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Power Management Calculator

Boat Systems Technology

Recently I have had the privilege to teach small groups of adults how to sail, and how to utilize and maintain the many systems found onboard a typical cruising sailboat. Teaching cruisers to manage battery usage and power consumption has been a challenge. The typical cruising sailboat is a complexity of DC and AC circuits, with DC batteries of various types & sizes and many different charging sources, also of both the DC and AC current type.


DC /AC Distribution Panel

A user-centered approach is one of the best ways to train cruisers in basic power management. There are many modern devices to help monitor battery life these days. But those devices will only help indicate when something is out of the ordinary, they will not prevent problems nor will they tell the owner or crew how to correct a system that is failing to provide enough battery power.

The management of power on a boat (where batteries are employed for any reason) involves managing Amp hours (Ah). One Amp hour is the amount of energy in a battery that would theoretically provide x number of Amperage for 1 hour of time. So 50 Ah of energy in a battery will light a DC bulb that draws 1 Amp of energy and theoretically continue to power it for 50 hours. The same 50 Ah would only run a pump drawing 5 Amps for 10 hrs. Another way to state it would be to say that a 1 Amp bulb run for 14 hours would eat up 14 Amp hours. So after using the light, 14 Amp hours would need to be put back into the battery somehow.


DC Voltage Guide

With a grasp on Amp hours, which is the currency of power management, we next need to understand a battery state of charge. We can use sophisticated monitoring devices, but we can also simply observe voltage at the battery terminals.

Then we need to understand that not all batteries charge alike, and think about the charge rate of a given battery. This is known as the Charge Acceptance Rate (CAR). Battery type (wet cell, AGM, Gel cell, etc) affects CAR, as well as the age of the battery, the temperature while charging, and how many cycles it has experienced. A cycle is when the battery is drained to any point and then recharged. Deep cycle batteries can be drained to less than 50% charge and recharged, shallow cycle batteries need shallow cycles. The CAR matters when recharging your battery because without a charging source that matches the CAR, the battery will not full recharge or maintain good health over the long run. So the first step in power management is to find out the type, overall condition, size and CAR of the batteries in a given system.

Example: Two (2) Gel Cell deep cycle 12 volt marine batteries each with 200 Amp Hour Capacity (connected in parallel) gives us a bank of 12 volts with 400 Amp hours.

Now we just need to figure out how much power we want to use, or will end up using in a given situation, and then we can figure out how to put that energy back once we've used it. For figuring out Power Use, creating a table is helpful, using the specific components of a given boat or system.

DC Amp Hour Consumption Table Under Sail
Device Amp Draw Hours Ah Total
Bilge Pump 5.0 1 5 Ah
Anchor Light 1.0 6 6 Ah
Masthead Light 1.5 0 0 Ah
Running Lights 3.0 1 3 Ah
Cabin Lights 2.0 3 6 Ah
Corded Spotlight 10.0 .25 2.5 Ah
Spreader Lights 8.0 0 0 Ah
Deck Lights 8.0 .25 2 Ah
Engine Blower 1.0 1 1 Ah
Cabin Fan 1.0 6 6 Ah
Laptop (180 Watts AC) 15 2 30 Ah
Radar (all components) 5.0 2 10 Ah
Depth Sounder 0.5 10 5 Ah
Autopilot 20 6 120 Ah
Chart Plotter 1.5 6 9 Ah
Knotmeter 1.0 6 6 Ah
Wind Speed 0.1 6 .6 Ah
FW Pump 5.0 .25 1.25 Ah
SW Pump 5.0 .25 1.25 Ah
Refrigeration 6.0 6 36 Ah
Anchor Windlass 50 .10 5 Ah
VHF (Receive) 1.0 6 6 Ah
Stereo 2.0 6 12 Ah
Total 273.6 Ah

We used a conservative amount of time-in-use for each component in the Hours column.

The actual amp draw of each device will be different from one boat to another. Each component has a published spec, usually marked on the box or in the manual (or in online specs) showing either Amp or Watts consumed. Let's assume for this example that with no charge sources engaged (solar, generator, engine) the system would use approximately 200 Ah in a 24 period. If we take away 200 Ah from the battery bank example we are using (400 Ah capacity) we get a battery bank that is 50% depleted. In other words our State of Charge is 50%. If we then actually checked the voltage at the battery terminals, using a voltmeter, we could look at the voltage table above and expect to find the battery bank to be at 12.10 volts.

We know how much DC energy needs to be replaced in our 12 volt battery bank. And we have a limited number of charging sources on a boat. So what is important is figuring out how long it will take to replace the energy given our charging sources.

This Charge Time Calculator can use the variables from a given scenario and estimate the total charge time. This is a simple JavaScript program built using published specifications from battery manufacturers. There are many more variables that affect charge time, but the ones used here are the minimum required to make an estimate. Most importantly is the amount of Amp Hours needed to be replaced. Using the table above, we can enter a rounded off 273 Amp Hours Used.

We also need to know what the Amp output is from our charge source, as well as the loads that will be on the system during charging, because they will siphon away energy that would otherwise go into the batteries. Enter 55 Amps for the alternator output, which is the average output on a stock diesel engine alternator for a 30 ft sailboat.

Next enter what engine loads the DC system will have to run while we are charging the batteries. For a 30 ft sailboat it would be approximately 10 Amps, which mostly goest to powering the engine console gauges and lights.

Next, enter the DC panel loads that the DC system will be asked to run during charging, such as occasional bilge pump, stereo, interior cabin lights, etc. This number will vary greatly depending on the boat and the activities of the crew, so we'll use a very conservative 5 Amps.

The inefficiency factor in the calculator is set with a minimum spec from battery manufacturers, but can be higher in charging systems that have faulty connections, or if the batteries to charge are old or have physical internal damage.

Lastly, we will choose the battery type, because the calculation is different based on battery technology used.

If you calculate using the above variables, you should get 9.82 Hours of charge time to replace the 273 Amp Hours. That's a long time to run the engine. Instead of 55 Amps for the alternator, use 100 Amps to see what difference it makes in charge time.

The conclusion you should draw is that sizing the alternator correctly is very important to be able to charge your batteries in a timely fashion to increase your enjoyment of the boat and keep the batteries healthy.

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