Hop Up How To

This is not a discussion of how to improve your car motor, but rather how to improve and test every other piece of the electric system in your car. This discussion will show just how much can be gained from improvements to any or all of you electrical drive system.

For you to understand what is being discussed here, I am going to include some very basic theory in the most simplest of terms. Also, if your going to test your system, you will need to know how to use a multi-meter.

Disclaimer. I will not take responsibility for any damage to any of your testing equipment, of any part of your rc system. You do these tests and modifications at your own risk.

Now that I've stated that, the risks are minimal, as long as you don't go shorting across terminals with your meter.

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Theory

The most basic of electrical laws is "Ohm's Law", which states that :- The current flowing through any resistance is equal to the Applied Voltage divided by the resistance to the flow.

Current (A) (Amps) = Voltage (V) (Volts)

Resistance (R) (Ohms)

or A = V

derived from this is :- V = A x R , and R = V

The concept of voltage, current and resistance can be likened to water flow. If you have a tank on a hill connected to a pipe with a restriction on the end like a tap, then the further up the hill the tank is, the more pressure you will have. The Pressure = the Voltage. The tap (the restriction) is the resistance to current flow, and the flow of water is the current. Obviously, the greater the pressure, and the less the restriction, the more water is going to flow, as the formula suggests.

The power dissipated (lost) in a resistor is :-

Power (P) (Watts) = Voltage (V) (Volts) x Current (A) (Amps)

or P = V x A watts.

If we substitute the previous equations for V and A, we get :-

P = V² watts & P = A² x R watts

where V² = V x V

These formulas will be used quite a bit in the following discussion

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Basic Motor Theory

Question. What's the difference between a dc electric motor and a generator?

Answer. Almost none. In a motor you apply a current to rotate the shaft. In a generator you turn the shaft to produce an electric current.

I'm not going to go into all the theory here, but try to imagine this :-

If you apply a voltage to the terminals of a dc motor, current starts to flow in the coils (dependent on the applied voltage and the resistance of the coils) (Ohm's Law).

The current produces a turning force on the shaft (torque) proportional to the current (theory not supplied), and if the torque is high enough, the motor begins to rotate.

Now here's the bit that's a little harder to understand . When the shaft rotates, the motor also becomes a generator producing its own internal voltage. This voltage is of the same polarity of the applied voltage and has the effect of reducing the applied voltage, which in turn reduces the torque. The internal voltage is termed Back EMF.

So, if you have a fixed load on a motor (torque), then the speed of the motor is proportional to the voltage applied because :- To produce a particular torque, the motor requires a certain amount of current. (torque is proportional to current) To produce this current you need a voltage applied to the coils equal to the current in the coils multiplied by the resistance in the coils (which is fixed for a particular motor) (Ohm's Law). Voltage applied to the coils is equal to the voltage applied to the motor minus the Back EMF. The Back EMF is proportional to the RPM of the shaft.

Therefore, torque is proportional to current, and at constant torque, RPM is proportional to the applied voltage.

If you didn't understand the reasoning, just remember the statement in bold above. It's all that is required for the rest of the discussion.

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How do you make your system work better?

From the motor theory above, it can be seen that supplying more voltage at the same current, or more current at the same voltage will produce more motor performance, directly proportional to the voltage gain at the same current.

So, to improve performance, we could just add another cell to the battery, but that's not always allowed. We could just use higher voltage batteries, but that's not always the answer (see battery discussion later).

A lot can be achieved by reducing all the little voltage drops that occur all the way from the battery to the motor, which means reducing all the resistances in the system to achieve the highest possible voltage at the highest possible current.

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How to test your system

For some of the how much can I gain calculations later, you really need to know how much current you are drawing off the battery. This is extremely hard to measure while the car is racing, and impossible to get any usable readings while the car is off the track.

The following method gives a fairly accurate estimate.

How to test your current draw

Charge a battery which you know its discharge capacity (not charging capacity) Test the battery on a discharger from fully charged to flat before the charging it for the test. Note the capacity (usually in milliamp hours)
Put the battery in your car and run it on the track till flat. Take note of the time taken to flatten the battery.
Do the average current calculation as follows :-

Take the capacity of the battery (milliamp hours), and divide by 1000 to give Amp hours. This figure means that this battery could supply this many amps for 1 hour before going flat.

Divide 60 (minutes) by the number of minutes taken to flatten the battery.

Multiply this figure by the Amp hour figure for the battery to give average current drain in Amps.

