smart battery status indicator

[--BawA--]

I have to design an indicator which continuosly monitors the battery voltage and display on 16x2 lcd ..
The battery is 12V 7ah lead acid
This project will use a pic16 series microcontroller ..
Now the problem is if i design a voltage divider and gives its output to the MCU. The mcu will show the battery percentage .but as i connect a load to the battery ,the terminal voltage drops ( say from 12v to 11v). Which results in decrease in battery percentage (say from 80% to 60%)..which is undesirable.
So how to design the circuit or modify the software,in order to get absolute battery percentage ..

 

[victordion]

You can think that 11V to 12V is 100% and rescale the percentage number according to that

 

[--BawA--]

But i have to map the battery voltage like 10.2 should correspond to 0. And 13.5 should correspond to 100%.
So suppose at no load battery voltage is 12v then the battery percentage will be approx 65 %. ..this is the real status of battery ..but consider a case in which no load battery voltage is 13.5v and after applying a load the terminal voltage drops to 12v.now again the lcd will display 65% but its not a real status of battery..

 

[victordion]

Suppose
when real battery is 100%, open circuit voltage is 13V, output voltage measured with load is 11V
when real battery is 0%, open circuit voltage is 10V, output voltage measured with load is 7V
Then you can display the remaining battery to be (x - 7)/(11-7) * 100% where x is your measured voltage
It may be not that linear, but who cares? You may have noticed that your cell phone battery falls slow at the beginning but drops like hell when it approaches 0%

 

[BradtheRad]

As you see, volt level can vary widely, depending on the load attached.

OTOH, if you can measure outgoing amperes, and elapsed time...

Then you can calculate A-Hrs. This is the best way to determine a battery's state-of-charge.

 

[--BawA--]

if you can measure outgoing amperes, and elapsed time..

How to do this. I mean how to design this circuit which can measure outgoing amperes and elapsed time.??

 

[BradtheRad]

How to do this. I mean how to design this circuit which can measure outgoing amperes and elapsed time.??

1) Install a sense resistor in a wire which carries current through the load. Connect the MCU so it can measure voltage across this sense resistor. (You may need to amplify the voltage.)

2) Multiply by a scaling factor, to translate into amperes.

3) Multiply by time. This gives you A-Hrs. Or else calculate it as Amp-minutes, or mA-minutes, etc.

4) Keep a running tally of mA-minutes. Display the total.

5) Set everything back to zero when you recharge the battery.

If you are really inventive, you can arrange the same components to measure A-Hrs going into the battery when recharging, as well. (If you can take into account that the sense resistor will show opposite polarity.)

 

[Audioguru]

The battery voltage drops when it has a load because you are using an old-fashioned cheap little lead-acid battery.
I use modern Lithium-Polymer batteries that are tiny but can supply 50A or more and the voltage does not drop.
Here is a little one (larger ones have higher capacity and higher output current):