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Old Dec 01, 2012, 12:03 AM
carl1864 is offline
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Possible to rip a low voltage cutoff out of an ESC?


I've never been able to find a simple cheap lipo low cutoff device, after countless searching. I had the idea the other day, I started wondering, is it possible to just rip one out of some cheap budget ESC?

The reason I wonder is simply there are many times I wish I could use a 3s lipo battery to power other miscellaneous 12 volt devices. Only problem is theres no protection to save my batteries from over discharging.

I would have thought something simple like this would be sold all over the place, but i can't find any. I've found various circuits before, but the circuits are simply more complex than I'd want to deal with.

So is it possible to gut an ESC to accomplish what I'm trying to do? Or is there some better way thats still simple and cheap?
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Old Dec 01, 2012, 01:23 AM
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The low-voltage cutoff in an ESC isn't really a specific device or component; it's just a bit of the ESC's programming that tells it to do something when the voltage drops to a certain point.

I don't think I've ever seen a device on the market that does what you're looking for.
Old Dec 01, 2012, 04:08 AM
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What exactly do you want this thing to do when the voltage gets low ? Most ESC LVCs only work to cut (or reduce) power to the motor they're driving. If you're not driving your devices via an ESC or similar that doesn't help you.

OTOH if you just want it to warn you when the voltage is low then Lipo alarms have been available for years....Google will find you plenty of them. Some of them even have outputs which allow you to drive e.g. a relay or semiconductor switch to cut off power.

Steve
Old Dec 01, 2012, 04:52 AM
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I am tempting to the similar thing as well. I actually plan to use a ESC as a
microcontroller board with FET builtin. My usage case is have the 3S battery
to drive the LED for my microscope. It just cut the LED off if battery is too low.
The plus side is I can even adjust brightness of the LED using duty cycles.

However, even if the LED is off. The voltage regulator and the MCU is still
running, that will slowly drain the power. So it is better to have a FET in front
of the ESC as well. I want it to be manual on and auto off.
Old Dec 01, 2012, 06:54 AM
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Originally Posted by carl1864 View Post
is it possible to just rip one out of some cheap budget ESC?
Generally not. Most ESC's use a microcontroller (and supporting code in firmware) to monitor battery voltage, and only stop the motor (they don't completely disconnect the power). You could use a brushed ESC to power your device, but you would have to supply a servo signal to turn it on and off, and it would still draw a small current even when 'off'.

The simplest way to provide true low voltage protection is with a relay and zener diode. The relay coil is connected to the battery via the zener and its own own contact, so when the voltage drops too low it opens and disconnects itself as well as the load. The zener diode reduces voltage to the relay coil so that it will drop out at the desired voltage. To turn the power on you wire a momentary push-button across the relay contact and zener, and to turn it off you wire a normally closed bush-button in series with the coil.

The main downside of this circuit is that the relay needs a significant amount of current to stay operated (~15mA for a 12V relay with 280 Ohm coil). There may also be issues with vibration causing the relay to drop out prematurely. Replacing the relay with a couple of FETs and resistors could create a solid state version which uses virtually no power.

Another relatively cheap and easy solution is a PCM (Protection Circuit Module), which can be installed inside the Lipo battery or connected via its balance connector. These usually cut off at a rather low voltage (typically 2.4V/cell), so should only be used as a backup in case you forget to turn the power off manually.
Old Dec 01, 2012, 03:16 PM
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So is there any other way to do it?

This seems like something so simple that would be all over the market, that lots of people would want, but apparently not. I'd simply like it to cut power completely once the voltage drops under a certain point.

Could be useful for any range of things, such as temporary LED's, Temporary wireless security cameras, or almost any other 12v device you'd like to be able to power temporarily, being most rc enthusiasts have a highly abundant supply of lipos.

I use lipos to power 12v devices all the time, but I always have to worry about not forgetting to unplug them. Or I'll be worrying about lipo damage, only to unplug way earlier than I have to, and finding I only used 10% capacity. Would be nice to take the worry out of things and know I could just leave things plugged in all day, and if they happen to run the battery too low it will shutoff.

I do know there are certain homemade circuits out there, but they are more complex than I'd like, wish there was a simpler way.
Old Dec 01, 2012, 08:14 PM
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Originally Posted by carl1864 View Post
This seems like something so simple that would be all over the market,
PCM's are standard in most consumer batteries. They are cheap ($7 for a 15A 3S unit) and simple to wire up. They protect against over-charge, over-discharge and over-current, making the battery virtually idiot-proof.

We don't use them in model aircraft because we don't want the battery to cut out due to momentary current surges, there is a weight penalty, and it raises the cost of the battery (it then becomes our responsibility to make sure the battery does not get over-charged, shorted out, or over-discharged!).

If you think the circuit in a PCM looks too complicated, consider what it has to do:-

1. Monitor the voltage of each cell, accurately detecting both low and high voltage.

2. Disconnect the load permanently if the voltage goes too low, but switch back on when the charger is connected.

4. Disconnect if the charging voltage goes too high.

5. Disconnect the load permanently if over-current is detected.

6. Draw insignificant current.

It would take a lot of discrete parts to do all this, but luckily IC's are available which have most of the circuitry integrated, so the only extra components required are two FETs and a few resistors and capacitors. Of course all this comes preloaded on the PCM board, which you just have to wire onto the battery.

If you don't need over-charge and over-current protection or individual cell monitoring then a single voltage detector and switching FET will do the job. However that still requires a few components. Here's an example (it's for a single cell, but could easily be modified to work on 3S by changing some resistor values).
Old Dec 01, 2012, 10:33 PM
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So where can I find one of those common $7 PCM's you mention?

Since I'd only plan to plug it inbetween the battery and load when in use, I think most of those 6 features such as ones dealing with charging, would be unnecessary, since i'd still charge it on the normal charger.

Only feature I need is simply, disconnect the battery when it reaches a certain voltage, say 10.5 or 11 volts for a 3s (which leaves plenty of safety margin anyways)
Old Dec 02, 2012, 04:15 AM
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A Google or eBay search on 'PCM Lipo' should find a few .....
Old Dec 02, 2012, 08:39 PM
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Awesome. Thanks so much. That seems like exactly what I need. I've spent so many hours over the past couple years repeatedly searching every few months, and never found the answer until right now. Much appreciated.


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