I have a discharger circuit that I intend to hook up to a 3V battery. Total resistance of the circuit (depending on how fast I intend to discharge it) will vary from 0.04 to 0.01 Ohms (that's 75A and 450A respectively).

Can I hook up a simple voltage divider circuit in parallel to measure the 'health' of the battery? (Diagram below) Or will I run into some trouble?

enter image description here



  • 2
    \$\begingroup\$ How do you intend to measure the voltage? You "measurement" device can have an input resistance comparable to the resistor values, effectively changing the voltage divider. \$\endgroup\$
    – Eugene Sh.
    Nov 18 '14 at 18:13
  • \$\begingroup\$ Indeed. 5Mohm will introduce considerable error when used with a typical 10Mohm DMM. \$\endgroup\$ Nov 18 '14 at 18:20
  • 1
    \$\begingroup\$ It's a 3V battery - why don't you just measure it directly using an MCU with 5V ADC facilities? \$\endgroup\$
    – Andy aka
    Nov 18 '14 at 18:20
  • \$\begingroup\$ @EugeneSh. I was going to read it via this guy's IO pins: sparkfun.com/products/9669 \$\endgroup\$
    – jules0075
    Nov 18 '14 at 18:24
  • \$\begingroup\$ What kind of battery are we talking about? You have a battery which can supply 900 watts of power (3V at 300A) for a meaningful length of time, but it's only 3 volts? \$\endgroup\$
    – supercat
    Nov 18 '14 at 19:28

The divider has an output impedance of 2.5M ohm, so it should not be connected directly to the ADC or you'll get considerable error.

Why not use 10K + 10K 1%?

Edit: According to the manual you linked in your comments, the maximum source impedance allowable for the board you have in mind (it appears to use a PIC) is 10K ohms.

enter image description here

So 10K + 10K is fine, and you could use a higher value if you like (as much as 20K+20K if you always have the battery connected) or stick with the 10K+10K if you might put a switch in there to reduce drain on the battery.

  • \$\begingroup\$ Tbh I chose random values thinking it wouldn't matter. Can you please explain why 10K resistors would work better? \$\endgroup\$
    – jules0075
    Nov 18 '14 at 18:52
  • \$\begingroup\$ Thanks, otherwise no problems? \$\endgroup\$
    – jules0075
    Nov 18 '14 at 21:29
  • \$\begingroup\$ See edit above. \$\endgroup\$ Nov 18 '14 at 21:37
  • \$\begingroup\$ @SpehroPefhany, do you mean source impedance of 5K? \$\endgroup\$ Nov 18 '14 at 22:45
  • \$\begingroup\$ 10K resistors will look like a 5K ohm source impedance to the ADC input, well within the acceptable range. They will draw more current from the battery, of course, about 150uA. – Spehro Pefhany 4 hours ago \$\endgroup\$ Nov 18 '14 at 23:21

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