14 Jun 23
Voltmeters and Ammeters
Accurate and sensitive meters enable the best use and understanding of a system.
Analogue (moving needle type) meters are cheapest and for some applications with fluctuating current, eg. wind turbine output, preferred.
Digital voltmeters allow much greater resolution and so provide more information about state of charge etc..
Voltmeters are connected between positive and negative. If connected directly to a battery a small (eg. 100mA) fuse should be fitted in the wire to positive.
Ammeters are usually connected in a break in the positive wire of the circuit being measured. For currents above 25A ammeters making use of an external shunt are used. The shunt is usually placed in the negative wire to the load or charge source being measured, and the meter mounted remotely using smaller signal wires.
Note - To measure energy generated or consumed kilowatt hour (kWh) meters are needed. OFGEM approved kWh meters are available to enable credits to be obtained for renewable generation -
These digital meters measure Ampere hours (Ah) or energy used or generated in a system, so giving an accurate indication of the battery state of charge at any given time. Useful for assessing stored battery power, spare capacity, performance or fault detection.
Starting at zero for a fully charged battery Ah consumed are shown as a negative display, charging Ah are added until the reading is back to zero.
Measurement is via an external current shunt which is usually sited in the main negative feed to/from the battery. Signal cables are then taken to the remotely mounted display.
They act like a battery ' fuel gauge '!
Using one of these meters increases awareness of battery state of charge and electrical usage, translating into longer battery life and smarter use of your power resource, - every battery system should have one! For optimum use:
- Locate where easily seen eg. in a living area rather than the battery room.
- Educate yourself about how much each appliance uses by observing amps reading with appliance off and then on.
- Check Amp-hours reading daily to see that you are keeping within your energy budget.
Battery charge indicators
These are included in many charge controllers and give an easy to understand indication of how full batteries are, whether they need to be charged, and whether to turn off your charging source. While in no way as useful or accurate as proper metering they give a simple measure of system performance.
These are used to measure and record generated or imported/exported electrical energy. To provide a basis for payments they must be fully approved in accordance with the Measuring Instruments Directive (MID).
When used to record generation they are located between the inverters and the point of connection to the supply.
Used to measure how much is being exported to or imported from the grid in order to control battery charge/discharge and increase self-consumption. Often these are installed on existing solar PV systems to evaluate the value and possible size of any battery storage system prior to purchase
Specific meters should be used that are suitable for the brand of inverter battery system that is being used or considered.
Current Transformers (CT 's)
Often used in conjunction with kWh Meters or Energy Meters to enable larger AC currents to be measured than would be possible through direct connection. They are placed around the wire or conductor carrying the (primary) AC current that needs to be measured and produce a proportional (secondary) current which is measured using smaller wires connected to the meter.
Split-core CT's have a two-part core which allows the transformer to be placed around a conductor without disconnecting the conductor first.
Safety Note: These must never be operated open-circuit (secondary coil wires disconnected) once installed, whilst on a current-carrying conductor, as dangerously high voltages may occur under these circumstances.
CT's should be used that are compatible with the relevant meter.