Boat Batteries

What Boaters Must Know About Boat Batteries

Without them, you are dead in the water. Boat batteries not only get you started, but they also give you the power to do all of the fun things you love about boating.

Most boats have two types of batteries. Starting batteries and deep-cycle batteries. What makes a starting battery different from a deep-cycle battery is the thickness and number of the lead plates inside the battery.

Starting Batteries

Starting batteries have one important job. To start the motor. They provide short bursts of high current to turn the starter motor. Once the motor starts the batteries job is done.

Starting batteries have a large number of thin, porous plates. This creates a large surface area that is good for producing a high current. The drawback to thin plates is that they don’t store a lot of energy and will break down when deeply discharged. Deep-cycling a starting battery will quickly destroy it.

Getting the Right Starting Batteries

When buying a starting battery the first thing you need to find is your motors minimum requirements. How many CCA/MCA and how much Reserve Capacity your particular motor needs. You can find this information in your owners manual or at the manufacturer’s website.

Motor Size: The larger the motor the more current it takes to get it started. As an example, here are the minimum CCA’s for a few popular motors.

  • 150 HP Yamaha outboard minimum CCA 380
  • 5.0L Volvo inboard minimum CCA 650
  • 5.9L Cummins diesel inboard minimum CCA 1100

Before buying a battery with the minimum requirements for your motor think about how you use the boat. If you cruise all-day the minimum requirements for your motor might be all you need. But if you start and stop the motor often, you should look for a battery with higher reserve capacity.

Starting Battery Terms You Should Know

Cold Cranking Amps (CCA): A measurement of how many amps the battery will produce when cranking an engine for 30 seconds at a temperature of 0°F.
Marine Cranking Amps (MCA): A measurement of how many amps the battery will produce when cranking an engine for 30 seconds at a temperature of 32°F.
Reserve Capacity: How many minutes it takes a fully charged battery to fully discharge with a 25 amp load at a temperature of 80°F.

boat batteries keep the lights on

Deep-Cycle Boat Batteries

Deep-cycle batteries are designed for capacity and a steady flow of current for long periods of time. Steady current is good for running electronic devices but is not good for starting motors.

Deep-cycle batteries have fewer, thicker, and less porous plates than starting batteries. Thick plates increase capacity and are better at standing up to the stress of repeated deep discharging.

Battery Capacity

A deep-cycle batteries capacity is shown in amp-hours. But it’s capacity depends on how many amps are taken from the battery over a given time.

To make it easier to compare one deep-cycle boat battery to the next, manufacturers use what is known as the C Rate. The C Rate shows how many amp-hours the battery will produce over a set time period. Several C Rates are given. Usually 5 hours, 20 hours and 100 hours. Each of these C Rates shows the capacity of the battery at different discharge currents (amps). The C rate is listed as a C, followed by the time (C5, C20, C100), then the capacity in amp-hours. The C Rate used most often for comparison is C20.

The C20 rate tells you the number of amp-hours (Ah) the battery can produce when discharged over a 20 hour time period. Dividing the number of amp-hours by the stated C rate tells you how many amps the battery can produce each hour for 20 hours.
For a 100 Ah C20 battery: 100 / 20 = 5 amps
For a 120 Ah C20 battery: 120 / 20 = 6 amps

Calculating Capacity

The capacity of a battery depends on the current being taken from it.

As you can see, a 100 Ah C20 battery will give 100 amp-hours of capacity when discharged for 20 hours at 5 amps. However, that same battery will have less than 100 Ah capacity when discharged at 6 amps and more than 100 Ah capacity when discharged at 4 amps.

If this isn’t complicated enough, the difference in amp-hours produced at different amperages isn’t even. A C20 100 Ah battery, when discharged at 10 amps, will give you less than 50 Ah, and fewer than 10 hours of time.

To find out how long deep-cycle boat batteries actually last at any given discharge amperage we need Peukerts Law.
Peukert's formula

Peukerts Law is a mathematical formula that uses a number to show how efficient a battery is. The closer the number is to 1, the more efficient it is. With it, you can accurately calculate battery capacity when discharged at any given amperage.

The chart below shows the Peukert number for different types of deep cycle boat batteries. AGM batteries are the most efficient, flooded batteries are the least efficient.

Peukert Number
AGM Gel Flooded
1.05 – 1.15 1.10 – 1.25 1.20 – 1.60

How Many Amp-Hours Do You Need?

Battery capacity depends on how many amps are taken from the battery over a time period.

To find out how much amperage you use, start by listing all of the electronics running when the motors are off. Navigation lights, radio, stereo, cabin, deck lights and so on.

Next, you need to find out how many amps each device uses. You can find this information in one of three places.

  1. The nameplate on the back or bottom of the device.
  2. In the owner’s manual or specification sheet that came with the device.
  3. Online at the manufacturer’s website.

Sometimes the power a device uses will be listed in amps and sometimes in watts. We will be calculating the amount of power being taken from the batteries in amps. To convert watts into amps divide the number of watts by the voltage. The voltage will be 12-volts or 24-volts depending on your boat.
50 watts / 12-volts = 4.16 amps.

Add up all of your devices to get the total number of amps.

Now, how long do you use them?

If you stay on your boat overnight you might depend on your batteries for 4 to 6 hours a night. But if you spend your day cruise the waterways with your family you might only use them for an hour at a time.

Now that you know how many amps you use and for how many hours, you can find out how much capacity you need. You can also find out what type of battery is best for your needs. This is where Peukerts Law comes in handy. Here is an example.

Tom and Terry own a 34′ cruising yacht. They spend weekends exploring the waterways and sleeping on their boat. The longest they run all of their electronics while anchored is 6 hours.

Electronics Used Amps
Navigation Lights 2.5 amps
VHF Radio 3 amps
Stereo 5 amps
LED Courtesy Lights .36 amps(6pcs X .06A)
Cockpit Downlights 1.25 amps (5 pcs X .25A)
Underwater Lights 10 amps (2 SCR-16 X 5A)
Spreader Light 2.6 amps (2 X 1.3A)
Galley Lights 0.24 amps (4 pcs X .06A)
Cabin RGB Strip Lights 3.5 amps (2 pcs X 4′ strip)

When you add everything together we come up with 28.45 amps per hour.

To estimate how many amp-hours of capacity they need we multiply the number of amps per hour by how long they run everything per night.
28.45 X 6 = 170.7A

To get the most life out of a deep-cycle battery it is best to not discharge it more than 50% so we multiply the total amps used by 2.
170.7A X 2 = 341.4

It looks like Tom and Terry will need at least 400 amp-hours of battery capacity. 4, 100Ah C20 flooded batteries should work for them.

To be safe though, lets use Peukert’s Law to find out how many amp-hours of battery capacity they actually need.

Do The Math

Multiply the load in amps times the batteries specified discharge time in hours.
28.45 x 20 = 569
Divide the batteries specified amp-hours by that number.
400 / 569 = 0.7
Multiply that number by the specified discharge time.
20 x 0.7 = 14.05
Divide that number by the Peukert number.
14.05 / 1.4 = 10.04 hours

Yes, 4, 100Ah C20 flooded batteries will work. However, using a more efficient AMG battery will give them a few more hours of time for a total of 12.8 hours.

Of course, this is just a rough estimate. It does not include power draws such as a refrigerator (around 1 amp/hour) or navigation lights being left on all night (around 15 amps). It also over-estimates the power used by lights. You probably won’t use the spreader light or galley lights the whole time. All other lights may be dimmed as well.

I hope this information helps you understand your boat batteries and how they work. If you find it useful please share this post on your favorite forum or social media website.

Learn more about batteries at Battery University.