battery monitoring circuit

[controllerman]

Hi.
i want to check my battery status by using the circuit which i have attached.I am using 12v 4.5Ah battery.when i fully charge the battery it gives more than 12v.What i exactly want is that when battery voltage is 12.5v all the LED's will be ON.when battery volts becomes 12v two LED's will be ON,when battery volts becomes 11.5 one LED will be ON and if battery volts becomes less than 11.5v No LED will be ON.Reference voltage i am using is 5v.
My question is that how i find the values of resistances so that when voltage of battery increases/decreases then LED's will light up accordingly.
thanks

[Hero999]

The comparator will turn on when the voltage at the + input exceeds 5V.

You need to rearrange the potential divider formula.

http://en.wikipedia.org/wiki/Potential_divider

It can be a bit of a fiddle to calculate resistor values that are standard values.

It's also a good idea to add a high value resistor from the output to the positive input to provide hysteresis which will prevent oscillation when the voltage is near the threshold.

[Alex Tsekenis]

Apart from agreeing with Hero's post have a look at these links:

http://www.elecfree.com/Battery%20Monitor%20for%2012V.php
http://www.electronics-lab.com/projects/automotive/009/index.html
http://www.mhennessy3.f9.co.uk/battmon/index.htm

You can also modify your circuit to use a duo-colour LED. Green when the battery is high, orange when half full and red when empty.

Also, there is the Bible of batteries that you can reference for an open terminal voltage VS remaining charge for lead-acid batteries:

http://www.amazon.co.uk/Handbook-Batteries-David-Linden/dp/0071359788/ref=sr_1_1?ie=UTF8&s=books&qid=1261596112&sr=8-1

[Hero999]

What comparator ICs are you using?

With the circuit as you've drawn it, you need a device which can source current, most comparator ICs such as the LM339 have open collector outputs which can only sink current.