# change the range of a battery using a varistor

Discussion in 'Power Electronics' started by vinnichenzo, Feb 17, 2012.

1. ### vinnichenzo

1
0
Feb 17, 2012
Hello,
I am a bit of an electronics novice but have a neat idea I want to try out:

I am using Lithium ion batteries for a portable stereo project i am working on and want to include nice old vintage car fuel gauges to indicate the remaining life of the battery (Full to empty etc.)

So i know that the fuel gauges work with car batteries and indicate the remaining fuel by the 12v input draining to zero as the float increases the resistance when the fuel tank empties.

So i need to try and replicate that drain, but on a span of 12.6v to 10v... (the lithium batteries life-span)

So the question is: How do i chance the range of 12.6v to 10v over to the desired 12v to 0v range?

I started research non-linear resistors and came across VDR (varistors), but couldnt find much information on the calculations and which type i would need to acheive my goal.

Does anyone here have an idea on the calculations needed or a good resource? or a better idea of how to change the range?

Any info would be muich appreciated. Thanks

2. ### Harald KappModeratorModerator

9,413
1,924
Nov 17, 2011
A VDR is not a good solution. It is rather imprecixe and is (as far as i know) mainly used to protect from overvoltage (high voltage = low resistance).
You need to do this calculation:
Vout = (Vin-10.6V)*6
where Vin = battery voltage
Vout = voltage to meter
This can be accomplished by
1) adding a zener diode in series with the battery voltage. I can't find a 10.6 V zener diode, so either use an 11 V zener or a 10 V zener in series with a normal diode. This will do the "-10.6 V" part of the calculation.
2) add an amplifier (OpAmp) to achieve *6 amplification.

Unfortunately this is not straight forward for a novice project.

Harald

3. ### BobK

7,645
1,663
Jan 5, 2010
Also, the voltage you read will depend on load, and is not a good indicator of battery life left since it does not go down linearly. The correct way to do a battery fuel gauge is to integrate the power drawn from the battery, and the charge put into to. There are ICs that will do this.

Bob