Boat Electrics; Surveys

Electrical Boat Surveys

Stray Current (an explanation)
Stray current is electricity that is flowing where it’s not supposed to — through water, fittings on your boat, wet wood, damp surfaces, etc. It can be a shock hazard and it can cause corrosion (technically known as electrolytic corrosion). Stray current corrosion is caused by a power source such as your batteries or the shore power connection. It is unlikely for serious corrosion to be caused by stray currents flowing through the water, without a metallic path to your boat. Because of the relatively high driving voltages, stray current corrosion can act far more quickly than the corrosion caused by dissimilar metals in contact (galvanic corrosion).

Note: The word electrolysis is often mistakenly used to describe various kinds of corrosion. Electrolysis actually refers to the bubbling off of gases that occurs with electrolytic corrosion.

Common Ground Point (ship’s ground) Grounds from batteries, engine, switch-panel negative bus bar, bonding system, auxiliary power generator, underwater ground plate, ship’s 120 Volt safety-ground, and LORAN signal ground all meet at one point.This point must be a heavy bus bar or bracket with bolted connections.Note: When referring to 12 Volt wiring, ‘ground’, ‘negative’ and ‘ground return’ are all equivalent terms.

  • Easy to access and located as far above bilge levels as practicable.
  • Labelled as Common Ground Point.


WARNING! The hydrogen gas in and around lead-acid batteries is explosive and the acid can burn skin and eyes. Avoid sparks and wash well after handling your battery.

  • Acid (electrolyte) level is up to plastic liner inside holes. Letting the acid level go below the top of the plates will kill a battery quickly. Use distilled water to top up batteries. If distilled water isn’t available, tap water is OK if it’s clear, not ‘hard,’ and not highly chlorinated. Let the cold tap run for a minute to clear metal ions out of the pipes and use a well rinsed, glass or plastic container to transfer.
  • fully charged specific gravity is 1.245 to 1.300 in each cell.
  • In a partially discharged battery, specific gravity of each cell does not vary by more than 0.050 from the other cells. Battery cells sometimes charge with uneven specific gravities but after discharging about 25% (from a full charge) they should even out.Note: If it’s been awhile since charging the battery, acid may have settled to the bottom leaving a lower specific gravity electrolyte on the surface. If you overfilled the battery then the electrolyte may be diluted. Either of these situations can result in abnormally low readings and they don’t necessarily indicate a weak battery cell.
  • ‘At rest’ battery voltage is 12.1 to 12.8 Volts. A battery is ‘at rest’ when it isn’t being used and hasn’t received a charging current for at least 12 hours. A voltage above 12.8 Volts indicates that the battery is still settling after a charge. A voltage below 12.1 Volts indicates either a weak cell or a battery charge below 50% of capacity.Note: Standard batteries have their life span shortened drastically by deep discharges, even to the 50% level. True deep cycle batteries (see below) function well with 50% discharges.Engine cranks properly for 5 seconds with each battery alone — battery voltage is above 9.5 Volts and steady while cranking. Perform this test only after engine has been running so that protective oil has circulated. Disconnect coil ‘+’ wire or engage diesel fuel shut-off mechanism to keep engine from starting. It’s possible for batteries to fail this high current test while still being able to provide good storage capacity at lower currents.Note: If engine doesn’t crank properly and battery voltage remains high, then there is a problem with the starting circuit or starter motor.
    Note: Starter or electric winch motors will normally ‘pull’ a battery’s voltage down to 9 or 10 Volts while they’re operating. The battery should recover most of its ‘at rest’ voltage within seconds.
  • Batteries draw not more than a few amps of charging current once they are charged except during conditioning (see below), water loss is at most a few ounces (50 to 100 ml) per cell, per year. Significant water loss indicates a problem. If the water loss occurs evenly in the cells, alternator or battery charger voltages may be too high. Water loss in only one or two cells indicates weak or shorted cells.
  • Batteries are true deep cycle type if used for anything but starting.
  • Specify that you want ‘golf cart’ batteries because most marine/RV ‘deep cycle’ batteries are only marginally better than automotive batteries for deep cycling. True deep cycle batteries will provide many hundreds of charge/discharge cycles instead of only a few dozen.Note: Avoid discharging deep cycle batteries below 50% of their capacity. A 50% discharged battery has an ‘at rest’ voltage (see above) of 12.1 to 12.2 Volts.
  • Top surfaces clean and dry.
  • Cables in good condition – ends are soldered and correct size for terminal connectors. Check cables for broken or corroded strands, especially at the ends.
  • Only one cable to each terminal. In particular, avoid small wires in battery compartment. Run them to the battery switch and switch-panel negative bus bar instead.
  • No connection depends on spring tension (i.e., no alligator clips).
  • Connections cleaned and sealed.
  • Positive terminals have insulating cover. Negative cables go directly to Common Ground Point. Many systems have the negative cable running directly to the engine as part of the starting circuit. This means that other negative connections need to be at the engine, or in the battery box, which can cause corrosion problems.
  • Positive cables go directly to nearby battery switch.
  • No batteries wired in parallel.Paralleled batteries tend to fight each other when they are at rest — this causes premature discharge and a shortened life span. It’s OK to parallel batteries temporarily with the battery switch, while charging, starting and running the engine – just avoid leaving the switch on ‘BOTH’ when no power is being drawn. If you require a large battery capacity, connect several 6 Volt or even 2 Volt cells in series instead of wiring 12 Volt cells in parallel.Note: Two batteries are in parallel if their positive terminals are connected and their negative terminals are connected.
  • Ventilation is provided for cooling and for venting the gases produced by batteries. Batteries produce hydrogen, oxygen and corrosive sulphide gases. The lighter-than-air hydrogen must be able to rise naturally through a venting system, with or without a blower.
  • Batteries can be conditioned with an equalizing current. After a normal full charge, conditioning consists of applying a reduced charging current (2 to 5 amps for most batteries) either for a few hours or until battery voltage rises to 15.5 – 16.5 Volts — this takes the lead sulphate ‘crust’ off the battery plates and helps maintain full storage capacity. Check the acid level when finished because this process causes bubbling and fluid loss. Condition batteries every month when they’re being used heavily. Conditioning requires either an override on the alternator’s standard voltage regulator or a battery charger with a conditioning or ‘equalising’ option.Note: Don’t condition batteries when they are in parallel or one battery may take most of the conditioning current.
    Note: Shut off all electronic equipment during conditioning because of the high battery voltage.
  • Inlet vent below batteries.
  • Outlet vent as high as possible in battery compartment.
  • if using an electric blower for battery venting, the motor is not in the air stream.
  • Ventilation system is for batteries only.
  • Batteries strapped down and prevented from shifting.
  • Battery compartment protected against acid spills.
  • Easy to access and located as high above bilge as practicable.
  • If batteries are not being used, they are given a full charge at least once every 3 months.Lead acid batteries will self-discharge over a period of months so they should be charged periodically to ensure that they don’t completely discharge. This is especially important during freezing weather because a discharged battery can freeze develop cracks in the case.

(NOTE: if you are not able to understand any of the above then please refer to a Boat Electrical Surveyor)

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Matt West Boat Surveyor
Tel: +44 (0)7798 55 45 35

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