In a recent article on boats.com I talked about amperes in your boat’s electrical system. In that article I focused on the need to deliver enough amperage to your appliances, how to measure it, and the need to make sure controls are in place in the event of a short circuit where too much amperage could flow, causing damage if the problem isn’t dealt with promptly.

In this article I’ll talk about amperage again, but this time we need to consider the finite limits in available amperage on board your boat. We need to learn a bit about amp-hour ratings for things like alternators and battery chargers, and about how to size a battery bank to keep all your gear running at its peak performance for as long as you need it to.

Starting Batteries and Battery Ratings

It’s important to understand the different ratings used to determine battery capacity and also to understand that sometimes battery manufacturers will use specifications that may do more to confuse you than actually help in making the right choices.

This deep-cycle marine/RV battery is rated for 500 MCA (marine cranking amps) and 80 amp-hours. The 140-minute reserve capacity indicates the number of minutes a battery can deliver 25 amps of current without dropping below 10.5 volts. An interesting rule of thumb is that typically if you divide a given reserve capacity by two, you will derive the approximate amp hour capacity.

The most common battery rating is the CCA or “cold cranking amp” rating. In the marine realm you may also see MCA or “marine cranking amps." These ratings describe amperage values that are useful for determining if the battery or batteries in your system have enough power to get your boat’s engines started, but not much else. Even that is a bit subjective, as boatbuilders and engine manufacturers rarely provide cranking amperage data today. The engine folks will tell boatbuilders what size wire to use and the maximum distance they want the batteries from the engines, and recommend what battery “group” size they require, but they rarely talk in terms of cranking amps any more.

The important thing to remember here is that the CCA specification is derived at 0°F and that MCA values are derived at 32°F. I’ve personally always felt that the MCA ratings were created by marketing folks to confuse consumers. Using MCA ratings enables the battery folks to use higher cranking numbers in their specifications. People are generally impressed by bigger numbers, but consider that two batteries, one with 800 CCA and one with 800 MCA actually show a fairly significant difference in actual cranking amps. At 0° F, the MCA rated battery will not be able to deliver 800 amps, but something considerably less.

A clamp-on meter set to measure amperes will help you determine the electrical demands of your system.

To determine your boat’s cranking amp requirements, check with your amp clamp around the largest red wire attached to your starter motor. Have someone crank over the engine for you and note the amperage shown on the meter. (Your clamp may have a “peak hold” function on it, which makes the measurement easier.)

Knowing the maximum amount of amperage your starter motor draws under ideal operating conditions gives you a benchmark to work with. As always, make sure your battery and all of the connections to it are in good condition before you conduct this test.

Deep Cycle Applications and House Loads

The next most common specification provided by some battery manufacturers is the amp-hour (Ah) rating. For servicing most on-board DC loads and appliances this is the specification you need to have. But understand that this specification can be misleading, too, unless you know the time component associated with the rating. Generally, the industry uses what's known as a “20-hour rate," but not always. Another way of expressing this is with a “C” rating, which is increasingly in use today. The C stands for “capacity” and will be followed directly by a number. C-20 or C-100 are examples of what these ratings look like, and for deep-cycle applications, representing the ratings in this manner is more realistic. In those examples, the C-100 represents a value at a 100-hour rate. The other consideration is how much amperage the battery is able to deliver during that specified time interval. For comparative purposes, it's best to use the C-20 rating, which is further defined like this:

For a fully charged 12-volt battery, a specification that reads 100 Ah at a 20-hr rate means that the battery can theoretically deliver 5 amps of electrical current for 20 hours before reaching a minimum voltage level of 10.5 volts.

