Frequently Asked Questions

Read through our frequently asked questions to find answers to most of your questions about our smart marine solutions. If you still cannot find what you’re looking for, feel free to contact us and we’ll respond to you in a timely manner.

What size battery bank should you have?

If you are a cruiser, the answer to this one is quite simple – The largest one you can afford in terms of space, weight and money. This is equally true no matter what the electrical load is! To see why, let’s take a look at two different approaches to estimating the “correct” battery capacity.

How do I analyze energy consumption?

It is common practice among the energy gurus to perform an energy analysis of a cruising boat and advise a battery bank capacity based on the results. To perform such an analysis, assumptions are made about the length of time the lights, appliances, navigational equipment, water maker, etc. will be operating and the amount of current they will draw while they are on.

An amp-hr (current draw x operating time) total over a 24 hour period is then calculated for every device which consumes electrical energy. For example;



Usage per 24 hrs

Three reading lights drawing 1 amp each will be used an average of 3.5 hours per day.

(3 x 1) x 3.5 = 10.5

10.5 amp-hrs

The refrigerator draws 6 amps while it’s running and runs an average of 18 hrs per day.

6 x 18 = 96

96 amp-hrs

Making accurate estimates for some items is relatively easy, for others such as electric autopilots it is nearly impossible to estimate. Nevertheless, to complete the analysis the total estimated energy consumption of all such devices is added together to provide an estimated average total daily energy consumption in amp-hrs.

As you can imagine the totals will vary tremendously from boat to boat, however for illustration purposes, let’s assume that our particular cruiser calculates a total estimated use of 140 amp-hrs per 24 hours.

In the next step of the analysis it is assumed that our cruiser will not want to discharge their batteries more that 50% since it is commonly believed that doing so will disproportionately shorten their life span.

The cruiser must now decide how often they wish to run their engine or generator to recharge their batteries. This in combination with their estimated daily energy consumption forms the basis for calculating the battery bank size required. For example;

Recharge Frequency Desired

Bank Size Required:

140 amp-hr once per day
280 amp-hr once every two days
560 amp-hr once every three days
840 amp-hr

So it is seen that by using the energy analysis method, the calculation of appropriate battery bank size is made purely on the basis of energy usage and the frequency of your recharge cycles.

Why energy consumption is irrelevant when sizing your battery bank?

While it can be helpful to have an idea of how much energy you are likely to use in a day, it should not form the basis for sizing your battery bank. Why? Because it leaves out the single most important consideration in battery bank sizing – the recharge, or acceptance, rate.

The energy consumption analysis gives you an idea of how often you will need to run your charging engine but gives no indication of how long the engine will need to be run.

For most cruisers, particularly those who have already had the unpleasant experience of sitting in their island paradise amid diesel fumes waiting for their batteries to recharge, minimizing their engine run time is the top priority.

The proper size alternator for the battery bank

To fully appreciate why the acceptance rate is so important it is first necessary to gain an understanding of the relationship between alternator size and battery bank size. It is only possible to select right alternator(s) after the capacity and type of battery bank has been determined.


While the power output of a battery is rated in several ways, the only rating generally useful to the cruiser also happens to be the most common and is referred to as the 20 hour amp-hr rating. This number represents the total amount of energy that the fully charged battery can release when discharged at a stable rate over a 20 hour period.

A good quality 8D size (20″x11″x10″) lead-acid type deep cycle battery would have a 20 hour rating of 220 amp-hrs @ 12 volts. Gelled electrolyte type batteries (gel cells) and absorbed glass mat batteries (AGM) are usually slightly less with about 200 amp-hrs in an 8D size.

Remember when determining the capacity of your battery bank, use the manufacturers ratings and add the amp-hr rating of all batteries which are connected in parallel and discharged and recharged as a single bank.

If you choose to use multiple house banks (not recommended) count only those banks which are discharged and recharged simultaneously. Do not simply add up the ratings of all the batteries on your boat.


