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Old Sep 05, 2011, 03:17 PM
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Joined Sep 2004
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DIY Lipo pack monitor - question

Hi all,

I'm working on a design for a lipo pack monitor similar to this one:

http://www.tcdynamics.com/Prod04.htm

I was going to buy one of these a while back but I ended up waiting on it (off to college...), and of course, now they seem to be unavailable.

So, I set off to design my own which (in theory) will work exactly like that one. He says he uses 'all analog circuitry', so I did some research and found that a window comparator is probably what he used for the monitor. Basically, the circuit has 3 outputs, 1 for when the voltage input is above one reference voltage (Level 1 on his graphs), 1 for when it is in between the two references, and 1 for when the input is below the second, lower, reference (Level 2). Each of these outputs is connected to one color of the RGB LED, and the PWM pulses from the ESC drive the LEDs and create the different colors for different levels.

I have my circuit design working in a simulator, but I am now faced with the question of what to use for the reference voltages. I can infer a general region for them based on his graphs, but I don't know for sure. I don't know enough about lipo discharging to make a good (safe) guess.

Can anyone who understands lipos give me some advice as to how to determine the reference voltage levels I need to use? I don't really have any sort of test bench where I can monitor the voltage in order to determine what I need. I only have the motor in my heli...

I don't have a computer with my schematic program, so its all on paper at the moment. Later tonight I'll be back home and I can post my schematic.
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Old Sep 06, 2011, 09:01 AM
Retired CFI Helicopter
Hughes500Bob's Avatar
Southeast PA
Joined Jun 2007
1,609 Posts
"WE" like to have our lipos at 3.7 volts per cell after resting. That's around 80% capacity discharge. 80% discharge and no more gives good battery life.

Where to stop at depends on LOAD. Every load is different based on their motor, prop, weight, aerobatic flying, lazy flying, etc.

Example: When bashing my RC truck I set my BM6 adjustable alarm to 3.4 volts per cell. Cells bounce back to 3.7 volts after resting.
When rock crawling my RC truck I set my alarm to 3.6 volts per cell. Cells bounce back to 3.7 volts after resting.
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