(All articles reproduced in this collection originally appeared in the Techtalk series in the club newsletter Telltale. Titles, authorship, and publication dates are reproduced as originally published.)
by George de Witte, Techtalk Series, NSC Telltale, September 2004
Since today is another sunny but windless day in the middle of August, I thought it would be useful to write an article about battery management for members who are considering long term cruising. Battery management is the simple task of estimating how many Amp-Hours (Ah) all the electrical widgets and gadgets on a cruising boat consume on an average day. After the electrical consumption is determined, one needs to make sure that the capacity of the house battery is sufficient to supply the estimated need. A good rule of thumb is that the storage capacity of the house battery, expressed in Ah, is at least twice the daily electrical consumption in Ah. Secondly, one needs to consider a strategy to charge the house battery to replace the consumed Ah. All boats come with an alternator as part of the engine, and it is usually the primary source of charging. The trick, however, is figuring out how much charging current your alternator actually supplies to the boat’s battery bank.
For instance, Whiskeydream is equipped with the popular Yanmar 2GM20 diesel engine, which features a factory-standard Hitachi 55 Amp alternator. Somewhat naively, I assumed that this alternator would deliver 40 Amp of current at a respectable engine RPM (Revolutions per Minute) of say 2000 (equalling 4000 alternator Revolutions Per Minute). But after a few months of cruising, I got somewhat suspicious about the Hitachi alternator, which was reinforced by the cruising grapevine as well. So I installed the Simple Ammeter (more about that later in this article) and sure enough – the alternator only supplied 40 Amp into a nearly dead battery with the engine compartment at room temperature. More typically, the charge current dropped to 20 Amp after 0.5 hour and eventually dropped to 10 Amp after 1 hour or more of charging time.
We had estimated that our daily electrical needs were approximately 70 Ah (see Table 1 below as an example for our cruising budget). So it is obvious that battery charging was fine as long as we motored all day, but would be a problem once it was possible to sail most of the time, which would be the case in the Bahamas and further south. So, some form of supplementary charging system was needed. The options are a high output alternator with a smart regulator, a solar panel, or a wind generator. We chose the solar panel, mostly because it is quiet and we had a convenient place to install it. But that is just a personal choice.
Another caution: using your diesel engine solely for charging is a very poor practice. An alternator delivering 20 Amp at 13.5 Volt produces 270 Watts of electrical power or approximately 1/3 HP. (1 HP = 750 Watts). So the diesel engine runs with zero load for all practical purposes. It is well known that diesels like to run hot (under load), or carbon deposits will build up on the pistons. Since diesels are high compression engines, it does not take much carbon build-up to produce excessive compression, resulting in premature wear of bearings, connecting rods, or camshafts. Hence, another reason for an alternate power source is to keep batteries charged.
Some Electrical Basics
Marcel Laroche has written excellent tutorials on basic electricity on boats in a series of Telltale articles in 2000/2001 (which are still accessible on the NSC website), so I’ll stick to the basic relationships in this article. The first relationship allows you to determine the current drain from a widget if the power rating is known, from the relationship:
Current = Power÷Voltage
Or in electrical units:
Amps = Watts÷Volts
For instance, the current drain of a 10 Watt cabin light can easily be calculated assuming a 12 Volt battery voltage as 10÷12=0.83 Amp. This equation is important as it allows you to calculate the daily current needs of a range of widgets for your battery management exercise. For instance, if you train your crew to have 2 cabin lights on at night for a maximum of 5 hours, then the current consumption is 0.83×2×5=8.3 Ah. Note that 5 lights for 2 hours would result in the same drain on your battery. This simple fact may be useful in your crew training efforts.
The second relationship, which you may remember from high school physics, is Ohm’s Law, which goes as follows:
Voltage = Current×Resistance
Or in electrical units:
Volt=Amp×Ohm.
I will illustrate this with a wire example. Boat wire of 16 AWG (American Wire Gauge) has a resistance of 0.004 Ohm/foot, so a 100 ft wire run to the top of the mast (50 there and 50 back) has a resistance of 100×0.004=0.4 Ohm. If this wire feeds a 10 Watt anchor light (0.83 Amp current draw), then the voltage drop across the wire according to Ohm’s Law is 0.83×0.4=0.33 Volt. So the anchor light only receives 12.00-0.33=11.67 Volt of the available battery voltage.