Remember this figure for later.

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How to test the rest of the system for voltage drops

For this you will need something to use as a fixed drain with a reasonable current draw (5 to 10 Amps) and a charged battery. One of the best things is a car light bulb, where you connect up both low and high beam together.
Replace the motor in you system with the light bulb.
Now we have to measure the voltage drop and the current at the same time. This requires 2 meters, but a reasonable result can be obtained using just 1, first testing the current with the meter in series with the light bulb (in the circuit i.e. system connected to one wire of the meter, the other wire of the meter connected to the bulb, the other terminal of the bulb connected back to the system), and then using the meter out of the circuit to test the voltage drops. Otherwise current can be measured with a calibrated clamp meter or a small multimeter in series on 10 Amp range.
Connect up the battery and the speed control to the radio, and apply full throttle. The bulb should glow and the amp meter should show the current.
While keeping an eye on the current, take readings of the current, and the voltages between the positive terminal of the battery and the positive terminal of the battery plug, the negative terminal of the battery and the negative terminal of the battery plug, from one side of the plug to the other (both positive & negative sides) i.e. take the reading between the positive on the battery plug and the positive on the mating half and between the negative on the battery plug and the negative on the mating half. What you are trying to do is measure each voltage drop along each wire from the battery to the motor. All these readings will be in millivolts. At no time should you connect a multimeter on the millivolt rage (mV DC) between the negative and positive wires. Some meters will be destroyed!!
Check each section of wire and also the speed control between negative in and negative out, and between positive in and positive out. Only do this at full throttle. Make sure you are at full throttle, otherwise the voltage could destroy your meter.
For each section calculate the resistance by dividing the voltage in volts by the current in amps. Because the voltage should have been in millivolts, the values you get will be in thousandths of an ohm. the range should be in the order of 0.0001 ohms to 0.1 ohms.
Add the positive and negative values together for the speed control. This is the speed control total resistance. This value is the most important value for the performance of a speed control. It is usually stated in the specs of most good speed controls. Note- In some speed controls the positive wire from the battery is not connected to the motor through the speed control, instead it is connected more directly to the motor, and either a branch wire returns to the speed control, or a wire returns from the motor to the speed control. In either case the connection from the branch or from the motor to the speed control has no effect on the performance of the circuit. Only the positive wire from the battery plug direct to the motor is in the circuit.
For a comparison with better quality wire, you could measure the length of each piece of wire in the circuit, and work out its resistance per meter. i.e. divide 1000 by the length of a section of your wire (in millimeters), then multiply this figure by the resistance calculated for that piece in step 7 above.
Record all values.

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How to test the internal resistance of your battery packs.

Battery packs have resistances too. If a battery had no internal resistance the voltage of the battery would not get lower when you draw current from it.

Charge the battery pack.
Connect a load and draw about 10% of its capacity off. At this point NiCd and NiMH batteries should be on the plateau voltage. This is done so that the voltage remains fairly constant throughout the test. Note- When you discharge a NiCd or NiMH battery pack, the voltage starts off at about 1.5V per cell. After a very short discharge, the voltage drops to about 1.2 to 1.3 V per cell. The battery remains at this voltage to near when the battery becomes flat, at which time the voltage drops rapidly.
Measure the battery voltage with no load.
Connect a current drain like the one used in the previous test (headlight bulb)
Measure the loaded battery voltage and the current drain simultaneously (2 meter method, one measuring current and the other measuring the voltage across the battery). To be really accurate measure the voltage across the battery, and not across the battery plug.
Disconnect the current drain and recheck the no load voltage. It should be the same as before. If not, either average the before and after voltages or repeat the test till a satisfactory result is obtained.
Calculate the internal resistance :- subtract the loaded voltage from the no load voltage. Divide this figure by the current draw of the loaded battery. This will give you you the internal resistance of the battery in Ohms (Ohm's Law)

This method gives a very accurate measurement of the internal resistance, and is a good way to determine the performance or age of a cell or battery pack.

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What does this all mean for you??

Batteries

If your batteries tested in the 0.07 Ohms or greater area, then you can forget about competing with anyone that has purchased new batteries. If you take the battery resistance and multiply by the average current draw that you worked out before, then you have the average battery voltage drop. High load points (hard acceleration) can be twice this amount.

Consider. Your battery pack has a plateau voltage of 7.2 V unloaded, and an internal resistance of 0.07 Ohms, and your average current draw is 20 Amps (typical for stock motors). The voltage drop is 0.07 x 20 = 1.4V. your available voltage at average current draw is 7.2 - 1.4 = 5.8 V.

