Home

Location map

Club Day Results

Club News

Championship Standings

Members Info

Wombwell 2009

RC Tips

Large Scale Info

1:10th Car Info

About Servos

For Sale

How It Works

 Safety Info

 SERVO'S AND LIPO'S

What is this 'balancing' business?

 

Each cell of a Lithium Polymer battery has a nominal voltage of 3.7v, and they are connected in series. This results in a 2-cell LiPo being referred to as a 7.4v Lipo, and a 3-cell Lipo being referred to as an 11.1v LiPo. LiPo cells must maintain a voltage between about 3.0 volts and 4.2 volts, and allowing a cell to go much above or below this voltage range can result in damage to the cell. The only way to assure that each cell stays within this voltage range is to charge them individually to a specified voltage (balance charging), or to bring each cell to the same voltage as the other cell(s) in the pack (balancing). A special connector on LiPo battery packs, called a 'balance connector', makes this possible. Some devices simply balance the cell voltages - they do not actually charge the cells. I personally use a Balance Charger that monitors and charges the individual cells of a LiPo. I do not use this charger everytime I charge a LiPo - I sometimes use a simple 'Wall Charger'. And to be honest, I went a long time without ever balancing my LiPos, and never had a problem. However, I do suggest that you consider balancing your LiPos, as they will likely last much longer. You also might consider reading through some RC Forums on this subject to get some other opinions about it.

2. What is a 'C' rating?

The 'C' rating of a LiPo indicates the maximum safe discharge rate. The 'C' refers to battery Capacity measured in milliamp hours (mah). The 'C rating' is a number that is multiplied by the capacity of the battery to give you the discharge rate. (It would be simpler to just indicate the maximum safe discharge rate in amps on the battery pack, but why make it simple?!) The way to determine the maximum discharge rate in AMPS is to take the capacity of the LiPo (mah) and multiply by the 'C rating', then divide the result by 1000. For example, a 1200mah 12C LiPo has a maximum safe discharge rate of 14.4 amps (1200 x 12 = 14400, divided by 1000 = 14.4 amps).

Keep in mind that amp output tells only half the story. You must also know the voltage to determine the total energy output in watts: Amps x Volts = Watts. So if your 1200 12C Lipo has 2 cells (7.4 volts), then the total energy output would be 106.56 watts (14.4 amps x 7.4 volts = 106.56 watts). If the LiPo has 3 cells (11.1 volts), then the total energy output would be 159.84 watts (14.4 amps x 11.1 volts = 159.84 watts). More watts will produce more thrust - all things being equal.

If you want to know the true usable energy in a LiPo, you would hook up a meter between the battery and an electrical load (motor/prop) and run from a fully charged state to motor cut-off, measuring the energy used in watt hours (wh).

3. How fast can I charge a Lipo?

Most LiPos are designed to be charged at a 1C maximum rate. This means that the charge rate (milliamps) must not exceed the capacity (mah) of the LiPo. So a 500 mah Lipo should not be charged at a rate greater than 500 milliamps (0.5 amps), and an 1800 mah Lipo should not be charged at a rate greater than 1800 mah (1.8 amps). THIS IS VERY IMPORTANT!!! I have a friend who can tell you why - he nearly burned his house down when a 500 mah Lipo burst into flame while being charged at a 1000 milliamp rate. BE CERTAIN OF YOUR CHARGE RATE!!! You might consider charging at a lower rate than 1C if you're not in a hurry, as this will help extend the life of your LiPos.

 

An insight into Servos

How Do They Work ?

     Servos are controlled by sending them a pulse of variable width. The control wire is used to send this pulse. The parameters for this pulse are that it has a minimum pulse, a maximum pulse, and a repetition rate. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has exactly the same amount of potential rotation in the clockwise direction as it does in the counter clockwise direction. It is important to note that different servos will have different constraints on their rotation but they all have a neutral position, and that position is always around 1.5 milliseconds (ms)

   

    The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse width Modulation. The servo expects to see a pulse every 20 ms. The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral position).

     When these servos are commanded to move they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position. The maximum amount of force the servo can exert is the torque rating of the servo. Servos will not hold their position forever though; the position pulse must be repeated to instruct the servo to stay in position.

     When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counter clockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs. The minimal width and the maximum width of pulse that will command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maximum and minimums. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide.

   Another parameter that varies from servo to servo is the turn rate. This is the time it takes from the servo to change from one position to another. The worst case turning time is when the servo is holding at the minimum rotation and it is commanded to go to maximum rotation. This can take several seconds on very high torque servos.