Voltage monitor for car battery

[malc-c]

Yes you can, but first you have to lower the supply voltage for the picprocessor. This can be done by using a LM78L05 and some capacitors.

I don't think that would work. The regulator will simply give you a 5v supply from the 12v battery, it will never fall below 5v because the car battery would never get this low. The simple way is to use a voltage divider made by connecting two resistors accross the 12v line and then connect the pic to the junction between the two.

If you used a 58K and 41K with a 12v supply you would get 5v, however the state of a car battery is such that it's voltage will vary under charge and drain, so you would need to make allowances for that. You might find that under charge the voltage could be around 13.8v, falling to 10v under load.

[chuck]

use an op-amp set up has a devider you can then put upto 20v in and get 0-5v out and for safety tie a 5V zener diode to prevent damage to the pic or just a resistor devider

I personal use op-amp has i can measure up to 60V

[peterdeco1]

Hello. I've been using the attached circuit and the following code to measure the battery condition on a 12V sealed rechargable gell cell. The code goes like this:
batterycondition var byte

adcin 1, batterycondition
if batterycondition < 192 then beep

Perhaps tweaking the adcin value would work for lead acid. It may have a different discharge curve.

[passion1]

use an op-amp set up has a devider you can then put upto 20v in and get 0-5v out and for safety tie a 5V zener diode to prevent damage to the pic or just a resistor devider

I personal use op-amp has i can measure up to 60V

Chuck / Jerry

Thanks for the advice! Could you be specific with regard to where the 5V zenere diode must be placed. (I'm rather new to electronics!)

THANKS!

[weirdjim]

I don't think that would work. The regulator will simply give you a 5v supply from the 12v battery, it will never fall below 5v because the car battery would never get this low. The simple way is to use a voltage divider made by connecting two resistors accross the 12v line and then connect the pic to the junction between the two.

If you used a 58K and 41K with a 12v supply you would get 5v, however the state of a car battery is such that it's voltage will vary under charge and drain, so you would need to make allowances for that. You might find that under charge the voltage could be around 13.8v, falling to 10v under load.

The OA didn't say use the regulator for the sensed voltage, but for the supply voltage. He also recommended a 3:1 voltage divider for the sense. Personally, I'd use a 4:1 because the car battery can get up into the mid-14 volt range with some charging systems.

I have no idea where you get 58k and 41k resistors; these aren't standard RETMA values. I'd use a 100k and a 22k-10k variable in series to set the voltage accurately, or do some other tricks in software for calibration.

Jim

[Dave]

passion1, Use a 10.2K 1% and a 2.55k 1% for the voltage divider. This gives you 25.00 volts full scale or a 20% division. I often use these values when not being concerned about the 1mA. current draw @ 12 volts.

Dave Purola
N8NTA

[mackrackit]

I will but in here
I use a 3 to 1 divider to monitor my solar battery bank. Two 10Ks in series from (+) and one 10K from (-). if peak voltage is 15 you get 5. Voltage drops to 10, you get 3.3.
This works for me.

[passion1]

Hi All

Thank you very much for the advice!

[malc-c]

The OA didn't say use the regulator for the sensed voltage, but for the supply voltage.

Sorry, mis-read the post

I have no idea where you get 58k and 41k resistors; these aren't standard RETMA values.

Whislt not standard a quick google will show that they do exist, or simple enough to make up the value.

[BobP]

Hi,

Don't forget to suppress the voltage spikes! A cars electrical system can generate spikes easily above 50v. Try to take the measurement points as close to the battery as possible. Also damp the takeoff point from the resistor divider with a small capacitor (10uF). I would also use some inductors in the supply line to protect the regulator and PIC.

Bob

[passion1]

Hi,

Don't forget to suppress the voltage spikes! A cars electrical system can generate spikes easily above 50v. Try to take the measurement points as close to the battery as possible. Also damp the takeoff point from the resistor divider with a small capacitor (10uF). I would also use some inductors in the supply line to protect the regulator and PIC.

Bob

Bob

Thanks for the advice! Could you be specific with regard to where the capacitor must be placed as well as the inductors. If possible, inductance values would be appreciated. (I'm rather new to electronics!)

THANKS!

[chuck]

Passion1

here is web link with values and op-amp

http://www.winpicprog.co.uk/pic_tuto...ogue_board.htm

Has for the zener diode you just put the block end to say PORTA.0 and the ohter to ground.

hope this helps,

chuck

[BobP]

Hi,

As per your request please find attached a quick and basic suggestion for values I would use.
As you have 10k resistors I used these.

Bob