The guy racing next to you has just bought a pack of the new GP batteries GP370SCHR cells. The listed plateau voltage of about 1.26 V per cell and 0.004 Ohms (worst case) per cell, giving a pack voltage of 7.56 V and resistance of 0.024 Ohms. Voltage drop at 20 Amps is 0.48 V giving an output voltage of 7.08 V, 22% more voltage than you at the same current draw which means 22% more speed at the same motor torque. Along with that his battery pack remains in the higher voltage ranges longer because it has much more capacity and will be no where near flat at the end of the race.

Another interesting point. Only a few years back, the best battery you could buy was a Sanyo Cadnica NiCd N1700SCR. All good battery packs used this cell. Its plateau voltage is around 1.25 V per cell unloaded and its internal resistance was 0.004 Ohms, where as a GP NiMH GP330SCH cell (common recent cell) has a plateau voltage of 1.28V, but an internal resistance of 0.006 Ohms. At around 17 Amps the N1700SCR delivers more voltage than the GP330SCH, and for all currents higher than this the performance gets better. However, the GP330SCH wins out because it has nearly twice the capacity. This means you can draw nearly twice the current for the same length of time. So you gear the car higher, lose some efficiency, make everything hotter, but you're still there at the end of the race having drawn more power from the battery.

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Speed Controls

First, I would like to convey that variable frequency speed controls are a good sales gimmick. The manufacturers of VF (variable frequency) speed controls claim the you get more acceleration and higher top speed. All speed controls have no frequency when they're at full throttle. They are just a turned on switch, no frequency, no PWM (pulse width modulation), just simply on. So for top speed or maximum acceleration all speed controls have the same frequency, i.e. none.

There is, however, an efficiency gain at variable throttle for higher frequencies, which comes about due to the internal construction of the motor. Basically, the lower the frequency the hotter the motor gets, but for any car motor, anything above 2 kHz (2 kilohertz) causes negligible heating and minimal efficiency loss. With the capacity of current batteries, efficiency in model cars has become a non issue. It has become almost impossible to drain a battery during a race.

The resistance of a speed control can be a significant factor. If you purchase a speed control of the type normally supplied with basic model kits, then you will find that drawing in excess of 20 to 25 Amps causes the speed control to get hot. Heat means resistance, and resistance means poor performance. Most of these speed controls have large heatsinks to reduce the problem. Speed controls of the basic type typically have resistances of 8 to 20 mOhms, especially if they are forward/reverse controllers, and some even higher. 8 mOhms at 20 amps is a 0.16V loss and for a 20 mOhm controller is 0.4V loss. At 40 Amps the loss is 0.32V and 0.8V respectively. Stock type (26 turn) motors can reach this sort of current under heavy acceleration, and "mod" motors generally run in this area on average.

Most top of the line controllers have a resistance of around 1 mOhm, my own Power Master and Skymaster speed controls have ~0.7 mOhm.

To estimate the improvement you could achieve from changing your speed control perform this calculation :-

Take the total resistance of your speed control that you worked out before, subtract the resistance of a new speed control (both in Ohms), and multiply this figure by the average current figure from your tests. The answer will be the amount of voltage you could gain by changing your speed control.

Example Your current speed control has a resistance of 8 mOhms and your average current draw is 20 Amps. You change to a Power Master Mk VI.

(0.008 - 0.0007) x 20 = 0.146V

Assuming the available voltage at 20 Amps is 6V, this represents a 2.4% increase in performance. This might not seem like much, but 2.4% could mean a pass down the straight.

Example 2 You have a speed control that is a couple of years old. You test it to have a resistance of 1.2 mOhms. You are running a mod motor and know your average current draw is 50 Amps. You see a new speed control that is rated at 0.6 mOhms on resistance and want to know how much you'll gain.

(0.0012 - 0.0006) x 50 = 0.03 V

This represents a gain of approximately 0.5%. Hardly worth the upgrade???

If you are running a forward/reverse controller, then consider this. Besides the fact that you are not allowed to use reverse in competition racing, F/R controllers have mosfets switching on the positive wire as well as the negative wire, which equates to twice the internal resistance for a controller that's nearly twice the size. In your speed control resistance test you will have seen that the voltage drop positive in to positive out was nearly identical to negative in to negative out.

