Motorhome and Caravan News Australia

This is the second (and final) part of my introduction to basic electrical theory – for owners of motorhomes and caravans.  In this section we will look at watts (and we all know that solar panels are rated in watts) and also the connection of batteries in both series and parallel. But before we launch into all of that, let’s take a quick review of what we learned in part one…

(If you have not read part one, you can get to it from by clicking here)

Volt = unit of measurement of electrical pressure (very similar to water pressure).

Amp = unit of measurement of the rate of electrical current flow in a conductor (much like measuring the flow of water in a hose).

Ohm = unit of measurement of a conductors ability to resist current flow (like measuring the resistance that a partly closed tap excerpts on the flow of water in a hose).

Remember, these are just new words for concepts you already understand!

Now, let move forward and take a look at watts…

Ok, so what are watts?

The short answer is that the “Watt” is a unit of work done over time. More watts, then more work is getting done quickly, less watts means less work.

So how does this relate to water?

Think of it this way … Say you had that 10,000 litre tank filled with water and you had a wide open valve going to a fire hose and you decided to have some fun and point the hose at all your mates standing 2 metres in front of you. I bet that you could knock them all over in no time and make a big mess at the same time.

Now suppose you only had just 100 litres of water and a small garden hose. Well the results would not be as spectacular.

I really doubt you could knock ya mates over with the stream of water from a garden hose and after a few seconds of drenching them, you would be dropping that hose and running away from some wet and angry friends.

The same is true with electricity. Lots of volts (pressure) and amps (flow) with little ohms (resistance)means a lot of watts worth of power. Low volts and low amps with high ohms means much less power. Increasing either volts or amps while leaving the ohms the same will increase watts. Here is a point of reference to help you gauge how much work a watt of electrical power is worth…

745 watts is equal one horsepower.

So horsepower is just another way of measuring work done – just as you can measure weight in either kilos or pounds. So that means that a 100 watt bulb is actually a little over 1/8 horsepower.

OK, so watts represents power (work over time) and to get lots of watts you need lots of volts AND lots of amps.

This is an important concept to understand when it pertains to the performance of your electric fridge, pump, light or even your power tools.

Just a little bit more math

The watt is represented by the letter P in electrical formulas (P is for Power)

Watts are equal to the number of Volts times the number of Amps  (V x I = P)

or

The number of Amps times the number of Amps times the number of Ohms (I x I x R = W)

or

The number of Volts times the number of Volts divided by the number of Ohms (V x V / R = P)

In a commercial wattmeter, volts and amps are measured and simply multiplied automatically for you.

So how does this relate to the solar panels we have bolted to the top of our caravan/motorhome roof? Well as we add more panels, we are adding more watts of solar. Because of the way we will connect these extra panels (in parallel – more on this in just a moment), we will be leaving the voltage the same, using the extra watts to boost the current (flow of electrons into the battery) and charge our battery more quickly.

About Batteries!!!

Batteries  aren’t rated in AMPS or Watts; they are rated in Ah, what is that?

Well they are actually rated in amps (sort of) that’s what the “A” in Ah is for.

The hour part of the amp hour rating is simply that, its how many amps a fully charged battery can provide for a solid one hour. At least that is the theoretical meaning (reality is a little different)! So a 100 Ah battery should provide (in theory) 100 amps steadily for exactly one hour. So what if you change the load resistance (electric tap) so that it only draws 50 amps?

Well, that battery would supply 50 amps now for 2 hours since it is flowing out only half as much as before.

Think of those trillion trillions of electrons coming out half as fast. It will take twice as long to get them all out.

It works the other way too. If you draw 200 amps out of your 100 Ah battery then you would exhaust the battery at twice its rated capacity and it will be flat in just half an hour!

So the amp hour number is simply a measure of capacity or size. It’s just like comparing a 10,000 litre water tank to a 500 litre tank! You know that the 10,000 litre tank will last longer dumping out at the same rate.

So you now have some practical information here. You can see why a 200 Ah battery will run your microwave for twice as long as a 100 ah battery.

Let’s take a look at how we can connect multiple batteries…

Batteries in Parallel

Here is another interesting bit of information. When you parallel two similar batteries (done by connecting plus to plus and minus to minus) they each help each other handle the load so that you now have double the capacity. So two 100 Ah batteries in parallel act like one big 200 Ah battery. With batteries in parallel we are not changing the voltage at all – we could add 100 batteries in parallel and still have just 12 volts .

Three similar batteries in parallel are triple and so on… Usually this is done for cost, availability, space or handling reasons (just try lifting a 1000ah battery). Reminder – paralleling a battery doesn’t increase the voltage. The voltage remains the same. It is just like having two water storage tanks side by side and combining them to flow from a single tap.

The tap will run twice as long but the pressure will be the same as with one tank.

Batteries in Series

Now suppose you stacked one big water tank on top of the other and connected them together. The weight of the water would be double so the pressure would double. This is the same with batteries. If you hook two batteries up in series, (plus to minus) the voltage will double. Three will triple and so on.

In the case of motorhomes and caravans, batteries are connected in series to get the desired system voltage. For example, a caravan might have two large 6 volt batteries connected in series to give 12 volts. A large motorhome might have two large 12 volt batteries connected in series to provide 24 volts (it is just as valid to connect four 6 volt batteries in series to give the same 24 volts).

It is also common (particularly in very large motorhomes) to use a combination of series and parallel to create banks of batteries that are then connected together to give a very large capacity. Once connected, these banks of batteries (electrically speaking) look like and act like, one huge battery.

Quick review of what we have learned…

• Watts is “Work being done” (just like horse power).
• Watts can be calculated by multiplying volts by amps (P = V x I)
• Batteries in parallel – voltage remains the same – AH capacity increases.
• Batteries in series – voltage increases – AH capacity remains the same.

Here is the full version of Ohms law in the format that I used when I was learning it as an apprentice electrician.

Here is how it works…. Look at the inner circle, use the coloured quarters as a “to find”.  For example – to find WATTS, look at the blue P – WATTS inner section. Any of the formulas in the three blue outer sections can be used to find watts.

Conclusion

Congratulations you have now learned the basics of electrical theory. But this is not just a theory, I use this stuff every time I am installing, fault finding or repairing anything electrical. Understanding this stuff is not difficult and a basic grasp can be of enormous help when designing, installing and repairing caravan and motorhome electrical systems.

I hope you have both enjoyed reading and learned something from this tutorial. As always, you are invited to comment by using the “Leave a Reply” box below. Thanks for reading.

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Tags: battery, Caravan, Electrics, Energy, Motorhome, Power, Solar

This entry was posted on Tuesday, July 6th, 2010 at 8:46 am and is filed under Caravan, Electrics, Motorhome, Power, Solar. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.