How Do Boat Alternators Charge Deep-Cycle Batteries?

Many boat owners with alternators also own boats or RVs that use deep-cycle batteries. While a car alternator can charge a deep-cycle battery, it is not the optimal method due to potential differences in charging requirements and rates between car batteries and deep-cycle batteries. Deep-cycle batteries are designed to provide sustained power over a long period and can be deeply discharged repeatedly. They require a slow and steady charging process, whereas car alternators are designed to deliver high currents for short periods to quickly recharge a car's starter battery.

Charging a deep cycle battery with an alternator

Deep-cycle batteries are designed to hold a charge for extended periods. While a car alternator can charge a deep-cycle battery, it is not the optimal method due to potential differences in charging requirements and rates.

Understanding Deep-Cycle Batteries

Deep-cycle batteries are lead-acid batteries designed to be regularly discharged and recharged. Unlike car batteries, which only need to be discharged briefly, deep-cycle batteries can withstand long periods of discharging. They are commonly used in applications that require constant power sources like boats, RVs, golf carts, and solar panels.

Deep-cycle batteries slowly release their stored energy over an extended period, enabling them to be used for long durations without needing frequent recharging. They are also less prone to the "memory effect," which can reduce performance over time in other battery types.

Alternators and Their Role

An alternator's primary role is to maintain the charge of a vehicle's starter battery and power its electrical systems. While it can technically charge a deep-cycle battery, there are significant differences between standard automotive batteries and deep-cycle batteries that make this less than ideal.

Deep-cycle batteries are designed to provide sustained power over long periods and can be deeply discharged repeatedly. They require a slow and steady charging process to ensure proper filling and conditioning. In contrast, car alternators deliver high currents for short periods to quickly recharge car batteries, which don't experience deep discharges.

Using a car alternator to charge a deep-cycle battery may result in improper charging, leading to reduced battery performance and lifespan. It is always best to use a proper battery charger designed for deep-cycle batteries to ensure optimal performance and longevity.

Charging Precautions

Connecting a deep-cycle battery directly to an alternator without a regulator is not advisable, as it can lead to overcharging and potential damage. A voltage regulator ensures that the battery is charged at a safe and controlled rate, protecting it from damage and maximising its lifespan. Without a regulator, an alternator could send too much current, causing overcharging, excessive heat, swelling, leakage, or even an explosion.

Additionally, charging a deep-cycle battery with jumper cables is technically possible but not the most efficient or safest method due to the risk of overcharging and potential damage. Jumper cables are designed to jump-start vehicles with dead batteries and can deliver a much higher current than deep-cycle batteries are designed for, increasing the risk of overcharging and potential damage.

Charging While in Use

A deep-cycle battery can be charged while in use as long as the charging system provides more current than the load is drawing. This is common in RVs, boats, and solar power systems where the battery needs to stay charged while being used. However, it's crucial to monitor the battery to prevent overcharging or undercharging, both of which can harm its longevity and performance.

Charging Duration

A deep-cycle battery can typically sit without charging for up to 30 days, but this depends on the size, type, and usage of the battery. Larger or higher-quality batteries tend to hold their charge longer, and a fully charged battery will last longer than a partially drained one. Storing the battery in a cool, dry place also helps maintain its charge.

Onboard Battery Chargers

When choosing an onboard charger, look for features like digital control of battery charging, automatic temperature compensation, and waterproof design. Warranties are also desirable, indicating the manufacturer's confidence in their product. It is worth investing in a reliable charger to ensure your deep-cycle battery is well-maintained.

What is a deep cycle battery?

A deep cycle battery is a type of battery designed to be regularly deeply discharged and recharged. They are commonly used in applications such as golf carts, RVs, and solar power systems. They may look similar to car batteries, but they serve very different purposes.

A deep cycle battery is a lead battery designed to provide sustained power over a long period and run reliably until it is 80% discharged or more. At this point, it needs to be recharged. It is important to note that although deep cycle batteries can be discharged up to 80%, most manufacturers recommend not discharging below 45% to extend the battery's life.

