Sizing Your Battery Bank

Sizing Your Boat's Battery Bank: Key Takeaways for Off-Grid Success

• Base Capacity on Daily Loads: To size your marine battery bank properly, first calculate your total expected 24-hour energy consumption in amp-hours (Ah) by evaluating the power draw and run-time of all onboard electrical items.
• Factor in Inverter Loss: If your onboard appliances run off an inverter, you must multiply their estimated amp-hour requirements by approximately 1.15 to compensate for AC conversion energy losses.
• Maintain a 50% Depth of Discharge Limit: To prevent battery damage and maximize lifespan, standard marine batteries (excluding LiFePo4) should not be depleted below a 50% state of charge. Therefore, the absolute minimum bank size should be at least double your daily energy needs, plus a 20% safety margin.
• Target 3 to 4 Times Your Daily Needs: If budget and space allow, creating a battery bank with 3 to 4 times your daily power requirements provides maximum flexibility, allowing you to go up to three days between charge cycles and enjoy a quieter anchoring experience.
• Understand Charge Acceptance Rates: Larger battery banks accept a higher volume of charge simultaneously, which significantly reduces engine or generator run-times. Charge acceptance varies heavily by chemistry:
  • Flooded (Wet-Cell): 10% to 25% acceptance rate (6–12 hours average charging time).
  • Gel: 20% to 40% acceptance rate (6–10 hours average charging time).
  • AGM: 30% to 50% acceptance rate (4–8 hours average charging time).
  • LiFePo4 (Lithium): 50% to 100% acceptance rate (1–2 hours average charging time).
• Recognize Critical Battery Ratings: Always evaluate a deep-cycle battery's Reserve Capacity (RC) for its ability to sustain heavy loads before dying, Cycle Life for its total lifespan potential before failure, and Amp-Hour (Ah) Rating for meeting baseline daily loads.
• Monitor Operational Voltages: Standard marine batteries are completely discharged at 10.5V and should generally be maintained between 12.45V and 12.75V for prolonged life, whereas Lithium Iron Phosphate (LiFePo4) batteries operate at a higher range between 12.8V and 13.6V.

Battery banks and how they work

A battery bank is a collection of multiple batteries connected together to store energy for various applications, such as running the refrigeration on your boat. It works by charging the batteries with electricity when you're at the dock, and then discharging that energy as needed when you've left the dock.

What size battery bank do I need for my boat?

When sizing a marine battery bank for off-grid use, the standard line has usually been to figure out your daily energy needs, and then base your marine battery capacity calculation on that number. While that number is definitely important, we believe that there is something else to consider – namely, how much time do you want to spend listening to your engine or generator running each day while you wait for the batteries to recharge?

With that in mind, this article will take a deeper look at how to calculate the size of a battery bank, including energy analysis, charge acceptance, marine battery depth of discharge and battery bank size to help you arrive at a more realistic conclusion of what your needs are, before you make your purchase decision. So let's come up with that number!

Conventional Energy Analysis

Analyzing your energy needs onboard is the most important step to determine how many amp-hours (a measure of electrical capacity) you will need in your battery bank. To do this, you need to first determine how many amps each electrical item on your boat consumes per hour. These items might include cabin lights, your iPhone, the TV, a laptop, refrigeration, the stereo, VHF, navigation instruments, running lights, etc.

To figure out how much they all draw – run each item individually while measuring how many watts or amps are being drawn. Times this by how many hours you think the item will need to run during a 24-hour period. For instance – you plan to run several cabin lights for approximately 4 hours every day. All of the lights together use 5 amps per hour – so they will use a total of 20 amps over 24 hours.

Continue on in this manner until you've figured out every single item that requires energy onboard, then calculate your total expected energy consumption in amp-hours for a 24-hour period. Many electrical appliances will state their theoretical amp draw in their manual or on their label, saving you the effort of running the actual test. Also of note – if your electrical items need to run off your inverter, you will need to multiply their estimated amp hours by approximately 1.15 to compensate for the loss of converting that power for AC usage. For more information on this topic, read our Inverter Basics article.

To say this will vary from boat to boat is a complete understatement – but for this example let's say that your total came to 100 amp-hours per day (which would be a low estimate for many modern boats). If we assume that we don't want to deplete our batteries more than 50% (since that can shorten their lifespan) then we need a battery bank with a minimum of 200 amp-hours.

But if you only buy a 200 amp-hour battery bank, it means that you will be spending a lot of time every day charging your batteries – especially since recovering from a very deep discharge can take significantly longer than a lighter discharge. That's why there is still quite a bit more to consider.

The Importance of Battery Ratings and Discharge

• Reserve Capacity: Arguably one of the most important battery ratings to consider is reserve capacity (RC). This is the number of minutes a fully charged battery can maintain a “useful” voltage, while operating at a 25 ampere discharge rate at 80° F, until it is completely dead. The higher this number – the greater the battery's capacity for deep discharge before recharging is necessary – highly useful when looking at our house bank!

