Circuit to measure internal resistance of a battery?
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.