Watt meter

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Watt meter

RC Electronics 101

The performance of radio controlled electric cars, planes and helicopters using electric motors is determined by their electrical systems.

With new tools like a Watt meter, hobbists now have the ability to understand, modify and troubleshoot their models in ways previously impossible with gas powered models.

What's the difference between an Ah and a Wh?
Will a larger battery make my model faster?
How do wiring and connectors affect my model's performance?
Why must I use connectors designed for high current?
What's the difference between continuous and intermittent current measurement specifications?

What's the difference between an Ah and a Wh?
An Amp Hour (Ah) is a measure of charge (measured in "Coulombs") whereas a Watt Hour (Wh) is a measure of energy. The two are related by voltage. So a 22.2 V battery stores twice the energy of 11.1 V battery with the same Ah rating. When only an Ah specification is given it is understood that the voltage that determines the energy this represents is that of the battery (storage device). In summary, the energy (measured in "Joules") stored in a 11.1 V, 1.7 Ah (1700 mAh) battery is 11.1x1.7= 18.87 Volt-Amp-hours (or Watt hours). Where a Volt-Amp-hour is 3600 Joules (J). So our battery has stored 18.87 Watt hours or 67,932 Joules.

What can that much energy do? 18.9 Wh / 190 W = 0.1 hours or 6 minutes. So a motor could draw 190 Watts for 5.7 minutes from this battery. Actually less time than that because battery capacity is less at high currents because of "Peukert's law", but that's another article.

Will a larger battery make my model faster?
A larger (bigger or higher Ah capacity) battery with the same open circuit voltage will make your model faster only if it has lower series resistance and can, therefore, deliver more current to the same resistance load. e.g. your motor. Think about it using a bucket of water analogy. A fixed diameter hole (resistance), a fixed depth (voltage) below the surface, will leak water at the same rate (current) for any size bucket.

How do wiring and connectors affect my model's performance?
Though typically very low, only fractions of an Ohm, wires and connectors have resistance. Consider a 9.6 V battery pack with a motor that draws 25 Amps. Ohms law tells us this circuit has 9.6/25 = 0.38 Ohms of resistance. Much of that is attributable to the battery and the motor. Let's say 0.05 Ohms is due to wires and connectors. If we could reduce their contribution 5X to 0.01 ohms the total resistance is now 0.34 Ohms and at 9.6 V gives 9.6/.34= 28 Amps - a 12% current improvement! So using fat wires and low resistance connectors can pay off. You don't want your battery's energy going into heating your wires!

Why must I use connectors designed for high current ?
Like the wires they connect, connectors have resistance that's measured in Ohms. That resistance depends on the type of metals making contact and the total contact area. Other things being equal, increasing the contact pressure (from crimping or spring tension) maximizes contact surface area and thus lowers resistance. Another benefit of higher contact pressure is reduced vibration induced (fretting) corrosion. This is due to the increased pressure reducing relative motion between the contact faces and that motion tends to create metal oxides that increase contact resistance. Properly designed noble metal contacts (e.g. Gold over Nickel) or contacts with a conduction lubricant (e.g. Tweek) minimize fretting corrosion. Increased contact resistance creates positive thermal feedback that destroys contacts.

Here's an example. Using the a contact resistance of 0.015 Ohms and 35 Amps current means that contact must dissipate (P=I^2 x R) 35 x 35 x 0.015 = 18.4 Watts of power! Insulate that with heat shrink tubing, and bury it in your plane and you have a small soldering iron getting pretty hot.

Unlike wires, a contact's resistance is developed over the tiny area that actually makes contact between the mating connectors. In a contact all that generated heat is dissipated over that small area rather than the whole bulk of a wire.

The heat increases the contact's resistance (since metal weakness at temperature reduces contact pressure) and more power in the system goes into the contact until something bad happens.

Note that the squared relationship of current to power dissipation in resistance means things change fast at higher currents. e.g. at 49 amps the contact is dissipating 36 Watts. The current in your circuit "I" is the voltage divided by total resistance. e.g. I=V/R. (in Amps, Volts and Ohms). If your contact resistance increases, some of your battery voltage is now wasted across your contacts instead of going to your ESC/motor. Your max current is less because there's more total resistance (assuming ESC wide open). What gets lost is Power.

If you measure the Amps through and Volts across your connector you can find its resistance (Ohms) and power dissipation (Watts) and know watt's what (sorry).

Yes, an RC "Watt meter" that can measure down to 0 volts would be a handy tool for testing these things. Armed with the above, you can measure just what your situation is and verify it isn't degrading over time.

What's the difference between continuous and intermittent current measurement specifications?
Specifying a parameter, e.g. current, as continuous means you can expect the device to handle it, well..., continuously. That means it shouldn't be damaged by operating at that amount. A parameter not obviously specified as continuous may not be! Convention may expect it to be so (e.g. a household light bulb has a continuous voltage rating that's unstated), but you should check it as failure to be so rated could cause damage or error if used continuously.

Intermittent ratings, ideally, have a time or duty cycle associated with them. E.g. 100 Amps of current for 20 s per minute means the device should handle 100 Amps for a total of 20 seconds in any 60 second interval. That is also called a 33% duty cycle over a minute because the device can handle the rated amount for 20/60 = one third or 33 percent of the minute.

Here's an example. When high currents flow in wires, their electrical resistance produces heat. That's how a light bulb filament works. A test device like a Watt Meter may produce 10 Watts of heating with 100 Amps running through it due to its internal resistance. That's like a small soldering iron. If on for a few seconds it would barely warm up. But after 10 minutes it might have melted the case! So it might have an intermittent rating of 100 Amps for say one minute in any ten or a 10% duty cycle over ten minutes. That same Watt Meter might be able to continuously handle 20 amps corresponding to 2W of heating continuously.

While we have discussed current, continuous versus intermittent ratings are relevant for many specified parameters. Other examples are: voltage, temperature, pressure, acidity and duration of a baby crying.

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