RC / Radio Controlled Cars & Planes FAQ

We hope the following questions and answers will help you use our products and get the most out of your electric RC modeling hobby.

You may also be interested in our RC Electronics 101 content that focuses more on general electric and electronic notions as they are applied to the RC Cars & Planes hobby.

 

Why do I need a Watt's Up Watt meter / power analyzer?

Here are some applications where you will benefit from having a Watt's Up.

RC Airplane Watt meter Applications

RC Car & Truck Watt meter Applications

Without an RC Watt meter you can only guess what's really going on with your electric model and questions like these go unanswered.

  • What RC battery, motor or speed control will work?
  • Exceeding your battery's discharge and charge rates?
  • Running too much current through your speed control or motor?
  • Getting a full charge on your RC batteries and maximum runtime?
  • Battery cells in balance?
  • Speed control cut-off set right?
  • Buying compatible components?

"Watt's Up" is a precision instrument and the missing tool to show you how electrical specs like Volts, Amps, Watts and mAh describe both:

  • your model's performance and
  • the prop, motor, speed control and battery you need to get what you want from you model.

For a fraction of what you've already spent on your RC cars & planes projects, the "Watt's Up" gives you answers to electrical questions that protect your investment and help you get the most out of it. "Watt's Up" replaces an ammeter, voltmeter, wattmeter, Amp-hour and Watt-hour meter and does more while costing less.

You don't need to be an electrical engineer to figure out which electrical specs are relevant to your goals for your model. With our watt meter, you can easily conduct your own experiments.

You're in charge because you can measure the actual current and voltage your model uses and the power it produces. You can check what YOUR prop, motor, battery and speed control are really doing and compare that to their specs.

"Watt's Up"'s Unique Features

  • Low voltage capture makes it easy to setup the cutoff voltage on your speed control or see the effect a prop or motor change has on your battery voltage.
  • Peak Amps and peak Watts capture shows you the maximum current and consequent power (instantaneous voltage X peak current) drawn and even if they only happen for 1/1000 'ths of a second. Way too fast to see any other way.

If you're having a problem, the "Watt's Up"'s measurement abilities mean you can find an out of spec current or voltage that may explain a crash or damaged component. The possiblilities are endless.

Electric RC cars & airplanes can be easily analyzed to improve performance and duration in ways their gas powered brethren can not. With a watt meter you can take advantage of that opportunity.

How do balanced battery cells benefit me and how do I measure balance?

You don't want battery pack cells going below their minimum safe voltages for safety and battery life reasons.

A battery pack whose individual cells are all balanced delivers the most energy since all cells are exhausted at the same minimum voltage. If any cell is "out of balance" it may reach the minimum safe voltage before the others and continued pack discharge will damage the cell.

If the cells in a pack all equally contribute to the overall pack voltage they are considered "in balance". So we can check each cells voltage at various states of pack charge and see if their voltages are the same. If they are, the pack is balanced. If not some individual cell reconditioning or replacement is necessary.

Cell balancing measurements need at least 0.02 volt resolution so we can tell the difference between a 1.22 and 1.20 volt battery. More resolution is better because the lower voltage cell will get exhausted first. The "Watt's Up" 0.01 Volt resolution is great for cell balancing where resolution is more important than accuracy. This is because we're comparing our batteries to each other seeking equality rather than wanting to know what particular voltage they're at.

There are two ways to measure cell voltages.

1. Measure each cell individually

2. Measure the pack voltage and a few cells and subtract the cell voltages from the pack total to get the remaining cell voltage. Let's call this the "Sum and Difference Technique" - SDT for short.

The SDT is a bad idea for some very technical reasons.

The short explanation is that it requires more accurate voltage measurement equipment than most people have in order to prevent "quantization errors" due to digital measurement effects. A more complete explanation can be found elsewhere TI Data Converter Applications or, in the future, in this FAQ (TBD).

We, therefore, recommend you measure each cell independently.

The Watt's Up needs to have a battery or other power source attached to the auxiliary connector so it can measure below 4 volts.

Protect the "Watt's Up"'s LOAD side leads so they can't short.

After powering up, simply apply the red lead to the plus side of the battery cell being tested and the black lead to the minus side. Read the battery voltage and write it down. Repeat for all cells in the pack. Your goal is to have all the cell voltages be as equal as possible when the pack is charged and discharged. If you want to load the cells when taking measurements, you can attach the load to the LOAD leads of the "Watt's up" while testing. This will also show you the load current being drawn.

How can I reset the meter's accumulated readings?

Meters in current production (e.g. versions 2.1 or higher) have a reset signal on their Auxiliary connector's pin 3 (The orange lead on our CONN100 cable). Briefly connect pin 3 to 0 volts (minus lead) to reset the meters accumulated measurements to zero. This is also described in the meter's user manual.

When using Auxiliary power, why do I see small measurement values when the SOURCE and LOAD leads have nothing connected or are shorted? Will this affect performance?

It is neither unusual nor a problem for there to be small values displayed in this situation. The "Watt's Up" has been optimized to provide highest performance for real measurements of connected power systems and loads, but with all leads unconnected may produce erroneous readings. With real connections, accurate readings will be produced. Note that shorting the SOURCE leads together and/or the LOAD leads together (DO NOT SHORT WITH A POWER SOURCE CONNECTED TO EITHER SET OF LEADS!) may show a very small voltage (e.g. 0.01 Volts). These small "shorted lead" voltages are well within specification and don't affect the accuracy of real measurements.