Keep in mind that this capacity issue is not a perfectly linear progression. This can get pretty esoteric when we start looking at some of the rules established in the late 1800’s by a German scientist named Wilhelm Peukert. He figured out that a battery’s capacity actually decreases as the rate of discharge increases. So, in the example above, that 100 Ah @ C-20 rated battery above will not actually be able to deliver 100 amps over a period of 20 hours if the rate of discharge is, say, 9 amps instead of 5. This all gets further complicated by the additional fact that we should never fully discharge any battery, as that will have a serious impact on the total cycle-life of the battery, i.e. the number of times a battery can be discharged and recharged during its life. Historically we have used a 50% discharge as the benchmark, meaning that we’d only count on 50 amps in a 100-Ah rated battery. This value has changed in recent years as we now have battery vendors claiming 80% discharge levels as being acceptable, with no negative impact on cycle life.

The bottom line with all of this is to use the C-20 rate as a point of comparison with any claims of deeper discharge capacity and cycle-life predictions to establish which battery can give you the longest potential service. Don’t forget price either. I like to calculate this using a cost-per-amp-hour value, which takes all of the metrics discussed here into consideration.

To make an accurate assessment of your boat's requirements for both mission-critical and less important demands, use a load requirements worksheet like this example from the American Boat and Yacht Council.

Determining Amp-Hour Needs

There are several factors you need to consider here. First is to perform a load analysis. With this you’ll be measuring how much current each of the DC electrical devices on your boat uses. The easiest way to do this is to use your amp clamp to measure the current draw on the positive feed wire on each of the circuits on your boat.

Now, you'll probably never actually run all of your electrical appliances at full amperage load at the same time, much less for a protracted amount of time. But it's still important to add up the demands of all your gear, first of all the appliances that the ABYC defines in their E-11 standard as "mission critical." These would be safety-related items like running lights, navigation equipment, and VHF radios. Things like anchor windlasses, cigarette lighter power sockets, cabin lights, and other gear only operated intermittently are considered less mission-critical. Then there are systems like electric refrigeration that are not mission-critical, but that run constantly and draw significant amounts of electricity.

The idea here is to establish your own list based on the specifics of your boat and the types of DC electrical equipment you have installed. Take 100% of the amperage values measured for what you consider to be your mission-critical equipment, then add the amperage value of the highest-drawing piece of equipment from your intermittent or less-than-critical equipment. (Alternatively you can use 10% of the total from that group.) You will then have established your total DC load. Only after you've done this can you accurately determine things like what size battery bank, alternator, or battery charger you need.

Battery chargers and alternators

Armed with knowledge of your actual amperage draw and amp-hour needs, you should be able to make some intelligent choices relative to engine alternator sizing in terms of amperage output, as well as how large a battery charger you need.

The goal is to have an alternator robust enough that with the engine running it can power all of your critical loads and still have enough extra power to recharge your batteries simultaneously. You do the math.

Bear in mind that installing a new high-output alternator on an older engine will probably require other modifications, too.

Keep in mind that it’s wise to build in some reserve. An alternator that's operating at full output for extended periods of time will not last as long as one that runs at no more than about 75% of its rated output for extended periods.

The same is true for battery chargers: Running one at high output for long periods of time will impact its life expectancy.

But the real issue with battery chargers is a thing called battery “acceptance rate." Historically, we used a value of 25-30% of a battery’s amp-hour rate as the maximum acceptance rate. Translated, this would mean that a 100-Ah rated battery would gain no advantage connected to much more than a 25- 30-amp rated battery charger.

Modern battery technology can mean a higher acceptance rate for charging, which in turn means that buying a higher-output charger might be a smart decision. However, bear in mind the other elements in the system that might also need upgrades. Photo courtesy of ProSport.

However, with much of the new technology available and in growing use by boaters today, recharge acceptance rates are considerably higher, approaching the 60% rate in many cases. So, this means that larger, higher-output battery chargers can be employed, and can recharge high-acceptance-rate batteries much quicker.

You should consider all of these points if you decide to upgrade to newer high-tech batteries on your older boat. However, bear in mind that you may then also need to consider upgrades for your alternator, battery isolator, voltage regulator and over-current protection.

Now that you know the basics of amp-hours you'll have a much better feel for the demands of your electrical equipment and the ability of your batteries and charging devices to keep up with those demands.

Written by: Ed Sherman
Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.