Expanding a panel meter around a certain range

[joblessalex]

So I have been salvaging old laptop batteries to build an electric scooter and I have found a way to measure capacity, but not series resistance. The long route is to individually calculate it all out one by one with a multimeter, or I could do something else... The method is to measure open circuit voltage, then loaded voltage (V1-V2)*R/V2 gives you the result. I have built a toggled load of a 3 ohm resistor that at a press of a button loads the cell. I want to use a standard panel meter to read the voltage across the battery terminals, but to confine the results to about 400 millivolts. Anything more than that and the battery is no good. I have started with a simple resistor divider across the power to the panel meter, but this gives a very small needle movement. I then added a transistor in line with the movement as well as the trimmer to calibrate it. This helped some, but it still won't give me full scale. I really don't know where to go from this.
TLDR
I want to twist a potentiometer to set the needle to P1, and have P2 be about 500 millivolts less than P1, no matter what voltage I feed it. (within the range of 3-4.2 volts) This 500 millivolt range needs to stay constant no matter what the main voltage is.
Sorry if you can't understand much of what I put here, been working on this all night.

[rstofer]

You can offset and scale with an op amp and 4 resistors.  I refer you to Chapter 4 of "Op Amps For Everyone"  Specifically Section 4.3
http://www.cypress.com/file/65366/download

Yup!  There's a bit of math but it's pretty much 'plug and crunch' as my grandson would say.  FWIW, 'b' may be zero for your application, you probably don't need to offset.  In my case, I was interested in battery voltage between about 11V and 15V and I wanted to spread the 4 volt difference across the entire range of an A/D converter.

The examples use a single supply op amp but you would still need to have a battery or something to power the circuit.

I have to wonder why you're not using a digital multimeter.

[Ian.M]

Its very difficult to offset an ordinary analog meter movement electronically without either an auxiliary supply to power the offset circuit, or severe compromises to the linearity and accuracy of the meter.  Sometimes its possible to derive the auxiliary supply from the voltage being measured without loading it excessively, but this is difficult for very low or high voltages.  A single LiPO cell doesn't have a high enough terminal voltage under load to  power most offset circuits.

[Alex Nikitin]

Assuming that the meter used can do 500mV full scale with a suitable series resistor, you may try to connect an adjustable micro-power reference (i.e. LM4041) in series with your meter. If the meter coil is sensitive, add a resistor across the meter so the full scale current would be about 1-5mA (the LM4041 needs about 50uA to operate, so for 5mA full scale the error would be only 1%).

Cheers

Alex

P.S. - The circuit is attached. R1 and R2 depend on the meter used, to provide 500mV and 5mA full scale, R3 is a variable resistor to adjust the offset "zero" voltage in 1.225V to 4.9V range. In your case you may want to set the full scale voltage with the battery connected and measure down from that with the load.