Will "backwards" or reverse connection hurt the "Watt's Up"? E.g. connecting a charger to the LOAD side and a battery to the SOURCE? What will it read?

No. Connecting in this manner causes current to flow in the reverse direction, but will cause no damage up to the rated current of the meter. However, while connected this way, the meter will not indicate the actual current nor any value derived from the current i.e. Watts, Amp hours or Watt hours. E.g. the Amps and Watts will read 0.

Any accumulated Ah or Wh displayed will not increase further until power again flows from SOURCE to LOAD, i.e. in the forward direction (meaning current is flowing in the black wires from LOAD to SOURCE).

About measuring current flow in both directions (e.g. bi-directional measurement)?

The "Watt's Up" and Doc Wattson meters only measure current flowing in the black (negative) wire from their LOAD side to their SOURCE side as labeled on the meter's case. Current flowing in the opposite direction is not measured and does not affect accumulated or peak readings. Current flowing in the red wires is not measured at all.

To do a net current flow "bi-directional" current measurements (e.g. measure both directions of current flow) at one point in a circuit, we recommend using two meters connected together and oriented LOAD to LOAD (or SOURCE to SOURCE) as that is a more flexible and easily interpretted arrangement, than a bidirectional meter usually offers.

Alternatively, the meters can be mounted in separate parts of the system to measure two currents simulaneously. For example, the charge current into a battery and the discharge current out. Those currents may differ and this arrangement will show that. In this case meters are connected as usual ensuring that the current to be monitored is flowing in the correct direction in the black leads.

These dual meter arrangements allow independent measurement of the two current flows and their peak values. It also allows their respective accumulated values (e.g. Ah, Wh) to be reset separately. A bonus is that the voltages of the two separate systems can be independently monitored as well as the currents.

Note that with the dual systems, the red wires can also be connected using the "3-wire" arrangement. The red wires only need to carry enough current to power the meters and measure the voltage at the desired points.

What happens when the value I am measuring exceeds the "Watt's Up" maximum value? Will the meter be hurt?

Exceeding the voltage or current maximum values can permanently damage the meter. Exceeding the maximum voltage at all will probably cause permanent damage. Infrequent and brief (a second or less) current spikes to 150% of the maximum value should not cause permanent damage. The maximum current (Amps) an individual meter will display varies from meter to meter. Some may show up to 110% of maximum (e.g. 110 A on a 100 A maximum specification meter) while others may have a maximum of 101 %.

You can exceed the Watts, Ah, and Wh maximums without any damage. All that happens is that the values "wrap around" above 6553.6. This means they begin counting again from 0. So if your system has really measured, say, 7000 Wh, the display will actually show only (7000.0 - 6553.6=) 446.4 Wh.

How do "inductive loads" like motors cause circuit damage?

Loose connections and long wires on circuits switching high currents can damage electrical components. The problems can be largely eliminated by ensuring that connections are secure/not intermittent (e.g. not just twisted together) and to a lesser degree by keeping wire lengths short and by twisting wire pairs together to reduce their inductance. Always disconnect first/connect last the meter from/to the load (or highest inductance connections) first. Another way to think about this is to keep a battery or power source connected to the meter when ever potentially inductive loads are connected or disconnected. The battery or power source helps to clamp any voltage spikes created.

Here's a little more about why this all happens.

Wires exhibit a property called inductance. Inductance is an indication of the amount of energy stored in a wire's magnetic field due to the flow of current. If you interrupt the current flow, say with a switch or pulling apart a connector, the magnetic field collapses and induces a voltage in the wire to try and oppose the drop in current. Depending on the specifics of the wire, current and materials near the wire (like iron) the induced voltage can be quite high. In fact, it can give you a shock and damage electrical components still connected to the wire by generating voltages that exceed their ratings. This is one of the reasons capacitors and diodes are needed on brushed motors in addition to reducing RF noise.

Cars use this ability to generate voltage spikes in their ignition coils to generate the thousands of volts their spark plugs need from the car's 12 volt battery.

Version 1 and version 2 "Watt's Up", WU100, Watt meter - which do I have and what's the difference? Are there upgrades?

The power up screen shows the "Watt's Up" version number. Also, the instruction card that comes with Version 2 has a photo of the "Watt's Up".

The "Watt's Up" Version 2 added two features compared to Version 1. Otherwise it is the same.

We no longer make or sell Version 1.
Please Note: Version 1 cannot be upgraded to Version 2 and our exchange/upgrade program has ended.

  1. Addition of peak Watts capture for a total of EIGHT measured values.
  2. Volts, Amps & Watts are displayed continuously and the five additional measured values are displayed sequentially in the "Data-Queue" position (lower left) of the display.

I have a question not answered here. What should I do?

If you can't find what you need in our specifications or on-line manuals, you are welcome to contact us for more information. Please see our contact us page for contact info.


To RC Cars & Planes.

 

No watt meter yet?

The"Watt's Up" user's manual contains lots of useful information on rc electrical parameters, measurements and system testing.

It's useful even if you don't yet have a watt meter.

RC Frequently Asked Questions