficient battery usage. However, it is not as simple as just dumping power back into a battery. How much power can be replaced and at what rate will vary with battery type. To make matters worse, most boats have multiple charge sources requiring each to work with the selected battery type. Most shore chargers will have settings to adjust to a select battery type, with the exception of lithium-ion. Engine-driven alternators with built-in regulators are set up for standard wet cell batteries and are not very efficient for deep cycle batteries. Most alternator regulators are designed for simple wet cell starting batteries. Any other type of battery will require the use of an external regulator that can be programmed for a particular battery type. This also would apply to solar and wind charge controllers. The better controllers will have settings for different battery types.

If you’re thinking of going with lithium-ion, it is best to consult with the battery manufacturer concerning charge methods. Because lithium-ion batteries can accept a charge so quickly, they have been known to cause overheating and damage to alternators. Some lithium-ion battery makers are now designing built-in charge controllers to work with standard chargers designed for regular batteries. This could help keep the cost down for switching to lithium-ion. 

No matter what type of battery is used, it is important to have the right charger. Over-charging and under-charging are the number one source of premature battery failure. Charging too slow or too fast can also affect battery performance. It is always best to consult with the battery manufacture about the best charging solutions.

Battery installations can affect a battery’s performance and lifespan as well. Worst case, a bad installation can lead to a battery explosion or fire, and a shortened battery life at best. Ventilation is key to any battery installation. Even sealed batteries will benefit from good ventilation to keep the batteries cool as they are charged and used. Most batteries will vent if over-charged. These gases are explosive, making ventilation even more important. Batteries packed closely together will retain heat, so it is best to leave at least 1 inch separation between two or more batteries mounted in the same location. This will allow airflow between individual batteries, reducing heat buildup.

All batteries should be properly secured to the vessel. Although ABYC standards allow 1 inch of movement, it is better if the batteries are secured to prevent any movement. Most batteries are heavy and, of course, boats move around quite a bit. Even slight shifting can chafe the battery case and loosen the conductors connected to the battery. Batteries coming loose from their mounts can cause spillage or worse, such as shorting leading to a fire. It is best if the batteries can be mounted low and aft in the boat as the vessel movement will be less in these areas. The common nylon straps sold in most marine stores are not strong enough for most installations. The battery should fit snugly into chocks around the base, and should have top clamps or heavy ratchet-type straps to hold them securely.

Cabling and connections
Cabling and cable end connections are an important part of any installation. Bad connections or cable ends can add resistance, reducing efficiency. Likewise, undersized conductors will result in losses. When installing new batteries, it is a good time to review the conductor size along with the end terminals. When upgrading batteries, keep in mind a larger bank or replacements with a higher capacity may require larger conductors to handle the additional load. This would include the charger conductors as well as the feeds.

Connections to the batteries and the boat’s electrical system are often a weak spot. Placing too many conductors on a single terminal, miss-sized ring terminals, and loose connections can all cause damage to a battery or create a possible fire hazard. ABYC now recommends all battery connections be made with standard hex nuts and not wing nuts as was common in the past. The reason for this is to encourage the use of a wrench to tighten the connections instead of merely hand-tightening wing nuts. It is best to use batteries with threaded studs, and not the automotive type with only a round stud for clamp-type connectors.

Another common mistake is multiple connections on a single terminal. The only connections to a battery should be the main cable to a battery switch and ground cable to a busbar, along with any conductors connecting batteries together in a bank. If more connections are needed, a busbar should be installed, allowing each conductor its own stud. Make sure the cable ends are correctly sized for the battery studs and try to avoid using stainless steel washers between connections as these are not as conductive as copper washers.

Battery banks
Batteries are often installed in banks of two or more to obtain higher capacity. When doing this, it is important to always use the same size and type of battery in a single bank. Mixing an AGM with a wet cell in the same bank is going to greatly reduce the life and efficiency of both batteries. As mentioned, charging is different for each battery type, and mixing them will result in poor charging and possible damage to the batteries. Any time one battery in a bank goes bad, it is always best to replace all the batteries. This is why it is usually best to not have banks larger than two batteries.

Battery capacity
Battery capacity is always a tricky subject. As noted, wet cells should never be discharged below 50 percent capacity, while lithium-ion can often be discharged to near 100 percent. It is clear the type of battery and its performance have much to do with how much capacity is needed. It is not enough to just calculate the needed capacity based on loads alone. Battery discharge levels, along with charging capacities and charge rates, also need to be considered. With most cruising boats, the battery is being discharged while at the same time being recharged. This further complicates knowing what the most efficient capacity needed for a boat is. Careful analysis is needed when determining capacity.

As can be seen, upgrading batteries is not a simple subject. Knowing that the batteries work as a system will help avoid some of the common mistakes with battery installations that I see as a marine surveyor. The best batteries in the world will not perform well if they are poorly installed and the wrong charging system is used. Not charging a battery correctly is the most common cause of premature battery failure, but the installation can shorten a battery’s life as well. When it comes time to upgrade the boat’s batteries, be sure to inspect and upgrade the complete system.

Contributing editor Wayne Canning is a self-employed marine surveyor, delivery skipper and freelance boating writer.

By Ocean Navigator