This is not a question of brand name but one of electrolyte type. Your choice here is common lead-acid, gelled lead-acid (gel cell) or absorbed glass mat (AGM).

Calculating your battery bank’s acceptance rate

The acceptance rate is the maximum rate at which a battery bank can be recharged. The acceptance rate of the bank is determined by it’s capacity, type and state-of-charge. Since we know (or can determine) the capacity and type of the bank, we will need to make an assumption as to it’s average state of charge under cruising conditions. Batteries which are deeply discharged have a higher acceptance rate than those which are more fully charged.

Surveys have shown that cruisers typically cycle their batteries between 50% and 80% of full charge under most conditions. Cruisers who have large capacity banks in comparison to their energy needs tend to cycle between 70% and 90%, while those with small banks tend to cycle between 40% and 70%.

For the sake of this exercise, we will estimate at the average acceptance rate when cycling the batteries between 50% and 80% of full charge. Under these conditions lead-acid batteries have been shown to have an acceptance rate equal to 25% of their total 20 hour amp-hr rating.

Stated another way, a lead-acid battery bank consisting of three 8D size 12 volt batteries @ 220 amp-hrs each (660 amp-hrs total) would have an acceptance rate of 165 amps.

One advantage in gel cell type batteries is that they have a higher acceptance rate than do the common lead acid type. Acceptance rate calculations made with gel cell batteries should be based on 40% of their 20 hour amp-hr rating rather than the 25% figure used with lead acid.

Because of their lower 20 hour rating, the bank of three 8D batteries described in the example above would have a total capacity of only 600 amp-hrs (rather than 660 with lead acid). However, they would have an acceptance rate of 240 amps instead of 165 amps.

The highest acceptance rate is obtained with absorbed glass mat batteries (AGM). Acceptance rate calculations made with AGM batteries should be based on 100% of their 20 hour amp-hr rating rather than the 25% figure used with lead acid or 40% used with gel cells.

Our bank of three 8D batteries (as described in the other examples) would have a total capacity of 600 amp-hrs just as would the gel cells. However, they would have an incredible acceptance rate of 600 amps instead of 165 amps (lead acid) or 240 amps (gel cell).

As you can see, once you know the capacity and type of your battery bank you can calculate it’s acceptance rate. Simply multiply the total capacity by 25% for lead acid batteries, 40% for gel cells or 100% for AGMs.

What about Lithium/LiFePO4 batteries?

More and more boaters are making the switch to lithium.  They are lighter and at least twice as efficient the traditional lead-acid, gel or AGM battery technology meaning you can purchase half then number of batteries you currently have.  However, this weight savings and performance efficiency comes at a premium as most lithium batteries will be at least 2x the cost of a traditional battery.

Also, the prices for lithium batteries vary drastically, from low cost Ali-xxx or eBay options to premium Battle Born, ReLion or Lithionics, but the bottom line is you get what you pay for. And, just like all other marine-grade components, this is not where you should be looking to cut costs or corners.

There is a ton of information to be found online about lithium batteries and specifically lithium batteries for marine use.  One of the best and most enlighening articles we’ve come across regarding lithium batteries for marine use comes from Rod at and can be read here.

What about the alternator?

Since the acceptance rate described the maximum rate at which a battery bank can be recharged, it stands to reason that the proper size alternator can only be selected once the acceptance rate has been determined. It is wasted money to charge a battery bank that has an acceptance rate of 70 amps with a 165 amp alternator.

Likewise, using a 100 amp alternator to charge a battery bank with an acceptance rate of 240 amps is pointlessly slow and inefficient. The goal is to get the output of your alternator (under actual charging conditions) to match the acceptance rate of your battery bank as closely as possible.

Most alternator manufacturers will provide you with the output curve of the alternator you are considering under hot conditions and at a variety of speeds. An alternator rated at 150 amps will likely only put out 130 amps once it gets hot and will only do that running at full speed.