From the above, it seems that one can calculate the current drain of any item if the power consumption in Watts is known. However, for many items such as refrigeration, VHF radio, SSB radio, Autopilots, Laptops, etc., the daily current drain depends on individual usage factors and cannot be directly determined. Usually, the spec sheets for these items give a range and thus make it hard to make a realistic current budget. Therefore, the best solution is to have an Ammeter on board, so that the charge and discharge patterns of individual items can be evaluated by turning them on and off one at a time and seeing what happens to the measured current.
It is my observation from boat shows that production boats in the 26-34 ft range usually lack an Ammeter. It can be purchased in the aftermarket, but they tend to be expensive, and quite often, there is not enough room near the DC Switch Panel for a tidy installation. So in the following section, I’ll describe how a simple Ammeter can be added to your boat without breaking the bank. It is based on the assumption that every boat has a decent Digital Voltmeter (DVM) on board with a 200 millivolt (mV) DC selection setting, for general troubleshooting. If you don’t own one, they are regularly on sale at a popular automotive store for about $25. It would be a bonus if your DVM could handle 10A DC measurements as well.
Simple Ammeter Construction
Most boats should have a DC wiring schematic similar to that shown in Fig 1. The Battery Switch is shown in Position 2, which is the position 99% of the time for the cruising type, so that the start battery is kept alive for emergency purposes. The thickness of the wiring connections in the diagram is intended to indicate heavy-duty #2 or #4 AWG boat wire.
The dashed connection between the house battery negative terminal and the ground post is the one of interest for our Ammeter. If you study the schematic for a bit, you will realize that under charging conditions, current flows from the battery to the ground post through this wire. Similarly, when the battery is the sole power source, all current used by the boat flows from the ground post to the battery’s negative terminal.
The concept of the Simple Ammeter is to replace the existing dashed wire with 4ft of #4 AWG boat wire. The idea is that #4 AWG has a published resistance of 0.24 Ohm/1000ft, or 0.96 milliohm (mΩ ) for a 4 ft length (milli is a metric prefix indicating “1/1000th of”). With connection resistance, the #4 wire resistance will be very close to 1 milliohm or 1 mΩ . The beauty of 1 mΩ is that 1 Amp of current produces 1 mV, or 46.8 Amp of current produces 46.8 mV across the battery wire, according to Ohm’s law. By connecting the 2 ends of the wire to a couple of test points near the DC Panel, we can now use the DVM in the 200 mV range to indicate battery charge or discharge directly in Amps on the DVM. The connecting wire, as indicated in Fig 1, can be very light gauge as it draws negligible current. The sign of the DVM indication will also tell you whether the battery is charging or discharging. Which way is which? You can soon tell by turning on a couple of lights. Test points can be purchased at local electronics surplus stores. Use proper marine-grade wire for boat wiring (it is tin-plated) as opposed to landlubber bare copper wiring, which will corrode over time in the marine environment.
If your DVM has a 10A DC selection, you can calibrate the Ammeter as follows: shut everything off and disconnect “the wire” from the Battery Negative terminal. Connect the DVM in 10A mode to close the just-opened connection. Turn on navigation and cabin lights one at a time until the DVM reads close to, but less than 10 Amp. Restore the circuit and measure the same current in 200 mV mode at the test points. The ratio of the two measurements represents a calibration factor, and if it bothers you enough, you can shorten the “wire” proportionally to increase the accuracy of the Ammeter. In my case, I had to shorten the “wire” by 4 inches for near-perfect Ammeter readings.
Now go and measure the current drain from the propane safety valve relay. Mine measures close to 0.8 Amp, about the same as a cabin light (so, turn the propane valve off if you are not making coffee for the captain).
Conclusion
From the above explanations, the usage of the Ammeter should be straightforward. Just one word of caution: When trying to measure the charge rate of your alternator, shut the DC panel off momentarily, as the alternator charge current will be shared by the battery and the DC load offered by the DC panel. By zeroing the DC load, you force all the available alternator current into the battery (with the Battery Switch in position 2, otherwise the start battery gets a piece of the action).
With this new toy installed, you should be able to manage your battery health on your long-term cruising trip. It is the same principle as your bank account: make sure that more comes in than goes out, and the whole thing should stay out of the red.
Table 1: Typical cruising budget for electrical power consumption
George de Witte