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113
TECH
tip
by BLUE SEA SYSTEMS
M
E T E R I N G
Battery State-of-Charge (SoC)
Knowing the State-of-Charge of your battery is like
knowing the amount of fuel in your fuel tank. To avoid
getting stranded with a dead battery, accurate battery
bank monitoring is essential.
The Voltmeter Method—
Voltage can be used to measure
of the SoC of your battery. The difference from a fully
charged battery to a fully discharged battery is only 1.0V
in a 12V system, so the meter must have good resolution
and accuracy. This method is generally sufficient to monitor
batteries which experience intermittent use, such as starter
or thruster batteries. However, a battery must not have
been charged or discharged for over 12 hours for this
measurement to be trustworthy. This makes this method
unsuitable for monitoring house batteries, which are in a
charge or discharge pattern much of the time. There are
emerging devices that use sophisticated algorithms to
determine SoC from voltage alone, but these are not yet
widely available.
The Amp-Hour Method—
A convenient and accurate
way to measure SoC is with an Amp-Hour Monitor. This
is a complex calculation of the energy available, energy
consumed, and the energy returned to the battery in
charging. SoC can be expressed as Amp-Hours used,
Amp-Hours Remaining until the battery is dead or Time
Remaining until the battery is dead. The advantage of this
method is that it works well for batteries in a constant state
of charge and discharge.
The Hydrometer Method—
A hydrometer measures the
specific gravity of the battery, but this can only be done
with flooded batteries and not with Low Maintenance,
AGM, or Gel batteries. Using a hydrometer requires
accessing the battery and although an extremely
accurate method of determining SoC, is not commonly
used due to the inconvenience.
ELECTRICAL MONITORS AND METERS
Knowing the state of the electrical system is critical to the safe operation of
a vessel. There are several ways to measure the state of an electrical system
using a basic analog meter to the more contemporary OLED Digital Monitor.
Monitoring an electrical system can be broken down into two categories:
DC and AC monitoring
DC Monitoring
Direct Current is typically derived from batteries, but can also be
produced by converting AC Current to DC Current using a battery
charger. DC values are typically measured in Volts, Amps and Amp-Hours
(State-of-Charge or SoC).
Amp-Hours (SoC)
One of the top reasons for a vessel being disabled and requiring assistance is
a dead battery. For this reason the State-of-Charge of the battery expressed in
Amp-Hours is arguably the most important electrical measurement made on a
vessel. (To learn more, see the adjacent Tech Tip)
Volts
Voltage is useful to understand the behavior of devices whose functionality is
dependent on voltage, such as an inverter which may cut off at 10.5 Volts. For
smaller boats which spend time in storage, voltage is a good indicator of SoC
before leaving. On larger boats, even with SoC monitors, voltage is a good SoC
indicator for the starting battery. DC voltage can also indicate if charging or
discharging is active. A voltage jump after starting the engine indicates the
alternator is working, even if the battery is well charged and takes little current.
Amps
Amperage measurement can be used in two ways:
1) The net amperage flow to and from the battery can be monitored to
determine how hard the battery is being worked.
2) A specific device’s amperage flow to (a load such as a pump) or from
a power source (such as an alternator) can be measured to determine the
device’s contribution of the net current flow to or from the battery.
AC Monitoring
Alternating Current, known more typically as household current, can
also be produced by converting DC current to AC current through the
use of an inverter. Typically the values measured are Volts, Amps, Watts,
and Frequency.
Volts
Most AC loads work well in a specific range of input voltage. Voltage deviations
are symptoms of problems needing attention such as: a failing shore cord
connection, inadequate dock power, or faults in a genset or inverter.
Amps
AC current gives an indicator of how much load is operating. There can be a
problem with overloading shore cords. Using an ammeter is ideal to measure
and choose which loads to have on at the same time and may indicate the
potential to trip a circuit breaker.
Watts
Watts is a measure of power, or voltage multiplied by amperage. Most devices
that may become overloaded do so because of Amps. Gensets have a kilowatt
(kW) and an amp rating. It is rare for a genset to be overloaded in Watts and
not be overloaded in Amps. Shorepower and inverter outputs will be sufficiently
measured by Amps without referencing Watts. The power (watt) measurement is
useful for comparing usage to the rated capacity of a genset or Inverter.
Frequency
Frequency is required for installations with gensets or inverters to determine
if they are working correctly. Cruisers may occasionally venture into locations
where the shore line frequency is different, or where it is locally generated and
can deviate from nominal. This can be the case in the Caribbean or on the west
coast of Canada or Alaska in small communities.