• Cycle Life: Also important is a marine battery cycle life – which is the estimated total number of times it can go from a 100% charge, to a full discharge and back to a 100% charge again before failing – so obviously, the higher the cycle life, the longer your new battery is going to last.

• AH Rating: Lastly, you want to look at the battery's amp-hour (Ah) rating – that is how many total amps the battery can reasonably deliver for 20 straight hours. This marine battery sizing rating is what you're taking into consideration to make sure you have enough amp-hours to cover your daily energy needs.

So we're basically looking for a deep cycle battery that has a high reserve capacity, good cycle life and enough amp-hours to meet our daily needs. But another factor to consider is how often we will need to charge.

The Importance of Charge Acceptance in Marine Battery Capacity

The charge acceptance rate is the maximum rate at which your battery bank can be recharged – which will be determined by the type of batteries, their amp-hour rating and their current state of charge. Here are the general acceptance rates of the main battery types:

• Flooded (wet-cell) batteries have an acceptance rate 10 to 25% of their amp-hour rating. So if you have two 8D size batteries with 220 amp-hours each wired in parallel, at the highest, your 440 amp-hour bank has an acceptance rate of 110 amp-hours (440 x 25%).

• Gel batteries have an acceptance rate of 20 to 40% of their amp-hour rating, depending on the quality of the battery (higher quality equals better percentage of course - you get what you pay for). So the highest rate for that same bank of batteries would be 176 amp-hours (440 x 40%).

• AGM (Absorbed Glass Mat) batteries are slightly higher yet, with a charge acceptance rate of 30 to 50% of their amp-hour rating (again, depending on the quality of the battery). That means that our bank of AGMs would hypothetically have an acceptance rate of up to 220 amp-hours!

• Lithium Iron Phosphate (LiFePo4) batteries have an astounding 50-100% charge acceptance rate - which is why they are gaining in popularity. At this rate our batteries would have up an acceptance rate of up to 440 amp-hours! Think how quickly they could recharge at that rate.

It's important to note that a LOT of other factors go into how fast a marine battery can recharge, including the charger you use, how much the batteries were depleted when you begin recharging, the battery charging profile (i.e. bulk, absorption or float cycles) and more - so make sure you do your research.

Is charge acceptance the most important aspect to consider?

No – it's not just about charge acceptance – it's also about the SIZE of the battery bank. The more amp-hours you have, the more charge capacity you get. So while one 8D flooded battery has 220 amps, with a charge acceptance rate of just 55 amps – three 8D flooded batteries in parallel would equal 660 amp-hours with a charge acceptance rate of 165 amps. Not only does that give you more amp hours in your house bank, but it will also allow for faster recharging.

How the engine alternator factors in

Now that we know our charge acceptance rate, and about how big our bank should be - how do we figure out how long we will have to charge at anchor? By taking a look at how many amps our alternator can provide along with our acceptance rate, we can calculate exactly how long we will need to charge to recoup that 100 amp-hours we plan to use each day.

Continuing with the prior example – while all engines are different, let's say the alternator in this example has the ability to charge at 120 amps. Since my single flooded 8D battery (220 Ah) can only accept up to 55 amps of charge (even when the alternator can give more), I would need to run my motor for two hours to recoup the 100 amps I plan to use each day. Conversely, with a larger 660 Ah bank (capable of accepting 165 amps per hour) I can put in the full 120 amps my alternator can provide – which means it will likely take less than one hour to recoup that same 100 amps!

Back to the basics of battery sizing

Now that we've overwhelmed you with all this technical information to consider, let's go back and review the basics. The most important factor in sizing your battery bank is figuring out your daily power needs. At a minimum, you should have enough amp-hours to at least double that number – with say a 20% margin for safety since you don't want to run your batteries down so much that you damage the plates or can't start your engine to recharge them.

If you have the room and the budget, we highly recommend you have 3-4 times the amp-hours you need – giving you far more flexibility in how often you can go between charge cycles. A bigger battery bank not only allows for faster recharging, it also means you can go longer between charges. Returning to our 100 Ah usage model – if we have a bank of 200 Ah, we have to recharge every day. But if we have a bank of 600 Ah we can potentially go up to 3 days before recharging. For those of us that prefer listening to the birds instead of the engine, that is a hugely significant difference.

How to get help converting Watts to Amp Hours

Sometimes when you're trying to assess how many amp hours all your various electronics are going to use, the power listed by the manufacturer is listed in Watts (W) instead of amps (Ah). It's quite easy to do a quick marine battery watt-hour calculation by looking up one of the many online calculators that will convert watts into amps. So as long as you know one or the other for a specific piece of electronics, you should be able to calculate what that item will need.