# Ignorant Old Man Needs Help with Voltage Divider

Discussion in 'The Projects Forum' started by Peter Hollenbeck, Feb 21, 2015.

1. ### Peter Hollenbeck Thread Starter New Member

Feb 21, 2015
2
0
I know some computer but almost zero electronics.
I want to measure the voltage of a small 12 volt battery bank. This is hobby related, not commercial. The batteries are charged by a micro hydro generator. Half the year the site is unattended.

I am using a 1wire voltage sensor, an iButtonLink MS-TV attached to a Raspberry Pi computer. The sensor has a limit of 10 volts so I made a divider to halve the voltage. Two weeks ago, with a test 12v power supply I played with various resistor values, from 1K to 1M. The 1K resistors appeared to give the best reading, about 6 volts. The higher the value the lower the output. Now, here in the wilds of British Columbia, working with a real 12v battery system, I am getting values ranging all over: 0.1, 0.82, 0.07, 10+, rarely anything reasonable such as 6.6. Within several minutes I just got 0.7 followed by 9.75. The inverter connected to the battery bank displays 12.1 volts and the output from my program now shows .71 volts. With the same divider connected to a 9 volt battery I get a consistent 4.77 or 4.78 volts.

I am mystified and would greatly appreciate suggestions.
Thanks,
Peter

2. ### crutschow Expert

Mar 14, 2008
13,462
3,353
What value of resistors did you end up using?
What is the purpose of the 12V inverter if you have a 12V battery?
You may have a noise problem. Try a 10uF (or so) capacitor between the voltage divider output and ground.

3. ### Peter Hollenbeck Thread Starter New Member

Feb 21, 2015
2
0
The inverter converts the battery DC output to AC to power computers, lights, etc. in the cabin.
I am using two 1K resistors because my experiments with a 12 volt bench supply indicated these gave the best divided output.

This setup is working perfectly with a 9 volt battery. Multimeter shows the battery at 9.6. Output from the divider is 4.75. Close enough. But with the 12 volt battery bank it jumps all over. By the way, the battery "bank" consists of two 6 volt golf cart batteries.

Thanks,
Peter

4. ### #12 Expert

Nov 30, 2010
16,655
7,293
Lead acid batteries don't get squirrely. Even when they die, they do it in an orderly manner. Look for a loose connection.