12 Volt Batteries, Power and everything else

Well, in principle it's the same - we need to break the circuit somewhere to measure the current. It also depends what current we want to measure. If we now bring in a few more practicalities, like fridges & lights that are drawing current from the battery, then we have a choice of which current we want to measure.If we just want to measure the amount of current coming in from the panels, then we place the Ammeter in position-1. If we want to measure the current going to the fridge and lights, we place it in position-2. And if we want to measure the current going into-and-out-of the battery, then we'd put it in position-3. If this all becomes a little brain-scrambling then escaping back to the water-analogy usually helps.So those are our two main electrical quantities - voltage and current - measured in units of Volts and Amps. It's worth pointing out the somewhat obvious, namely that electricity can't be seen, so meters are our only way of knowing what's going on. In a water system we can look at how full the tank is and get a rough idea, and looking at the water coming out of the pipe will give us an idea of the flow. In electrical circuits we're blind - meters are our only way of getting a fix on what's happening.SOLAR REGULATORSOur circuit included a solar regulator, and many of these have meters built in, allowing us to see the battery voltage, the solar current coming in, and the current going out to the loads like fridges and lights. The more sophisticated ones will also measure things like Amp-hours coming in and going out, and using that they work out a guesstimate of the energy left in your battery (as a percentage). There's more on this in the blog post about Solar Regulators.WATTS AND AMP-HOURSNow that we've started to mention Amp-hours and other things, let's progress to some more complex electrical quantities - we'll start with Power (P), which is measured in Watts (W). Closely related is Energy, measured in Amp-hours (Ah) and it may be just as well to try and untangle these two units right now. In terms of formulas, both are a simple multiplication. Power = Voltage x Current, and Energy = Current x Time, or using their units we have Watts = Volts x Amps, and Amp-hours = Amps x hours.POWER AND ENERGYTo explore the difference between power and energy, let's say we have a 12 Volt battery - I think most people reading this article would be familiar with that. Without getting into questions of deep-cycle or starting batteries, we'll just consider two battery uses: first, starting a vehicle, and second, running a 12 Volt fridge for a day.Starting the engine typically requires hundreds of Amps but it's for a very short time - running a 40-litre fridge for 24 hours usually averages a current of about 1.5 Amps, so the current is tiny by comparison, but it keeps on going. If we calculate the energy and power in these two examples, hopefully that'll give us an idea of how energy and power relate.THE STARTERSo if the starting current was 200 Amps and the time taken to start was 10 seconds[*], then to get the energy, we need to determine the Amp-hours. The current is easy, that's 200 Amps - now we need to get those seconds into hours - the number 60 springs to mind, right? So it's (10sec/60) to get minutes = 0.167 minutes, and to get hours we divide by 60 again, which gives us 0.00278 hours.So to get the energy in Amp-hours we just multiply the Amps by the hours, so 200A x 0.00278hrs = 0.556 Amp-hours.THE FRIDGENow how much energy does our fridge use? Well it's again the current multiplied by the time, so we get 1.5 Amps x 24 hours = 36 Amp-hours. So now we can see that the starter actually uses way less energy than the fridge. That's of course if all goes well and we start in 10 seconds - we all know what happens if we don't start the engine that quickly, don't we?! But I digress.Back to our two scenarios - let's now compare them in terms of power. Over a day the fridge uses an average of 1.5 Amps, so the average power it uses will be 12 Volts x 1.5 Amps = 18 Watts. The starter on the other hand uses a lot more current so the power will be 12 Volts x 200 Amps = 2400 Watts = 2.4kW.So the fridge uses very little power but for a long time, while the starter uses heaps of power but for a very short while. Let's try and relate this back to our typical 12 Volt systems.BATTERIES AND AMP-HOURSAt the heart of virtually any 12 Volt system will be a battery. You will probably have noticed that deep-cycle batteries, the ones used to run fridges, lights, pumps, etc. are specified in terms of Amp-hours. So that means a 200Ah battery will store twice the energy of one specified as 100Ah. Working out how long the battery will last on our fridge above is simply the Amp-hours divided by the Amps, so on a 100 Ah battery our fridge should last (100Ah/1.5Amps) = 66.7 hours, so a bit less than 3 days.AVERAGE CURRENTWhile we're talking Amps and Amp-hours let's just clear up a common misconception - this occurs especially when talking about fridges which cycle on and off depending on how much cooling is needed. For instance in hot weather a fridge will stay on for longer and be off for shorter periods - this will increase the energy it draws from the 12 Volt system. If we average that energy draw over say 24 hours then we get the average current drawn - in our case above that was 36Ah over 24 hours, so an average current of 1.5 Amps.AMPS PER HOUR?In the example of our fridge some folks may be tempted to say that the fridge is drawing 1.5 Amps per hour - this statement makes no sense - it would be like saying that a boat was travelling at 11 knots per hour - ?!?What they might mean, is that for every hour that goes past, we are drawing 1.5Ah of energy from the battery - that would make sense. Or they could mean that the average current draw, averaged over an hour, is 1.5 Amps - that would also be correct. But Amps per hour makes no sense in this context.RESISTANCEThe only other electrical quantity that needs a mention is resistance, measured in Ohms, using symbols R and the Greek capital-letter omega ?. Resistance, voltage and current are inter-related by a simple but powerful equation known as Ohm's Law. A usefully compact form of this is a triangle with V at the top. In this form we can get the equation V=IR or R=V/I or I=V/R so we get 3 forms of Ohm's Law in one diagram. We'll use this when calculating volt-drop in cables, a very important thing to do in 12 Volt systems.So, that's about all we need in terms of electrical quantities and their units to get us through most 12Volt systems. That wasn't so hard, was it?[*] 10 seconds may be a bit pessimistic, but it makes the maths easier.