# Circuit to measure internal resistance of a battery?

Dear All

I know very little about electronics! So please pardon potentially dumb questions!

My objective is to know when to (properly) discard a rechargeable battery and buy a new one. I want to be able to read the voltage of a rechargeable battery when freshly charged. Then I want to be able place the battery under load and measure the voltage as well as amperage again in order to compute the internal resistance. I intend keeping track of these results for this (and other batteries) over time (and also comparing the data to other similar batteries I have).

I have created a simple schematic of what I think I aught to do (attached to this post).

Known parameters are:
- Battery voltages vary from 3.2 to 4.2 V depending on model and age
- Battery capacities vary from 700 mAh to 1,200 mAh depending on model and age

What I would like to do is:
- Have one circuit where I can measure the unloaded voltage (and amperage)
- Change a switch and then place the battery under load - with an LED indicating when this circuit is on permitting me to measure the loaded voltage and amperage.

What I don't know is:
- What size fuse do I use (do I need one?)
- What size / voltage LED should I use considering the different voltages of the batteries?
- What resistor should I use for this LED?
- What resistor should I use to place the batteries under load?  How large should this load be to sufficiently stress the battery but not damaging it? (The 3 Ohm in the diagram is a total guess!)

I know I can go and read up on this on various websites - but, being honest - its so much easier to ask!

Your advice would be most appreciated.

iceng2 years ago

Straight forward for me.;

Just accurately measure the open voltage Vo

Then select an 1C battery load and quickly measure the voltage and current Vload and I current turn it off after.

The internal battery resistance R = ( Vo - Vload ) / I

Then you could read the published battery resistance supplied by the manufacturer...

mpilchfamily2 years ago

So ultimately you want to determine internal resistance. In which case it doesn't matter what the amperage reading of the battery is before and after a load. Try some research here.

http://batteryuniversity.com/learn/article/how_to_...

As for the LED. Pick your LED. Calculate your resistor based on the highest voltage you'll be using on the system and your good to go. If you don't know how to calculate that then google for an LED calc.