The discharge level is the “deep cycle” and stands in contrast to other types of batteries, such as starter batteries, which provide only short bursts of energy before recharging. A starter battery, also known as a cranking battery, discharges only a tiny percentage—usually 2 to 5%—each time it is used.

Deep cycle batteries have thicker lead plates compared to traditional lead-acid batteries, allowing them to provide longer sustained amounts of electricity but lower power output. These batteries can withstand 80% or higher depth of discharge (DoD), although maintaining the DoD above 50% will extend the lifespan of the battery.

There are different types of deep cycle batteries, including flooded deep-cycle batteries, sealed lead-acid batteries, absorbed glass mat (AGM) batteries, and gel batteries. Flooded deep-cycle batteries are the most common type and are relatively inexpensive and easy to find. However, they require regular maintenance, such as adding distilled water to the electrolyte. Sealed lead-acid batteries, on the other hand, require little maintenance and are spill-proof, making them a good choice for recreational vehicles that experience a lot of vibrations. AGM batteries have a glass mat that absorbs the electrolyte, improving performance and lifespan. Gel batteries add a gelling agent to the electrolyte, making it thicker and preventing leaks.

Deep cycle batteries are ideal for applications that require a steady supply of a lower amount of power, such as recreational vehicles, boats, golf carts, and electric vehicles like medical carts or electric bikes. They are also used in off-grid energy storage systems, industrial forklifts, motorized wheelchairs, and audio equipment.

Charging and maintenance

Deep-cycle batteries are designed to be regularly discharged and recharged. They are different from car batteries, which only need to be discharged briefly. Deep-cycle batteries can withstand long periods of discharging. They are commonly used in applications that require constant power sources, such as boats, RVs, golf carts, and solar panels.

When it comes to charging a deep cycle battery with an alternator, it is possible but not ideal. A car alternator's primary function is to maintain the charge of the vehicle's starter battery and power its electrical systems. Deep-cycle batteries, on the other hand, require a slow and steady charging process to ensure they are properly filled and conditioned. Car alternators deliver high currents for short periods to quickly recharge car batteries, which don't experience deep discharges. Using a car alternator to charge a deep-cycle battery may result in improper charging, leading to reduced performance and lifespan. It is recommended to use a proper battery charger designed for deep-cycle batteries to ensure optimal performance and longevity.

Connecting a deep-cycle battery directly to an alternator without a regulator is not advisable, as it can lead to overcharging and potential damage to the battery. A voltage regulator ensures that the battery is charged at a safe and controlled rate, protecting it from damage and maximising its lifespan. Without a regulator, an alternator may send too much current to the battery, causing overcharging. This is especially true for deep-cycle batteries, which are designed for slow and steady charging and discharging cycles.

There are various conditions that can affect how a deep cycle battery charges. For example, if your boat is stored in an extremely cold climate, the battery will have a lower capacity. On the other hand, if the battery is in an extremely hot climate, it will have a higher capacity, but the lifetime will be shorter as the temperature rises. Generally, a battery will need more charging in terms of actual voltage as the temperature drops. Therefore, it is recommended to use a battery charger that uses automatic temperature compensation and maintain your batteries properly all year round.

Deep cycle batteries are designed to reach a significant level of self-discharge and then be charged back up to 100%. This cycle is repeated many times over the battery's lifetime. For example, a battery starting with 100% capacity may be used down to 25% and then charged back up to 100% before its next use. There is a balancing act with deep cycle batteries in this sense. If you use a large amount of the battery's capacity each time, you may drain it faster. However, if you only use a small amount before charging, you will likely shorten the battery life. It is best to use the battery as intended and ensure it gets charged all the way up after use.

Deep cycle batteries vary in terms of charging duration. If you have a small boat with a cranking battery and a deep cycle battery, a 2-bank battery charger is recommended. If your charger has a higher amps per bank, the charge rate will be faster, and you'll achieve a full charge more quickly. For larger boats with more batteries, a charger with 4 banks and extra power is ideal. It is important to find a battery charger that can provide the appropriate power to get your batteries charged conveniently.

When looking for an onboard charger, consider features such as digital control of battery charging and automatic temperature compensation. Waterproof chargers are also recommended for boats. Warranties are always a good sign, as it shows that the manufacturer is confident in their product. It is important to invest in a reliable battery charger to ensure the safety and functionality of your boat's electrical systems.

