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Monitoring Instruments for on-board Power Supply

The electrical power supply on board of a sailing vessel is a safety issue and should be managed the same way water and food supplies are managed. But contrary to other supplies, battery capacity varies considerably with ambient temperature and with the rate at which the capacity is consumed. This makes accurate monitoring of battery capacity hard to achieve so that practically only a good estimate of the available capacity can be made.
A rational energy management requires knowledge of the current power consumption, knowledge of the remaining energy stored in the batteries ("SoC") as well as supervision of the energy flow from external energy sources into the batteries.

The parameters involved in the energy management of the battery supply are battery voltage, charging/discharging currents and State-of-Charge (SoC) of the battery.

Monitoring Battery Voltage and Current

The following schematic diagramm shows the principal elements of a 12V on-board power supply installation and how they are connected:


The example shows a system with three parallel connected batteries, a shunt resistance (Rshunt) in the positive supply cable and the connection to the battery charger. To measure voltage and current two (analogue) meters are connected as shown. The supply network of the boat is connected to this power system by a power switch allowing a complete disconnection of the supply system from the electrical loads. These power switches should be selected according to the maximum expected current ratings in the supply network.
The analogue meters are very sensitive direct current meters (d'Arsonval galvanometer). They have an internal resistance of about 1k Ohm and require about 100mV or 100uA for a full-scale deflection. Reduced voltage sensitivity is obtained by inserting a resistance in series with the basic meter. Reduced current sensitivity is obtained by putting a resistance in parallel with it (shunt).
The shunt resistance in the main supply path is used to measure the current drawn from the batteries. Power drawn from the battery will cause a voltage drop over the shunt resistance directly proportional to the electrical current:
Ushunt = Isupply x Rshunt
A typical value for the shunt resistance is 0.001 Ohm. In this case a 50 A charging current will result in a voltage drop of 0.050 V. This voltage drop is measured with a sensitive Volt-Ampere meter. Since there is a linear relation between Ushunt and the current flowing through the shunt, the milli-volt meter can be directly labelled in Ampere.

With a shunt of 0.001 Ohm each mV of voltage over the shunt resistor represents 1A of battery current.
The range of charging currents (e.g. 50 A) down to the stand by supply current for the gas alarm (e.g. 5 mA) requires a measurement range of 1/1000 and a resolution of at least 1/10000. This means that voltages in the range of 5 microvolts will have to be measured reliably.
Beware of digital instruments displaying 4 or 5 digits because measurements are not better than 1%
So resolution is about 0.5 A or in capacity: 12 Ah/day!!
--> non-linear A-mv conversion ???
This requires shielded (coaxial) cabling between the measurement instrument and the shunt resistor.
Shunt-free current measurements: no voltage drop: complex and expensive???

Monitoring Battery Capacity

Today sophisticated microcontroller-based battery monitors are available that also can be used efficiently for battery energy management. The principle of these monitors is to continuously measure the net current flow from or into the batteries and accumulate (integrate) the measured values over the time. This way not only a good estimate of the net energy flow into or out of the battery can be obtained, but also information about the rate of energy flow is obtained. This is an important parameter that influences the resulting capacity (Pleukert effect). Capacity also depends on battery temperature, which can also be monitored.
With this continuously collected data, an intelligent battery monitor can give a reliable estimate of available electrical energy at any given time. However, monitors that do not take temperature and Pleukert effect into account will show very unreliable results and their digital displays may pretend an inflated precision.

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