Monitoring your solar system.

System monitoring of some kind is essential to ensure reliable performance and long life of your solar system components.

The main component of your power station will be your battery and your battery capacity will be the most important thing to understand so system monitoring really means knowing the state of charge, or lack thereof, in your battery. You can choose simple like an accurate digital voltmeter or get a little more hi-tech and use a purpose built monitoring system.

Monitoring via a voltmeter only

A digital voltmeter can provide very useful system information and was once the only device I used to monitor my entire solar system.

Using only a voltmeter can be a bit confusing and a little time spent understanding battery terminal voltage is time well spent even if you decide to install something slightly more sophisticated to monitor your power system.

For the purposes of this lesson we will look at a 12 volt battery, technically speaking I should talk about 2 volt cells and "voltage per cell" but for the benefit of folk like me "plain speak" with a common voltage is a little easier to understand.
For a 24 volt system you will of course double all the figures here, for 48 it will be four times the 12 volt numbers.
Lets grab a hypothetical 12 volt battery "off the shelf" supplied (as all batteries should be) full of charge. It has a smallish capacity of let's say ... 120 amp/hours. As you will discover later on down the track, voltage variations under load and charge will differ a little depending on the capacity of the battery.

Grab a digital volt meter ...

But a word of warning here ...we are talking hypothetical, not actual and frigging around with battery terminals and charging devices as I am about to list could well be a tad dangerous without some basic safety rules being followed. In this case you don't need to actually grab a battery even if that is what I am saying! Just read on instead ...

If we measure the voltage of fully charged 12 volt battery with nothing connected it should be around 12.6 volts.

If we connect a reasonable load to the battery, let's say 6 lights, the voltage will drop to around 12.1 but the battery is still full of charge at this point. Let's turn off the lights so we don't send the battery flat ... The voltage should slowly return to something in the vicinity of 12.6

If we connect a charging source to the battery, for example an 80 watt solar panel that is in full sunlight or a battery charger that is plugged in, the voltage will rise to about 15 - 16 volts if the charge supply is unregulated and the battery is still full.

If the battery was flat, let's say about 60% had been used from it, we might see the following ...

No load: 12 volts
6 lights: 11.0 volts
Charge connected: 13 volts
If the charge source stays connected you would see the voltage rise slowly until the battery was full and the voltage had risen to around 15 volts.

The whole point of this is not to cause confusion, getting to know your battery capacity by voltage alone is not an exact science but a matter of observation over a period of time and a wide variety of charge and load conditions.
The voltage will vary widely from about 10 volts right up to 16 volts depending on battery type and charge and or load conditions. If you install a power system and monitor it via an accurate voltmeter you will learn (and quite quickly) what is happening just by observing differing voltages at different times. It is not exact! No one can come up with a perfect list of voltages relating to battery capacity and a batteries voltage will change no matter what state of charge it is in if you add a load or a charging source. That said, here is a simple chart for the open circuit (nothing connected) voltages of a 12 volt battery.

Full charge: 12.6
Half flat: 12.0
Fully flat: 10.5

If a lead acid battery ever measures zero volts (after a period of sitting idle) it is usually damaged beyond repair.

You could add a few more meters ...

To compliment your digital volt meter you could add an ammeter to measure input and perhaps another to measure output. For sure something to monitor the solar gain is a useful device, even just to see the differences in input over varying conditions. If you are going to add all sorts of different meters though, it may well be more economical to fit something a little more sophisticated than just a digital voltmeter and an ammeter to measure solar input.

Using a dedicated system monitoring device

There are a few very good devices on the market that will measure all inputs and all loads connected to your battery and come up with a set of figures in easy to understand formats to let you know exactly what is happening within the workings of your power station. They all work by measuring what goes in and what comes out of your battery. In just about all applications this involves fitting a device called a shunt to one of your battery leads.

What is a shunt?

Installed shunt for a Trace TM500 battery monitor

Basically speaking a shunt is a lump of metal with cable connections on either end so you can get your battery power to flow through it. More than being a lump of metal however it is actually a calibrated resistor of very low value that makes a minute "blockage" of current.

André Marie Ampère was a French bloke born in 1770 something and a pretty brainy mathematician. He continued on to become a founding theorist in electrical calculations and was one of the pioneers of electrical mathematics. We remember André because the unit of current called "amperes" of "amps" is named after him.
André figured out amongst other things that a resistor with a current flowing through it will have a voltage across its ends. The voltage will change precisely when the current flow changes and will vary in proportion to the current along exact mathematical lines.

Anyway to get away from history and back to the shunt (or resistor) and the precise voltage as measured across its ends, it is this information that a shunt will feed to a microprocessor to calculate the ins and outs of battery power and therefore the actual battery capacity that we so want to know about. The microprossor will display useful information on a screen for us to read.

Before we end this lesson on system monitoring, we can sum it up a little.
Firstly, it is possible to monitor battery capacity using accurate voltage and observation.
Secondly, we can monitor our batteries more precisely with a microprocessor connected to a shunt and this is the basis of a more sophisticated battery monitoring instrument.

Important info regarding fitting a shunt

Just before you go you should know about correct placement of a shunt. These devices NEVER go between the positive and negative lead. They go in line on either the battery positive or the battery negative lead so that current flows through them on its way to (or from) the electrical load.

A shunt is usually fitted in the negative lead of a battery. While it could go in either lead, the negative is chosen as the shunt will be "live" and is often not insulated. In a car or motorhome with a negative earth it would be total folly to have a live exposed lump of metal that was positive!

For further information on battery monitors and their functions (and cost) see our products pages

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