View Full Version : Need more power?

03-20-2006, 12:26 PM
Have just maidened a ModelTech Magic Extra with Tower Pro BM3520-7 brushless with 13x8 prop with 12-16c 1550 Mah 4s4p lipo's through a 60A speed controller. The plane will take-off in 10 feet & go vertical to approx 60 ft & then the power drops off. It will still cruise around ok for ten miutes or so but it isn't real happy in the power dept. No prop hanging but will still climb so so. The question i need to ask is should I rewire to go to a 6s1p setup to try or will I need to buy two more packs to get a 6s6p arrangement. There is no heat build up in the batteries at present so I thought I would try the 6s1p. Has anyone used these motors with good results? This is the first serious E-conversion I've tried so its all new to me & interesting to the other club members. A few have allready commented on the stealth aspect of flying electric.

03-29-2006, 04:55 PM
I don't think the problem is with the motor. It is the battery that supplies the power and if power drops, it is the supply from the battery which is at fault.

Matt Kirsch
03-29-2006, 09:25 PM

It sounds like you're a bit confused with the whole ?S?P thing... You don't need to have the same number of packs in parallel as you have cells in series. What is necessary is to have enough packs in parallel to handle the Amps being drawn by the motor in a particular configuration.

There are three pieces of information necessary to determine how many packs you need to put in parallel:

1. How many Amps the motor is expected to draw with the prop you chose, on the number of cells in series you chose. Or, the maximum current rating of the motor and the assumption that you are not exceeding this rating.

2. The capacity of the cells you are using to build the final pack.

3. The C rating of the cells.

Multiply the C rating times the capacity to get the current capability of a single pack. In your case, 12C times 1500mAh equals 18 Amps.

Divide the expected Amps by the current capability of a single pack and round up. According to Balsa Products, your motor has a maximum current capability of 45 Amps. 45 Amps divided by 18 Amps is 3.

Thus, you need a minimum of a 3P pack of these 1500mAh 12C rated cells to handle the motor's maximum rated current.

The fact that power is dropping off so quickly indicates that you are likely running way more than 45 Amps in this configuration, the battery packs are damaged, or the battery packs are of poor quality. A Whattmeter run for 30 seconds on a freshly charged pack will tell the tale.

Going to 6S1P will make the problem worse. Increasing Volts will also increase Amps proportionally, so if you were drawing 45 Amps on a 4S, you'll be drawing 68 Amps on a 6S. Trying to draw 68 Amps from a pack capable of producing only 18 Amps will only succeed in damaging the pack.

Also, keep in mind the rated capacity of the motor. Balsa Products only recommends 4-5 LiPoly cells, meaning a 4S or 5S pack. A 6S pack will likely cause the motor to sling a magnet.