Battery Low

My friend and I are building a RC airplane. We want to make a sensor to detect the battery level, in order not to drain the battery, or else I won't be able to recharge it. If the battery is low, I want it to either enter landing mode, notify me, or do something.

I have a 6 channel fly sky transmitter and receiver. http://www.hobbypartz.com/79p-ct6b-r6b-radiosystem.html I also have an 11.1 V Li-Po rechargeable battery http://lightake.com/detail.do/sku.11.1V_1800mAh_25C_RC_Lipo_Rechargeable_Battery_for_RC_Helicopter_-31807

please keep down the weight.

Wire 2 resistors on series. Connect 11.1V to one end. Connect other end to Arduino Gnd. Connect middle to Analog input. 11.1V to 6.8K (R1) to 4.7K (R2) to Gnd. With fully charge, 11.1V, analog input should be: Vout = Vin * R2/(R1+R2) = 11.1 * 4700/(6800+4700) = 4.54V You'll have to decide how low to let it go.

Basically a voltage divider?

Exactly.

Okay. I went to the following site and it explains a bit: http://code.google.com/p/ardupilot-mega/wiki/Voltage?wl=fr

A neat wee online voltage divider calculator I've used in the past can be found at: http://www.raltron.com/cust/tools/voltage_divider.asp

Makes it nice and easy to work out the resistor values you need.

So what do I do if I want something extremely light-weight for an airplane?

yosler: My friend and I are building a RC airplane. We want to make a sensor to detect the battery level, in order not to drain the battery, or else I won't be able to recharge it. If the battery is low, I want it to either enter landing mode, notify me, or do something.

I have a 6 channel fly sky transmitter and receiver. http://www.hobbypartz.com/79p-ct6b-r6b-radiosystem.html I also have an 11.1 V Li-Po rechargeable battery http://lightake.com/detail.do/sku.11.1V_1800mAh_25C_RC_Lipo_Rechargeable_Battery_for_RC_Helicopter_-31807

please keep down the weight.

Well on the electric R/C planes I built and flew long ago, the ESCs would give the control pilot an indication of low main battery by either cutting off, slowing, or pulsing the prop channel to give the pilot an indication that he/she should land soon before battery level dropped enough to effect the BEC's regulated +5vdc that could effect the radio and servos.

Lefty

yosler: So what do I do if I want something extremely light-weight for an airplane?

Well the voltage divider should be pretty light. As i saw in the docs about your CT6B it has a low battery led. Not sure if this is for both the control or also the reciver unit. I think you can use this as signal input for your arduino. If not use the voltage divider as already told.

You can use a led(not sure if visible maby a flash led?) or buzzer. Shouldn´t add to much weight. Or you programm something that gives you clear indication that battery is low. Maby only 50-75% speed.

the ESCs would give the control pilot an indication of low main battery by either cutting off, slowing, or pulsing the prop channel to give the pilot an indication that he/she should land soon

Is it the same as i suggest to programm with impact on flight speed?

Lefty: that's what I'd like for my airplane. How?

CT6B's light- for CT6B's battery. It's NOT for the airplane's. Also, I don't think the receiver can transmit!

There's also a problem doing that to the airplane, it might crash!!

PS. This is my first rc airplane.

Also, let's say I don't want an Arduino on board?

yosler: Also, let's say I don't want an Arduino on board?

Well not sure. I read something about the quadrocopters and some have buzzer and flash led to signal low battery. Im not sure if this is really usefull for a plane as it, at least what i think, flies much longer distances. But flash led should be visible as you always look at the plane.

if you dont want an arduino, not sure why posting here. I thought you would use the arudino to controll all motors/servos and maby add programms like flight stabilisation etc. Or do you mean not big arduino board but all the capabilities of the chip? There are many guides on how to make the arduino small withouth the board design. In general you take the chip (from uno for example) and just use it with the clock and the outputs. Read about the Atmega8 that dont even need a clock because it can use inbuild. NOt sure if the other specs fit your needs. (DAMN CANT FIND THE THREAD AGAIN about the Atmega8 sorry)

I’m still trying to find a way to get the Arduino on there.http://arduino.cc/forum/index.php/topic,120030.new.html#new http://arduino.cc/forum/index.php/topic,115581.15.html http://arduino.cc/forum/index.php/topic,115401.msg908229.html#msg908229
if I could get that, I’d mount one on.
I still do want to know how to do it if I’d find out.

First things first.

CrossRoads:
Wire 2 resistors on series.
Connect 11.1V to one end. Connect other end to Arduino Gnd. Connect middle to Analog input.
11.1V to 6.8K (R1) to 4.7K (R2) to Gnd.
With fully charge, 11.1V, analog input should be:
Vout = Vin * R2/(R1+R2) = 11.1 * 4700/(6800+4700) = 4.54V
You’ll have to decide how low to let it go.

Again, from where on the battery?
That wire —> R1 —> (if using Arduino,) Analog In [can I put it into a pwm pin?] —> R2 —> Arduino/11.1V battery GND?

Connect Vin/Gnd across the battery.
Vout goes to an analog input pin.
PWM pins are digital output pins.