Under realistic charging conditions, you may only be running your engine at 1100 – 1200 rpms. If your alternator is belted at a 2 to 1 ratio it will be spinning at twice that speed, or 2200 – 2400 rpms then it is quite possible that 150 amp alternator is now only going to putting out 80 amps or so.

Given this, how is it possible to get 200 to 300 amps of real charging capability? Sometimes it isn’t possible, but don’t give up too quickly. Very large alternators with outputs of 200+ amps are now quite common. Additionally, it is often very practical to use two or more alternators simultaneously to charge a single bank.

Some boats are already set up to have one alternator charge the engine start battery and a second to charge the house bank. Usually the engine start battery needs little if any charging. An automatic battery bank combiner can be used to allow both alternators to charge the house bank.

How much difference does this approach to battery sizing make?

Occasionally it makes no difference, but often the results are surprising. Take a look back at our very first example where our cruiser was required to replace 140 amp-hrs of energy per day. Let’s assume that, using the energy analysis approach, he determined that running the engine once per day would be sufficient. He would then be advised to install a lead-acid battery bank of 280 amp-hrs capacity (ie. 50% max discharge).

Now using your knowledge of acceptance rate calculation, you will know that the maximum rate at which this bank can be recharged is 70 amps (bank capacity x 25%). Therefore, it will take two hours of engine run time per day to replace the electricity that the cruiser is using (70 amps x 2 hours = 140 amp-hrs).

If, on the other hand, the cruiser wants to minimize their engine run time, they could increase their battery bank to far in excess of that recommended by the energy analysis approach to, say 600 amp-hrs.

With this bank the acceptance rate would now be 150 amps, making it possible to replace the same 140 amp-hrs in less than one hour, or only ½ the time required by the smaller bank. If all that sounds great but you don’t have room for that many batteries, use gel cells or AGMs. You’ll be able to get that high charge acceptance rate in a much smaller bank.

By gaining a good understanding of battery acceptance rates, it is clear how large house battery banks can be used to reduce the engine run time. It is equally easy to see why the popular practice of separating house batteries into multiple banks is not the most efficient use of energy.

Battery life

One cannot really discuss “battery life” without first defining the term. The same energy gurus who promote the energy analysis method of battery bank sizing usually describe battery life by the number of cycles (ie. discharge and recharge) which can be done before the battery fails.

Through their efforts, most people now recognize that depth of discharge has a direct impact on the number of discharge/recharge cycles a battery can do. As a result, many cruisers endeavour to cycle their batteries as “shallowly” as possible.

However, the number of cycles is only one measure of a battery’s life and probably not the best. Another way to look at battery life is by looking at the total number of amp-hrs which a battery will store before failure.

When viewed in this way, quite a different picture emerges as can be seen in the chart below.

Typical Cycle Life

(100 amp/hr Trojan Deep Cycle Battery):

Depth of discharge Number of cycles Total amp-hrs provided during service life
10% 6,200 62,000
20% 5,200 104,000
30% 4,400 132,000
40% 3,700 148,000
50% 2,900 145,000
60% 2,400 144,000
70% 2,000 140,000
80% 1,700 136,000

From our previous look at acceptance rate we know that batteries can be recharged much faster when they are permitted to cycle down to 50% and below. From this chart it is obvious that doing so extends the useful life of the batteries as well.


By first calculating how much energy your system will use, and then applying the principles discussed in this article to your battery and charging system, you will see just how much better off you are with the DC system rather than an direct engine-drive.

For those who do not mind engine running, there is still a place for engine-driven refrigeration. However as you will see once your calculations are complete, they rarely make sense on a cruising sailboat.

Correctly set up, your DC system will typically require 1/2 to 1/4 the daily engine running time of an engine-driven system. By properly planning and setting up your DC system and combining it with energy efficient appliances such as refrigeration and water makers, battery charge concerns really can become a thing of the past.

Battery Care and Maintenance

Here is some sound Battery Care and Maintenance information from Marine Smart Energy. Marine Smart Energy is a full supplier of new electrical products for the starting and charging of Marine equipment.

