6:00 AM Thu 19 Nov 2009 GMT
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'What does it all mean?'
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All sailing boats of any size use batteries, but do you know enough to choose the right one for your boat?
... to diagnose problems when they occur?
... to know that the 'expert' in that unknown port is telling you the right information?
Here
Larry Janke
begins a MULTI-PART SERIES on batteries that no longer-range sailor can afford not to read. First, 10 of the myths!
There are some myths about batteries which, despite common sense and education, seem to persist. At the outset I want to dispel them.
Myth 1.
Batteries discharge if left on a concrete floor; At one time there was some truth to this; batteries were made with wooden cases which after a time would get saturated with electrolyte which would then conduct electricity to ground. With modern plastic cases, unless the battery is filthy and wet with electrolyte, this will not happen.
Myth 2.
Batteries should be completely discharged; Not true. As discussed further on depth of discharge is directly proportional to battery life.
Myth 3.
Batteries have a memory; Again partially true of early nickel cadmium batteries, no longer true and not true of lead acid batteries. However, the accumulation of hardened sulfate can create a condition of reduced capacity which could lead you to believe the battery had a "memory"
Myth 4.
Batteries should be "trickle charged"; not true, the current output, (amps) of so called trickle chargers, is too high for long term float charging and will dry the battery out due to excessive gassing.
Myth 5.
Batteries boil; The bubbles you see when the battery charges is hydrogen and oxygen gas released when the battery will no longer accept the current (amps) being supplied to it. This current disassociates the water in the electrolyte into its elemental components hydrogen and oxygen which bubbles up. The actual boiling point of 1.265 specific gravity battery electrolyte is 230? F (110? C). The battery would be in serious trouble at this temperature and so would you.
Myth 6.
AGM batteries can be charged at very high rates; Not without damage to the battery and potentially dangerous consequences, see discussion of "thermal runaway" further on.
Myth 7.
Three Stage Chargers are the best way to charge batteries; Not always, see the discussion of battery charging further on.
Myth 8.>
Batteries should be "equalized" on a regular basis; Again, not always true, see discussion on page 25.
Myth 9.
Hardened Lead Sulfate can be returned to solution by equalization or pulse charging; Not true. Lead sulfate is insoluble in sulfuric acid and nearly insoluble in water. It never was in solution, it is the sulfate ion SO42- ions which were in solution.
Myth 10.
Batteries connected in parallel will discharge each other; Not true. The batteries will reach voltage equilibrium and remain there. What probably gave rise to this myth is the phenomenon of self discharge caused by local action within the battery itself. All batteries exhibit this action whether paralleled or not.
Watch for the next installment on Sail-World Cruising.
Copyright 2003-2009, Larry Janke
www.semarine.com
No part of this document may be reproduced, copied, republished or redistributed with out the express written authorization of the Author. The Author is available on a fee for service basis for design and consultation, and can be contacted at larry [at] semarine.com
For those who are really battery-challenged, here are some definitions you need to be familiar with:
1. Ampere (amp) the unit of electric current.
2. Volt. Or Electromotive Force (EMF) the potential difference across a conductor when a current of one ampere dissipates one watt of power. Or, as commonly stated one volt can push one amp through one Ohm
3. Ohm. The unit of resistance to EMF. 1 Ohm is equal to the resistance of a circuit in which an electromotive force of one volt maintains a current of one ampere: symbol, O
4. Watt. The basic unit of electrical power, the rate of doing work. Can also be measured in horsepower. 1 HP= 746 watts, or other units.
5. Amp Hour. A measure of the capacity of a battery. A 100 Amp Hour battery can deliver 100 amps at a specified hourly discharge rate. Usually 20 hours.
6. Direct current (DC). Electric current which flows in one direction only.
7. Alternating Current. (AC) An electric current whose direction reverses cyclically. In the US The common house current which we use every day is 120 volts at 60 cycles per second (Hertz). Europe and other countries use 220 volt 50 Hz.
8. Specific Gravity. A measure of density of a substance, water is said to have a specific gravity of 1, battery electrolyte usually has a specific gravity of 1.265, that is, it is 1.265 more dense than water.
The water analogy is sometimes used to explain electric circuits by comparing them to water-filled pipes. Voltage is likened to water pressure; it determines how fast the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit of time). The flow rate is determined by the width of the pipe (analogous to electrical resistance (Ohms)) and the pressure difference between the front end of the pipe and the exit (potential difference or voltage drop). The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor (amperes x volts = watts). For our purposes since we are talking about direct current the water analogy works, quite well, it does not apply as accurately to alternating current.
by Larry Janke
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Hogan Island, VIC
Windsurfing - Boards
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