Onboard battery chargers

When selecting an onboard charger, it is important to ensure that it is fully waterproof, resistant to corrosion, and able to be mounted in damp or wet locations. It should also be compatible with the chemistry and voltage of the batteries on your boat. For example, if you have a 12-volt lead-acid starting battery and a 12-volt AGM deep-cycle battery, you will need a charger with separate banks for each type.

Most onboard chargers will come with instructions on how to mount them, and the process is usually straightforward and can be completed in a few hours. When mounting the charger, it is important to leave some space between the charger and the batteries while still ensuring that the cables can reach the battery terminals. It is also important to make sure that the plug can be easily connected to an extension cord when charging.

There are several different types of onboard battery chargers available, including:

• Smart-regulators: These handle the alternator differently, protecting it by reducing the field voltage, which in turn reduces alternator current output.
• Dumb-regulated: A simple dumb-regulated alternator does bulk and absorption, limiting voltage.
• Super-dumb-regulated: This is similar to dumb-regulated, but with an additional thermistor self-protection feature that reduces alternator regulation voltage based on ambient temperature.
• Trickle charger: This is used to maintain the battery when stored for the off-season or between uses. It is designed to be left hooked up to the battery terminals to maintain a proper charge.
• Jumper box: This is used like jumper cables but does not require another vehicle, boat, or battery nearby to get the boat engine cranked.
• Bench battery charger: This can be used to keep batteries charged and maintained during the winter months, as well as to recondition a battery and act as a jump starter.

It is important to note that overcharging a battery can cause irreparable damage and drastically shorten its lifespan. While basic battery chargers need to be monitored to ensure the battery doesn't become overcharged, most onboard chargers will come equipped with protection against overcharging, typically through a float or maintenance mode.

Recommended battery chargers

When choosing a battery charger, it is important to ensure that it is compatible with your battery. Select a charger that matches the voltage of your battery. For marine batteries, 12-volt batteries are standard, but 24-volt and 36-volt batteries are also used. Using the wrong charger can harm your battery.

Additionally, the charger you select should have a setting that matches the chemistry of your battery. Marine batteries come in four different chemistries: flooded, AGM (Absorbed Glass Mat), gel, and lithium. If your starting and deep-cycle batteries have different chemistries, you will need to find a charger with a setting for both types.

There are two main types of boat chargers: onboard and portable. Onboard chargers are permanently installed on the boat and connected directly to the electrical system, offering automatic charging. These are recommended if you use your boat near shore power sources or marinas. Portable chargers, on the other hand, are compact and versatile, designed to be carried on board or used on land. They are preferred when flexibility and mobility are crucial, such as when boating far from shore power.

Smart chargers are an excellent choice for marine batteries due to their advanced technology. They employ a multistage charging approach, optimising the process and safeguarding the battery's longevity. They prevent overcharging, a common issue with traditional chargers, by gradually reducing the charging current as the battery nears full capacity. Smart chargers also allow you to monitor your charge via Bluetooth (for compatible lithium batteries).

• Trickle Charger: This is used to maintain the battery when stored for the off-season or between uses. It is designed to be left hooked up to the battery terminals and will maintain a proper charge.
• Jumper Box: Jumper boxes are used like jumper cables but do not require another vehicle, boat, or battery nearby to crank the boat engine. They are great for automotive or marine use and are more expensive than trickle chargers.
• Bench Battery Charger: If you have access to power near your boat slip or lift, this is a good option. It can recondition a battery, act as a jump starter, and maintain batteries when in storage.
• X2Power Single Bank Onboard Automatic Marine Battery Charger: This charger is available at Batteries Plus and is suitable for marine batteries.
• NOCO GENPRO Onboard Battery Chargers: These chargers come in 2-bank, 3-bank, and 4-bank options, providing up to 40 amps of charging power.
• Ionic Lithium Charger: This smart charger is specifically designed for lithium batteries, allowing you to take advantage of their benefits, such as faster charging and Bluetooth capabilities.

Frequently asked questions