Picaxe as low voltage alarm

[D396]

I have a 2s lipo that outputs about 7.4 when the voltage drops to 6v and want the picaxe to detect it and go crazy(lights,buzzer, etc). I am using an 08m. I have already made a battery monitor before but am seeking device as to what resistor values to use for the voltage divider and any other tips you may have.
Thanks.

 

[eclectic]

I have a 2s lipo that outputs about 7.4 when the voltage drops to 6v and want the picaxe to detect it and go crazy(lights,buzzer, etc). I am using an 08m. I have already made a battery monitor before but am seeking device as to what resistor values to use for the voltage divider and any other tips you may have.
Thanks.

Here's a starter:
http://www.picaxeforum.co.uk/showthread.php?t=17760&highlight=low+volt*+monitor

e

 

[D396]

thanks. The only problem is calibrating it. I have all the code except the cutoff value and I don't know which resistor values to use for the voltage divider. Is there a calculator somewhere for that? I also don't have the batteries with me so I can test anything yet.

 

[MartinM57]

I would have thought that a "PIACXE going crazy (lights, buzzer, etc)" would be the last thing you want to do, in terms of current consumption, when the powering LiPo batteries are detected as going flat

 

[eclectic]

thanks. The only problem is calibrating it. I have all the code except the cutoff value and I don't know which resistor values to use for the voltage divider. Is there a calculator somewhere for that? I also don't have the batteries with me so I can test anything yet.

http://www.kpsec.freeuk.com/vdivider.htm

And also search the Forum for the word
thevenin

 

[eclectic]

A couple of assumptions:

1. Your Picaxe supply is 5.00 volts
2. Your voltage divider is two matched 5kOhm resistors.

Task 1
Calculate the current and the impedance.

Task.2

A "full" 7.4v battery would produce a
3.7v reading at the junction.

Readadc10 would show (3.7/5) x 1023 = ?

A "low" 6v battery would produce a
3v reading at the junction.

Readadc10 would show (3/5) x 1023 = ?

Then a little breadboarding, and
you're almost there.

e

 

[srnet]

thanks. The only problem is calibrating it. I have all the code except the cutoff value and I don't know which resistor values to use for the voltage divider. Is there a calculator somewhere for that? I also don't have the batteries with me so I can test anything yet.

Well, it would be helpful to explain what type of lipo you are monitoring and what its being used in, what is the purpose of the monitor ?

Also how are you powering the 08M and are you using a external voltage referance ?

Another also, how accurate do you need it to be ?

 

[D396]

I am monitoring an RC 2 cell lipo. The monitor will make sure I know when the battery is low so I don't over discharge it. the 08m will be provided a regulated 5v. It does not have to be that accurate the battery cant be allowed to drop below 6v though.

 

[eclectic]

I am monitoring an RC 2 cell lipo. The monitor will make sure I know when the battery is low so I don't over discharge it. the 08m will be provided a regulated 5v. It does not have to be that accurate the battery cant be allowed to drop below 6v though.

In that case, the number range from post #6
should be more than adequate.

e

 

[srnet]

I am monitoring an RC 2 cell lipo

In what sort of RC application ?

Is the Lipo just used to power the RC receiver and a couple of servos, or is it an electric motor powered RC model of some type ?

 

[D396]

thanks. I did not see post #6 until now. That should work.

 

[srnet]

A "full" 7.4v battery would produce a
3.7v reading at the junction.

At ''full" a single lipo should be 4.2v.

At 3.7v its maybe 50- 75% discharged.

 

[eclectic]

At ''full" a single lipo should be 4.2v.

At 3.7v its maybe 50- 75% discharged.

Fair enough I know little about LiPo's
but was acting on the info given in post #1.

e

 

[D396]

2 lipos in series = 7.4 volts

 

[D396]

It is powering a traxxas slash. (electric rc car)

 

[manuka]

These Traxxas Slash darlings seem more fully spec'd than a normal car. Do they have air bags too?!

As the R/C car will no doubt be some distance away, just how had you intended checking the discharge level? Perhaps a winking LED? Radio link ? As momentary decreases in emf naturally occur under load you may get false alerts too.

FWIW a mate with a large LiPo R/C plane mentions he can "feel" when the battery is weakening by the resulting sluggish plane performance, even though his setup includes a battery fuel gauge. Does this apply for you too?

 

[srnet]

For high power Lipos, i.e. the relativly expensive ones, 3V cutoff might be a little low for long life.

But anyway, unless size is a major issue, using preset potentiometer would allow you to set the cut off voltage to exactly what you want, without the problem of calculating exact resistor ratios.

 

[D396]

I have a blinking LED. I never thought of using a potentiometer. I will defiantly do that.

 

[D396]

As for the slash. She's amazing hitting 30+ mph. I probably could tell if the lipo is draining but every one says to use an LVA.

 

[srnet]

I fly electric planes too, you can tell when the power is draining, and most ESCs cut the power at low voltage in any case. I usually dont fit LVC alarms here.

I fly small hand launch gliders too, I do fit LVC alarms in these since I want o know if I am going to loose control of the plane because the battery goes flat, before it actuall does.

For planes flashing lights might not be that visible, check on eBay and you will see that the LVC alarms you can buy use piezo sounders. Tuned to the right frequency these are very loud and can be heard in flight.

As for the potentiometer, you can fit it, adjust the LVC alarm point, and then work out what combination of resistors will give you the same result.

Also be aware that the ESC may not be putting out 5v, the ESC on one of my planes puts out 5.3v for instance.

 

[MFB]

Although describing a wireless based system, this link may contain something of use to you.

http://www.picaxeforum.co.uk/showthread.php?t=15967&highlight=MFB

 

[D396]

Ill try it without the LVA and if I need one I will make it.

 

[spiritbear]

why bother with a picaxe etc when you can use an LTC1440 with standby current of 4uA.

See here: http://www.discovercircuits.com/DJ-Circuits/9vbatmon1.htm

Just recalculate the resistors or use potentiometer for one to tweak the threshold voltage.