A good battery can provide four or five years of worry-free service with the right kind of care. But batteries can also die out fast, in less than six months if they are neglected. It is very necessary in order to keep a healthy battery, that the charge rate be maintained within the recommended boundaries.

For lead acid batteries, if neglected, the electrolyte level becomes very low and the battery starts to lose power, since part of each plate is above the water line. This keeps the rest of plate overworked. Tops of the plates exposed to air start to dry out and harden.

Once this happens the plates will lose most of their ability to accept a charge and produce electricity. Lead acid batteries generally require adding water about twice a year. Electrolyte levels should be maintained about a quarter inch above the tops of the battery plates.

Although AGM, Carbon Foam and now Lithium batteries don’t require adding water they also need proper maintenance for optimum performance and longevity.  As for the failures related to each type of battery here is an interesting article on “Battery Blow-Ups and Melt-Downs” from Coastal Climate Control. Article

Outside the Battery

The outside of the battery is just as important as the inside. For example, dirt or acid salts can build up on top of the battery. A conductive layer is formed causing a constant discharge drain on the cells.

A baking soda bath with water and dish detergent, 3 times per year helps keep outside battery losses at a minimum. Never allow this mixture to enter the battery as this mixture will neutralize acid and cause dead cells.

Corrosion of the cable terminals is a major cause of battery problems. (Spraying Corrosion Block here can help)

Check all terminals to see that they are absolutely clean and tight. Check for broken wires at battery cable connections.

Proper care of cables and terminals is as important as maintaining the battery.

Never store a battery for more than 30 days without recharging, or store a battery for any length of time in a discharged state. Sulfation starts when a battery becomes discharged. Sulfation is normal.

When sulfation happens, lead sulfate is formed from the self-discharge of the plates or the plates standing in a discharged state for a long period of time. Hard, bulky, crystal-like substances are created on the plates.

You must have a charger with a “EQ” Equalize Button, when you Equalize the battery you in essence over charge it (15.5 volts) to break down the sulfides deposits on the plates, thus giving the battery more life and the ability to hold it’s original rated charge.

The Equalize Mode is a controlled overcharge used periodically (example, once each month or once every other month) to help dissolve any recently solidified sulfate deposits on the battery plates. Older sulfation deposits normally harden onto the plates, and cannot be dissolved into the electrolyte.

Batteries that are in a discharged state for too long and are not being used, will lose the balance of their charge. It is not recommended that a battery go for more than 30 days without getting a charging current. What also happens is that the pores of the active materials get clogged from the large bulky crystals of sulfate.

The active material gets pushed out of the grids causing them to buckle. If too much internal expansion happens from a sulfated battery, the case will become bulged or cracked.

**WARNING** Batteries give off hydrogen gas constantly. Hydrogen gas is highly explosive. Battery acid is corrosive and can burn skin. Always wear safety glasses or goggles and use caution when working with batteries.

Battery Testing: Open Circuit Voltage Test

Before you can properly test any battery it must be fully charged. You can verify the state of charge two ways: (Doing Both is Recommended)

The simplest test is done with a digital multimeter.

The other test involves the use of a hydrometer.

Using the digital multimeter to verify the state of charge:

A fully charged 12 volt battery will read at least 12.6 volts on the multimeter when connected as shown below.

Open Circuit Volts: Percent with no loads or chargers connected

  • 11. 7 volts or less = 0%
  • 12.2 = 50%
  • 12.4 = 75%
  • 12.6 or more = 100%

Digital multimeter will read 12.6 volts on a fully charged 12 volt battery.

(2.1 volts per cell times “6 cells” = 12.6 volts)

Connect the digital multimeter to the battery terminals. If your reading is 12.4 or below you must recharge the battery before testing.

Using a hydrometer to verify the state of charge:

When testing batteries with removable filler caps, the hydrometer can be a very useful tool. Always use protective acid resistant gloves and safety glasses or a full face shield.