If you have a forward Brake controller, and it has power in as well as 2 motor wires, then eliminating the speed control internal connection can improve performance marginally. Simply connect the battery positive wire directly to the motor positive, and then connect the speed control positive wire to either the motor positive or from a point in the battery/motor positive wire. The speed control motor positive wire can then be discarded. Warning. Do not do this on a forward reverse controller. When you apply reverse the speed control will short circuit the battery and be destroyed.

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Wiring

All wires have a resistance which is proportional to their length. If you are still running the original wires that came with your speed control, battery or car, then you would probably have calculated a resistance of around 14 mOhms per meter of wire. A normal system could have in excess of 0.7m of wire, which gives a resistance of 9.8 mOhms, which is about 0.2V loss at 20 Amps. So shorten that wire as much as practical. Take particular note as to which end of the battery pack the wires are soldered to. Some packs have the wires running the entire length of the pack and exiting the opposite end.

A much better solution is to try a high current wire such as Dean's Wet Noodle. My tests on this wire show it to have a resistance of 5.2 mOhms/m, less than half of standard wires, which is why I use it exclusively on all my high current speed controls.

The total "in circuit" wire length can also be shortened by the employing 3 wire forward/brake speed control method described in Speed Controls above, the positive wire to the speed control having no effect on the motor circuit resistance.

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Plugs

All connectors are a source of resistance, so it is essential that the contact surfaces be kept clean, and you use a low resistance plug. My tests have shown that the standard plug that comes with cars and battery packs has a resistance of around 1.76 to 1.9 mOhms new per leg, and with use and damage can be many times this. I have seen these plugs get so hot that they have melted together and cant be separated.

Tests on an old used Dean's type plug gave resistance readings of 0.25 mOhms on one leg and 0.42 mOhms on the other, the plug obviously needing a clean. Even so, the resistance was so much better than standard.

Another plug which I have a strong dislike for are the gold plated bullet connectors that push into tubes soldered directly to the battery. The resistance of these plugs is extremely low, although I don't have any exact figures, but there is no way to prevent them being connected in reverse. I do a few repairs on other makes of controllers besides my own, and these plugs are the number one reason speed controllers need repairing. Number two is using too small a speed control for the application. If you are going to use these plugs try to perfect some system where the wires can't be reversed.

Motor plugs. Use a good type if you need to use them, otherwise solder your wires directly to the motor, and forget the original supplied bullet connectors.

Some enthusiasts are so committed to reducing resistance, that they eliminate all plugs, preferring to solder the wires directly to the battery just before the race. They solder to a copper tab which has been permanently soldered to each end of the battery pack. The possible gain in performance over a Dean's plug is less than 0.3% at 40 Amps current draw.

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The Bottom Line

The amount you can gain is dependent on :-

Your Current Drain. If you were drawing no current, or very little than the voltage losses would be next to zero on any system. If you were upgrading from 'stock' to 'mod' where the currents can double or even triple then the gains an be huge.
What comprises your current system as far as battery, wires, speed control and connections, and what's available to upgrade to.

Disregarding battery and motor and looking specifically at everything else. If you had an 'out of the box' system, the total resistance from battery to motor would be in the order of 27 mOhms which would give a voltage drop of 0.54V at 20 Amps and 1.08V at 40 Amps.

An upgraded system with top of the line speed control such as Power Master Mk VI, Dean's wire, Dean's battery plug and short wire connections would have an "in circuit" resistance of 2.55 mOhms and a voltage drop of 0.05V at 20 Amps and 0.1V at 40 Amps. That's about 8% gain at 20 Amps and 20% gain at 40 Amps.

Some Resistances from My Own Tests
Typical Wire Supplied 'Out of the Box'	14 mOhms/m (0.014 ohms per meter)
Dean's Wet Noodle 12 AWG	5.2 mOhms/m (0.0052 ohms per meter)
Standard battery plug	1.76 - 1.85 mOhms per leg
Dean's type plug (used)	0.25 - 0.42 mOhms per leg
Typical Basic Speed Control (forward/brake)	8 - 20 mOhms
Typical Basic Speed Control (forward/reverse)	16 - 40 mOhms
Power Master Mk VI speed control	~0.7 mOhms

I hope this web page has been of assistance to you in helping you improve your system.

The calculations and dialogue on this page are correct to the best of my knowledge, and I take no responsibility for problems arising from using the results 'as is'. You would be best to verify the calculations for yourself. If you find any errors, or you have any other results, please email me.

Try my performance calculator Car Performance.xls

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This website last updated Monday, 06 November 2006