Electrical Terms for the Layman

Electricity
Written by Russ Pagenkopf.

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Hoo boy, let's see if I can compress this down, it's still awfully long. For all the electrical geniuses on the list, keep it down, this is greatly simplified. :-)

Your battery is a storage device. It holds a limited amount of energy. Just like all batteries it will discharge over time, more quickly if something is drawing energy from it (clock, lights, etc.).

To recharge your battery you have an alternator. The amount of energy your alternator puts out varies from a minimum level at idle, to a maximum amount at a certain RPM (typically 5000).

Now the alternator really has two jobs, one to recharge your battery after you draw energy from it to start the bike and two, to provide extra energy to power accessories. Headlights, turn signals, brake lights, clocks,...

So the key component to your question is how do I know how much energy does my alternator put out and how does it get spread around?

First an equation.

(W)atts = (V)olts * (A)mps

Volts is pretty simple, the minimum number you should ever use in this equation is 12.8. Your battery isn't really a 12 volt (or 12v) battery, it's actually a 12.8v battery.

Amps is typically the number used to describe how much energy your alternator puts out. Most Trophies have a 25 amp (or 25A) alternator (everyone just shut up about the rumored higher amperage alternators, we're keeping this simple).

Watts is typically the number used to describe how much energy your accessories use. For example, headlights are 55 watt low beam/ 60 watt high beam (or 55w/60w). The energy that your accessories use is cumulative, so if you have two headlights, then 2 x 55w or 2 x 60w = 110w at low beam and 120w at high beam.

Now how do we tie all this together? Back to your battery. Remember your battery gives up energy to start your bike, so it needs to be recharged from your alternator. You can recharge it slowly or quickly depending on how many watts are available from your alternator.

Let's take a simple example. If your alternator is a one amp alternator then:

12.8v * 1A = 12.8w

But if your alternator is a 10 amp alternator then:

12.8v * 10A = 128w

Woo Hoo! Faster recharge! Yes and no. Yes, it will recharge faster, however there is a limit to how fast your battery will recharge and there is special limiting circuitry to handle this built-in to your bike. No need to worry about it, unless it fails of course, and that discussion is outside the scope of this discussion. ;)

Now to the nitty gritty of your original question. How many accessories can I connect up. Well, as we've seen from the discussion above that is a pretty simple thing to compute.

Since our Trophy has a 25A alternator and the voltage is fixed at 12.8v then you have ~300w (approximately*) to work with. So start adding up the wattage on your accessories (and everything is an accessory) and subtract it from 300w. If your total wattage is greater than 300 then your battery will try to make up the difference, at least until it goes dead. If your wattage equals 300 then your battery probably isn't recharging. And if your wattage is less than 300 then you're good to go.


 * Note: all good electricians leave themselves some fudge factor when doing their computations. In actuality, the formula says you have 320w, but differences in construction of the alternator, losses from heat and wire length and size, yadda, yadda, yadda. Err on the safe side, round available wattage down, round wattage and amperage used up.

One of the most important (in my opinion) accessories to add is a volt meter. It's an easy way to see just where your sitting on that energy (power) curve. Because of that circuitry we mentioned above, it magically limits the maximum voltage in the system to 14.4v. Higher than that and you've got a problem on your hands. Remember our 12.8v magic number? If your voltage meter read 12.8 or less than your draining your battery and you need to shut something off. Anywhere in between and you're ok. Remember though, that the closer that number is to 12.8 the longer it will take to recharge your battery.

The last thing we need to talk about today is wire gauge. This is a critically important topic. Screw this up and you'll be lucky if all you do is burn a wire out. I've seen bikes burn that weren't wired correctly. Remember our formula from above? W = V * A. Amps are the critical part of the discussion here. I'm going to use an analogy. Think about wire gauge (or size) as a hose. A large gauge wire might be a fire hose. A small gauge wire might be a garden hose. The volume of water you can push through a garden hose in a set amount of time would easily go through a fire hose in the same amount of time. But if you tried to push the volume of water you can push through a fire hose in a set amount of time through a garden hose in the same amount of time, what would happen? Catastrophe. Now electricity works the same way, try to push more amps through a certain gauge wire than it can handle and you'll either burn out the wire* or cause it to overheat and burn off the insulation and arc to some other piece of metal. Not Good.


 * BTW, this is how a fuse works.

So how do you avoid this problem?

Simple, one, you compute how many watts your accessories will use. Let's use our headlights for example. Since they're 55w/60w and they feed off the same wire we'll use the higher value of 60w. Now there are two lights so our total wattage is 2 x 60w or 120w. Using our formula:

W = V * A 120w = 12.8 x A

Rearranging we get:

120w / 12.8 = A or 9.375A

So our lights are drawing 10A (remember that we err on the side of safety and so we'll round up). There are many places on the web to find out what size wire you need for a given amperage. Here's one

Engineering Toolbox

With our 10A lights you should use a minimum 14 gauge wire, however, you probably should round up and use 12 gauge.

Finally, remember that amperage is cumulative too. If you have a single wire coming from your battery to feed all your accessories that wire must be able to handle the total amperage of all the devices. You might consider a junction block fed with a large wire from your battery and smaller wires to your relays and accessories.

If you're interested, I have an extensive diagram for how I plan to rewire my bike this winter using multiple relays to handle more accessories than I have wattage to handle. The trick is electrical management, ie. if A and B then not C, but A and C and D are ok. :)

Anyone got a spare bike they want to loan me for testing?

Russ Pagenkopf 98 Trophy BBBB