Explanation of the Charge Status as displayed by SmartGauge

The Charge Status as displayed by SmartGauge is not directly related to the available amp hours remaining in the straightforward manner that may be expected.

This is because SmartGauge displays the charge status as a percentage of the total available energy in a battery as opposed to the total available amp hours. They are not quite the same thing.

The remaining energy available in a battery is not only related to the available amp hours remaining. It is also dependant upon the battery voltage.

For instance. Suppose you are using an inverter (let's assume the inverter is 100% efficient for ease of calculations) to power a 100 watt light bulb. The batteries are fully charged. The battery voltage cannot be calculated because it depends upon too many variables. But let's take a guess at around 12.5 volts when powering the inverter and lightbulb.

At 12.5 volts, with our 100% efficient inverter and 100 watt lightbulb the current draw on the batteries will be 100 watts divided by 12.5 volts = 8 amps. So in 1 hour it will use 8 amp hours.

Now come back to the battery some time later when the battery has run down in charge status. The battery voltage will have fallen somewhat. Let's guess it at 12.0 volts. The inverter is still powering the same load with the same power but the battery voltage is lower. So in order to maintain the same output voltage the inverter will draw more current. Now the current draw on the batteries is 100 watts divided by 12.0 volts = 8.3 amps.

So with the batteries in a lower state of charge, in 1 hour it will now use 8.3 amp hours. But we have been powering exactly the same load with exactly the same amount of power.

This is one of the problems with amp hours counters. An amp hours counter would actually calculate that the same load uses more current when the battery is in a lower state of charge. This is, as we have shown above, correct. But it doesn't use more power or energy. And at the end of the day, it is the remaining energy that we are really interested in.

Power is measured in Watts (which for a DC circuit is volts * amps) and is defined as being an amount of energy per unit time. Energy is measured in Joules and is a measure of how much energy is available, or has been used. 1 Watt is 1 Joule for 1 second.

So attempting to show the available charge remaining in a battery purely from amp hours available is bound to fail as the available energy depends upon the battery voltage as well as the charge status.

The result of this is that when relying on an amp hours counter, the same power consumption apparently shows different amounts of power being used depending upon the state of charge of the batteries. Which is obviously ludicrous.

SmartGauge displays the charge status as the remaining energy available in a battery which cures this problem and gives a far more accurate measure than remaining amp hours.

The maths behind the difference between charge status as shown by SmartGauge and amp hours remaining.

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Page last updated 02/04/2008.
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