I certainly agree that it's good to know some electrical principles when attempting to troubleshoot electrical problems on boats. However, everyone can't be an expert in everything, and there are some pretty complex systems on boats these days, especially when you take account of all the electronic gear
There are some important misconceptions regarding the usefulness and the use of test equipment aboard, especially when it comes to lead-acid battery chemistry.....itself a complex topic.
Here's my take, for what it's worth.
(1) These are only useful with flooded batteries since AGMs and Gels are sealed.
(2) Hydrometers measure the concentration of sulfuric acid in the electrolyte and, by inference, the state-of-charge
of the cell being tested.
(3) hydrometer readings are affected by temperature, water
added, and other factors, and thus are not gospel.
(4) hydrometer readings may tell you little about the capacity
of the battery being tested.
(1)These are invaluable aboard boats for all kinds of testing. They are virtually imperative for the testing of batteries, since analog meters (those with needles) generally don't read close enough to descriminate finely between states-of-charge. The difference between a fully charged battery and one which is 25% depleted, e.g., is only 0.2 volts!
(2) While an expensive multimeter isn't necessary, one with a known calibration is -- at least, it's very good to have on hand. I've seen even expensive multimeters (notably, a Fluke 337 clamp meter) which was way off in it's DC voltage readings. See: Gallery :: Miscellaneous 2007 :: Multimeter02
(3) Most digital multimeters can only be used to measure small amounts of amperage (usually under 10A loads). Clamp-on ammeters, or multimeter clamp-on accessories can measure higher loads.
Voltage of Batteries and What it Means.
It can mean little or a lot, depending on how you measure and how you interpret.
(1) Any voltage reading on a flooded lead-acid battery which has been resting awhile over about 12.8 tells you that there's some charge on it, i.e., something is charging
it (solar panel, wind generator
, battery charger
, etc.). Wanna know if your alternator
is putting out? Put a multi-tester on the battery terminals while the engine
is running. If you measure about 13V or more, the alternator
is putting out something. Wanna know how much? Put a clamp-on ammeter over the lead from the alternator. Don't have one? There are clamp-on ammeter accessories which can be used with just about any digital multimeter.
(2) You can get a good approximation of the battery's state of charge by measuring it's voltage -- at the battery terminals -- after it's been resting overnight with no charge and no load on it.
(3) Similarly, if you take a battery off charge and put a modest load on it for a short time -- enough to bleed off the surface charge -- you can then measure its voltage and get a good idea of it's state of charge. However, if it hasn't been exercised in awhile you may get a false reading....voltage will drop faster than it's state-of-charge would suggest.
(4) Voltage will tell you little or nothing about the battery's capacity.
A fully charged battery, reading a normal 12.6-12.7 volts after resting overnight, may have a greatly reduced capacity, and when you put a load on it the voltage may drop precipitously in a short time.
Old technology. Used for many years in auto shops to put a relatively heavy load on the battery and see how the voltage holds up. Crude, but workable.
Digital Battery Analyzers
. Good ones are expensive ($600 plus). They work by measuring the internal resistance of batteries, and by computer applying an algorithm which computes remaining capacity in CA, CCA, AH, etc. May be used with batteries in any state of charge. Must be used carefully. Not definitive, but indicative and, in my experience, generally correct.