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Pinter75
01-28-2009, 10:24 AM
Howdy,

I have a few 11v 3 cell Lipos that I use in my R/C models, along with balancer chargers etc... I would like to re-purpose these to use in my bot projects.

Here is my sticking point. I am trying to find or make a low voltage cutoff module so I can use it with my Arduino and some DC motors. I have R/C ESCs that have inbuilt LV cutoff, and all the ones I can find on-line send a signal to the RX to control the throttle.

Now I could use one of these and send the throttle output to the Arduino, but I loose an input pin :-(

There must be a module that sits between the battery and power in - for example the arduino, or a motor circuit that will autodetect cells, and simply cut power when the cut-off is reached.

Anyone got any ideas?

(off the shelf module would be best)

Cheers.

jes1510
01-28-2009, 10:27 AM
I think most of the Lipo BEC's will do what you are looking for. Look for BEC or Battery Eliminator Circuit. They are commonly used with RC cars and airplanes.

I believe someone did an Arduino Li-po "backpack" as well but I can't remember where I saw it. It think the same person did the OLED/touchscreen for the arduino.

[edit] Found it: http://www.liquidware.com/shop/show/BP/Lithium+BackPack

JadeKnight
01-28-2009, 11:45 AM
If you're just worried about the DC motor control cutoff, the SaberTooth 5A DC controller has an internal LiPo cutoff that can be engaged. When engaged, the motors will not run if the voltage is too low (although it says the controller itself will still draw a small amount of power until the battery is pulled).

http://www.trossenrobotics.com/store/p/5103-Sabertooth-dual-5A-motor-driver.aspx
http://www.trossenrobotics.com/store/p/5104-Sabertooth-dual-5A-motor-driver-for-R-C.aspx

Pinter75
01-28-2009, 11:45 AM
Awesome, that lipo back back is great!

One quick question about the BEC modules, don't they still need to be fed into the RX to enable power down? I just want a "dumb" power off when volts hit x feature.

crabfu
01-28-2009, 12:47 PM
There are lipo alarms, if you don't need an actual cut off:

http://www.v2heli.com/newlialfor3c.html

-Crabfu

Adrenalynn
01-28-2009, 01:53 PM
The Sabertooth motor controllers just clamp, light the Err light, and go to sleep when they hit their limit with the Lipo switch on. They're pretty "dumb" about it - or if you prefer - they're pretty protective of your lipo. ;)

Pinter75
01-28-2009, 02:31 PM
I should say I am already planning to use the Adafruit Motor Control Shield.

Here is what I have been looking at:


LiPo Battery Protector (2-4 Cells)

Attach this clever device to your lithium polymer battery and protect your battery from over discharge and increase the life for your battery. This cunning device will reduce the power to your motor if the battery voltage drops to 3 volts per cell and will cut the power to the motor if the voltage drops to 2.6v or below! It even has a bank of LED lights to give a visual display of the battery's condition. it will also protect your battery from overheating by reducing the power to the motor, how clever is that!

The unit features:

4 LED Status display
Protection Function will reduce or cut power to the motor
Overheat Protection - If Battery reaches 60'C
BEC and Parallel charge short circuit protection
Weight - 13g

http://www.sussex-model-centre.co.uk/uploads/Leton_LiPo_Battery_Protector_4403050.jpg

It has pins out for the ESC and the RX. So how would I hook this up to an arduino or motor controller? Would I simply take the +V -V from the ESC or RX outputs into the Arduino, or the motor controller? I have a feeling that these need to "speak" to the RX or ESC before they do anything. This one at least does not need programming via the TX for cutoff, cells etc.

Because these are made for RC stuff, all it does is signal the ESC to throttle down or cut-off completely. It will still give power to the RX and servos (in my experience).

I have seen some schematics for an in-line monitor, but can't seem to find it any more...

Pinter75
01-28-2009, 02:34 PM
The Sabertooth motor controllers just clamp, light the Err light, and go to sleep when they hit their limit with the Lipo switch on. They're pretty "dumb" about it - or if you prefer - they're pretty protective of your lipo. ;)

^

That is exactly what I need, just without the motor control stuff. Ideally I want a small module that plugs into my Lipo, then into my motor controller or Arduino.

Does not matter if it has buzzers, or leds - I just don't want to undervolt my Lipo and have an exploding bot!

jes1510
01-28-2009, 02:50 PM
Then you just want a lipo BEC. The receiver output plugs into the arduino and the motor output would plug up to the voltage inputs on the Adafruit motor controller.

Pinter75
01-28-2009, 03:13 PM
Great, thanks for the help, I'll pick one up and do some testing.

brutus
03-15-2009, 12:29 AM
I just purchased a 2S 7.4V 600mah LiPo pack and after searching the net, stumbled across this thread.

Since we're talking about a microcontroller, I'm wondering for mine if this would be as easy as two 10K resistors? OP would need one 10K and one 20K but I'm thinking a very basic voltage divider fed into one of the analog pins and the voltage would be in range to be measured. (measuring one Cell effectively) This way you can simply take software action when ADC reads 3V or less.

Its also late, so I could be babbling..


void read_lipo()
{
if (analogRead(0) < 614)
{
while(1)
{
//Stall here and do nothing.. maybe light an LED or something to indicate why
}
}
}

ScuD
03-15-2009, 04:52 PM
It works, but I'm a bit apprehensive when it comes to software solutions on lipo safety.

At least make sure you use a watchdog timer to rule out a stalled microcontroller destroying the lipo when it's voltage drops too low undetected.