Q & A — March 2002
By Capt. Ken Kreisler
| Testing a bonded system for stray current, problems with gel cell recharging, and more.
Other than disconnecting the bonding conductor and checking each of my fittings for stray current with a voltmeter, how can I test my bonding system? R.N., via e-mail
Refer to the illustration above to evaluate your zincs for corrosion potential and therefore the level of protection being provided for your boat's below-the-waterline metal.
First connect a voltmeter's positive (+) lead to your boat's bonding system. Then connect the negative (-) lead to a zinc "guppy" and lower it into the water in the vicinity of an underwater metal mass. Start at the bow near any through-hull fitting up there and note the voltage. Repeat this all around the boat, paying attention to areas near the rudders, shafts, and struts, outdrives, or outboards, depending on the type of boat. If the zincs are protecting the metal parts, the readings on your voltmeter should be less than 0.2 volt. Readings from 0.5 to 0.8 volt indicate a severe lack of protection. Anything higher than 0.8 volt means there is a voltage field, probably stray current, in the water surrounding the boat, which may be coming from your vessel or another source. Ask your dockmates to perform this test to determine its exact location. The idea is to determine where it's not coming from. After that, the only logical explanation is that it has to be coming from the marina, usually dockside electricity. You may also want to alert your dockmaster of the situation so that the rest of the vessels can be checked.
If you discover that your boat is the source of the stray current, check all wiring. Look for loose or frayed connections, missing, burnt, or cracked insulation, and corrosion in the bonding conductor. In addition, do a visual check of your dockside electrical connections, again looking for any telltale signs of trouble including corrosion, loose wiring, or burnt or cracked insulation. And unless you are very familiar with working with electricity aboard, it is highly recommended that you call in a qualified marine electrician to fix whatever is wrong.
What problems are associated with charging a deeply discharged gelled-electrolyte battery? G.F., via e-mail
This battery's particular chemistry and construction--a closed and sealed case with no access to the electrolyte--can often present a charging problem when discharged to more than 80 percent of its capacity. (Individual manufacturers' specifications may differ.) The problem stems from the fact that this type of battery recharges inefficiently when allowed to deeply discharge and that most of the charge current is used to produce heat rather than the chemical reaction necessary for recharging. For gel cells, this can cause overheating and eventual battery failure.
A gel cell is at 80 percent discharge when the "open-circuit terminal" measures 11.80 to 12 volts, again depending on the manufacturer. Open-circuit terminal voltage is measured after the battery has been at rest--no load or charge source present--for one to three hours.
If the battery has been discharged more than 80 percent, the goal is to limit the charge current. While varying from battery brand to battery brand, current should be no more than four to 25 percent of capacity, depending on the battery. For example, if the bank's capacity is 200 amps, the charge current should be between 8 and 50 amps. Many charger systems are designed for gel cells and so do this automatically. If your boat has gel cells, you should look into one of these chargers to ensure your batteries will provide the proper service over the long haul.
This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.