Each cell of the battery can be individually tested for state of charge. A reading of 1.250 is usually considered good. (See hydrometer below.) If a cell is found with specific gravity below 1.150 the battery is considered dead. When one or more of the cells produces a reading that is .050 or more below that of the others, it’s a pretty good indication that the low cells are shorted. If all the cells read below 1.250 then the battery must be recharged before testing.

State of Charge – Standard Specific Gravity

  • 100% charged = 1.270
  • 75% charged = 1.225
  • 50% charged = 1.190
  • 25% charged = 1.155
  • Discharged = 1.120

What are the advantages of the Cruiser Marine Alternator producing 70% of amperage at idle?

Your original alternator most likely is around 35 – 65 amps but this amount of output is only achieved at motoring speed. At idle your original alternator will only be producing approximately 15- 30 amps. A Cruiser 100 amp alternator (for example) will produce approximately 70 amps at your engine’s idle. This is over 50% more output, which is a considerable improvement over your original alternator. This improvement has many other benefits which you can read here, below.

How does a Cruiser Marine Alternator reduce engine running time?

As mentioned above regarding the advantages of producing more amperage at your engine’s idle, this will directly affect the amount of time it requires to charge your battery bank. The more amps your alternator can produce the quicker the batteries can be charged and the less time you have to run your engine(s). Your batteries get charged quicker so you also reduce the amount of wear and tear on the moving components of your engine. Read below to view how the Cruiser Marine Alternator reduces fuel consumption.

How does the Cruiser Marine Alternator reduce fuel consumption?

Remember how reducing your engine’s run time by producing more amps at idle was affected? Well simply put less engine run time equals less fuel used! If you can run your engines for shorter periods of time you will use less fuel, and that is money in your pocket!

Why do I not see a cooling fan on the outside of the Cruiser Marine Alternator?

The Cruiser Marine Alternator uses 2 Internal Fans for twice the air flow capabilities as compared to the traditional single exterior fan found on most original marine alternators. This works in conjunction with our Non-Coated casings for exceptional cooling properties.

Why is there no coating or paint on the Cruiser Marine Alternator casing?

The Cruiser Marine Alternators are cast from aluminum with no coating. Any coating that is put on a casing will actually decrease the heat dissipation properties of the aluminum. This is especially true in marine applications as opposed to automotive applications where there is an abundance of air flow available. Most marine engine compartments are cramped, confined areas with little or no air flow to externally cool the alternator. The Cruiser Marine Alternators use the natural heat dissipation of the casing in combination with dual internal fans to regulate the heat produced during operation.

What is the difference between a “marine” alternator and a regular alternator?

The main difference between a Marine Alternator and a standard alternator is in the ignition protection. Most marine engines are found in a compartment that is enclosed and have very little air flow. With these enclosed areas the gas fumes can easily and quickly build up and create an ideal situation for the fumes to be ignited.

An alternator can in fact create a spark and ignite the fumes which is what a marine alternator is designed Not to do! The Cruiser Marine Alternators are built to meet or exceed CSA, USCG, ISO, CE & SAE Ignition Protection Standards. They are designed to eliminate sparks that can ignite built up fumes.

Why do I need to activate my warranty?

Marine Smart Energy warrants your alternator for 1 year from the date of purchase. There is no activation or warranty form required. Some units may be found on a brand new boat that you have just purchased.

In this case your 1 year warranty period with Marine Smart Energy begins on the date you take delivery of your new boat, allowing you to get the full term of the warranty and not have the warranty run out while sitting in a ship yard or in inventory.

Can I purchase an extended warranty?

Yes. Marine Smart Energy offers a (2) year and (4) year extended warranty on all Marine Smart Energy products. An extended warranty is added to the (1) year included warranty. A (2) year extended warranty adds (2) years to your manufacturers warranty giving you (3) full years of coverage.

The (4) year extended warranty gives you a full (5) years of warranty. Extended warranties must be purchased during or before the end of the included (1) year manufacturers warranty.

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