voltage monitor

[aftab]

Hello friends,

I would like to make a voltage monitor for my battery pack.
It has 4 li-ion batteries.

When the voltage drops below 3.0 in any of the batteries it will blink a led.

My understanding is that I need an analog to digital converter IC that will
measure the voltage. Just want to make a very very simple circuit.

I have seen some examples here using maxim ICs. Up to this
point I have bought all my ICs from e-bay :-) Could not find the maxim ICs
in e-bay. What about using pic12c671 ?

Any suggestion where to start ?

Thanks

[mackrackit]

All you need is a PIC with ADC. Here are two examples of how to use the ADC .
http://www.rentron.com/PICX2.htm
http://www.rentron.com/serial.htm

If the voltage that you start off with is higher than 5 volts, use a voltage divider to bring it down to 5 and adjust in soft ware.

[w7ami]

The simplest and fastest would be to use a PIC with comparator. You would only need two resistors used as a voltage divider to bring your battery voltage down to something the PIC can handle. You can set the comparator reference voltage in code. You also can program the comparator to trip an interupt when the divided battery voltage drops below the reference voltage. You will need to look at the reference manual for the PIC you have to find out how to set the comparator and turn on the correct interupts.

Terry

[samtheboxer]

The simplest and fastest would be to use a PIC with comparator. You would only need two resistors used as a voltage divider to bring your battery voltage down to something the PIC can handle. You can set the comparator reference voltage in code. You also can program the comparator to trip an interupt when the divided battery voltage drops below the reference voltage. You will need to look at the reference manual for the PIC you have to find out how to set the comparator and turn on the correct interupts.

Terry

Sorry to interrupt here, but im trying to build the same circuit to monitor a lipo battery on my RC heli. do you have a schematic for the voltage divider or what this circuit might look like using a 12v lipo batter? i am really lost on this project and cant quite get a handle on it!

from what i gather, the chip will compare the input voltage from the battery to the internal voltage and detect the difference? am i close?

[mackrackit]

Sorry to interrupt here, but im trying to build the same circuit to monitor a lipo battery on my RC heli. do you have a schematic for the voltage divider or what this circuit might look like using a 12v lipo batter? i am really lost on this project and cant quite get a handle on it!

from what i gather, the chip will compare the input voltage from the battery to the internal voltage and detect the difference? am i close?

This link will explain voltage dividers.
http://www.omega.com/techref/das/divider.html
The divided voltage will the go to an ADC pin of the PIC. The voltage will then either be compared to the voltage that the PIC is running on or a reference from another source(voltage divider).

[w7ami]

Samtheboxer,

Dave has already sent you a link that explains what a voltage divider is and how to figure the correct values of resistors to use so I won't repeat his info.

Whether you use an ADC or comparator you will need to use a voltage divider to divide down the battery voltage into a range that is safe for the PIC.

I suggested using a comparator and interupt because it doesn't require clock cycles from the PIC unless the battery voltage has dropped below the pre set threshold. I was thinking that he just wanted some sort of failsafe that would shut things down if the battery voltage dropped to low but do other things in the mean time. I am using just such a feature on a project that I am working on now. After re reading the original post I see that what was really wanted was to circuit that just monitored the battery voltage and for that the ADC approach is better.

Which circuit and code you use depends upon what it is you are wishing to accomplish.

Terry

[samtheboxer]

Samtheboxer,

Dave has already sent you a link that explains what a voltage divider is and how to figure the correct values of resistors to use so I won't repeat his info.

Whether you use an ADC or comparator you will need to use a voltage divider to divide down the battery voltage into a range that is safe for the PIC.

I suggested using a comparator and interupt because it doesn't require clock cycles from the PIC unless the battery voltage has dropped below the pre set threshold. I was thinking that he just wanted some sort of failsafe that would shut things down if the battery voltage dropped to low but do other things in the mean time. I am using just such a feature on a project that I am working on now. After re reading the original post I see that what was really wanted was to circuit that just monitored the battery voltage and for that the ADC approach is better.

Which circuit and code you use depends upon what it is you are wishing to accomplish.

Terry

Terry - I have attached a schematic of my circuit using the voltage divider. I drew it up really quick using paint, so it doesnt look pretty. Also, the battery input should be +12 and -12, not +12 and -11. I also figure out were to put that voltage divider ground, im just running it into the chip ground. (i will post a proper schematic here shortly, i am working on it now.)

So, i think ive got the circuit down but now i need to work on the program. All i want to do is run a program flashing some LEDS as a strobe light on my helicopter. When the LIPO reaches 9.5v (or so, just not below 9V) i want the chip to change the pattern of the LEDS to warn me that my battery is getting low and its time to land. So far, i understand i need to use the comparator on the 12f629 and compare the battery voltage from the divider to the internal voltage in the chip (supplied by a constant 5v).

Here is my frustration, I am new to all this and might be in over my head, but im not giving up. I understand how to blink a light and different flash patterns, but thats about as far as ive gotten. No matter how hard i look i cant seem to find a program to dissect or an example of what i want to accomplish and how to accomplish it code wise.

I hate to ask, but would it be possible for someone to write the picbasic pro code for me, a working program, so that i can run it and figure out how it works? I feel like if i just has a working program, a lot of it would make sense to me........

[GioppY]

Any suggestion where to start ?

Perhaps you can start from here:
It is for car battery; but you can adjust for any voltage
http://www.pctuner.net/articoli/Pic/..._-_Datalogger/

Regards
Gianni

[BrianT]

Li Ion cells are easily damaged if they drop below 3 volts. Run them flat and they lose most of their capacity and instead of being a 1500 mAH capacity, they drop to maybe 1/4 of that capacity. In a series string of 4 cells I suggest you need 4 analog channels on your PIC, say a PIC16F88. You can measure the first cell directly but you need voltage dividers off cells 2, 3 and 4 to ground so you can measure V1, V2, V3 and V4.

V1 is the first LiIon cell and must not be allowed to fall below 3 V. V2-V1 gives you the voltage of the second cell. V3-V2 gives you the voltage for the third cell, etc.

The PIC16F88 needs no crystal and would be a very cheap solution for up to 7 series cells.

HTH
Brian

[aftab]

Hello,

Thanks everyone for the tips and links. I am getting the parts now from e-bay.
Rentron is always my favourite site for tutorials for amateur like me(thanks mkit)

On way is 12c672. And trying to get 16f877 - seems a bit more expensive.

From Wikipedia got valuable info on li-ion cells(thanks briant).

Got $1.59/each (X 8pc) charging circuit from batteryspace.com. The charger works fine but charging is stopped once the voltage reaches 4.2v and stops discharging when <3.0v. It is not current limited and the batteries do not get warm in the process of charging. So, I guess it does not charge the batteries to their full capacity. Interested people might find it useful for ready-made low cost chargers.

This micro controller world is really fascinating.

thanks
Aftab.