MY NEW VOLTMETER

Harold Brochmann

Most boats’ electrical panels have a voltmeter. On all but the newest boats these are analog – the kind with a needle pointer behind a clear plastic cover. Usually they cover the range of 0 – 16 Volts, and are connected to the center tap of the battery switch so as to show the voltage of the battery which is presently online.

This arrangement has some shortcomings. The first is that analog meters provide notoriously imprecise readings because the scales have very poor resolution. In other words, you can’t really be sure whether the meter is reading 12.3 V, 12.6 V, or 12.9V and so on. The second shortcoming is that the way most of them are wired makes it impossible to obtain open-circuit battery readings when the engine is running.

Accurate and precise open circuit voltage readings can tell you at what state of charge your battery is. The voltage drops shown when a known load is applied can tell you whether your battery terminals need cleaning or whether your battery has served its useful life. The voltage shown when the engine is running will tell you if your alternator is functioning as it should. For these diagnostic applications it is essential to have voltage readings of a higher degree of accuracy and precision than is obtainable with the usual panel mounted analog voltmeter.

Some time ago I bought an inexpensive digital voltmeter ($35) for this purpose. Over time, however, it became increasingly obvious that this meter, despite its implied four-digit precision, gave very inaccurate readings. In other words, the readings obtained with this meter are, to put it plainly, useless for diagnosing marine electrical problems. The incorrect information lead to much head-scratching and wasting of time. I assume that the reason why this meter gave incorrect readings is that it relies on an internal battery to provide a reference voltage. Thus the condition of this battery – how long has it sat on the shelf, how long has it been in use, what temperatures it is – determines the voltmeter reading. Not good enough.

Enter Brochmann’s unoriginal switched panel mounted digital voltmeter. There are two types of meters available: the light-emitting diode (LED), and the liquid crystal display (LCD). The first of these is preferred because the display is easily read in the dark. I got my LED panel voltmeter for installation on GYPSY from RP Electronics in Burnaby.

The cost of the meter, the 5V regulator chip required to power it, and a SPDT toggle switch, was less than $30 (well, before handling charges, postage, GST and PST !).

The first of the diagrams shows how the original meter was connected; the second, the new arrangement. S1 is the heavy-duty battery selector switch. S2 is one of the original panel switches. S3 is the new toggle switch. Note that all the wiring to the new meter as well as the voltage regulator chip and the capacitor can be size 22 AWG because it carries very little current.

The installation was not difficult; the most unpleasant part was the cutting of the hole in the electrical panel to accommodate the new meter. This was accomplished using a “dremmel” tool. Dremmel is a brand name. I purchased a considerably less costly tool at the local auto supply store. It did the job just fine. The instructions that come with the tool do not mention that safety goggles are a must – which they are.

The meter could also have been mounted externally on the panel.

The voltmeter installation went so smoothly that in my enthusiasm I removed the existing analog voltmeter and replaced it with a 0 – 50 Amp ammeter, wired so as to show the current being delivered by the alternator. Note that in the new installation the alternator output goes directly to the ship’s battery, bypassing the battery selector switch. Running this wire (shown as a dashed line in the second diagram) from the engine compartment to the electrical panel was a little awkward. It should be size 10 AWG and has the advantage that now no harm will come to the alternator diodes if the battery selector switch is accidentally turned to the “off” position with the engine running.

Normally, I leave this switch in the “all” position except when the engine is stopped. In the “all” position with the engine running, both batteries will be charged at the same rate as they would be if they were connected separately one a time because the alternator maintains a regulated applied voltage regardless of how much current is drawn.

And while I was at it I installed a filter capacitor to absorb any stray noise in the wiring. There is such a capacitor on the alternator already, but capacitors of this type have a tendency to deteriorate with age. A friend donated the ammeter, while the 1000 microfarad capacitor (a far higher rating than necessary) cost around $2 at RadioShack.

First I left the two batteries on a shorepower charger overnight to make sure they were fully charged, and then let them stand with no load for a few hours to let the surface charge dissipate. Both batteries now read 12.67 Volts. The meter gives readings to 2 decimal places. This level of precision is probably a bit of wishful thinking, but that’s ok. Call it 12.7 Volts if you want. This resting voltage, by the way, is slightly temperature dependent, and will be lower on a very cold day.

Now GYPSY’s engine was started. The voltage rose to that set by the alternator’s voltage regulator… 13.45 volts. It did not matter whether either or both batteries were selected, and the engine speed had a very slight effect on it.

As soon as the engine started, the ammeter showed an initial current surge and then settled down to 10 Ampere or so. This lasted for just few minutes while the energy used by the starter was replaced. The energy required to start the engine is actually very little. I don’t know how much the starter draws, but let’s assume that it’s 100 Ampere for 15 seconds, which is probably in the right ballpark. At 10 Ampere charging current, and assuming a 20% loss, it should, in theory, only take four minutes for the batteries to recover completely. Theory and practice are often not quite the same. But these numbers here are close enough to illustrate the point.

After a few minutes the ammeter settled down to a constant 3 – 4 Ampere. It’s a little hard to tell exactly because, as suggested, the resolution of analog meters isn’t that great. If the batteries were already fully charged, where was this energy going? Well, that is one of the questions I intend to answer in the next article in which I also report on my research into battery charging.