Tecktalk - A Simple Ammeter

Telltale Article - by George de Witte

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’s. 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 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 of say 2000 (4000 alternator RPM’s). 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 + charging time.

We had estimated that our daily electrical needs were app 70 Ah’s, (See Table 1 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 Bahama’s and further south.  So some form of supplementary charging system was called for. The options are a high output alternator with a smart regulator, a solar panel or a wind generator. We choose for 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 Watt of electrical power or app 1/3 HP. (1 HP = 750 Watt). So the diesel engine runs with zero load for all practical purposes. It is well known that diesels like to run hot i.e. 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 camshaft. Hence another reason for an alternate power source to keep batteries charged.

Some Electrical Basics

Marcel Laroche has written excellent tutorials on the subject in a series of Telltale articles in 2000/2001, which are still accessible on the NSC website, so I’ll stick to the basics in this article. The first one 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:   Amp = 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 0.83x2x5=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 = CurrentxResistance
Or in electrical units: Volt=AmpxOhm.

I will illustrate this with a wire example. Boat wire of 16 AWG gauge has a resistance of 0.004 Ohm/foot, so a 100 ft (50 go +50 return) wire run to the top of the mast has a resistance of 100x0.004=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 0.83x0.4=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 the time and see 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 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 Pos 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 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 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 mOhm. The beauty of 1 mOhm is that 1 Amp of current produces 1 mVolt or 46.8 Amp of current produces 46.8 mVolt across the battery wire according to Ohm’s law. By connecting the 2 ends of the wire to a couple of testpoints 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. Testpoints can be purchased at local electronics surplus stores. Use proper marine grade wire for boatwiring (it is tinplated) 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 the 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 i.e. about the same as a cabin light. I’ll bet you did not know that. So there is another item for crew training. Turn the propane valve off if you are not making coffee for the captain.


From the above explanations the usage of the Ammeter should be straightforward. Just one word of caution. When trying to measure 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 pos 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