Measuring internal resistance of a battery...

Newone

New Member
Hi folks,

I am needing quite a bit of help as usual! (I am still new to electronics and picaxe)

Basically, I am needing to measure the internal resistance of rechargable batteries (sealed lead acid). I am needing to then display this reading on a LCD module. This will mean that I am able to monitor the status of the guts of the battery.

Now for the crunch!!

I need this project to have the ability to monitor batteries that are either 12 or 24 volts. This is not all!!! These batteries will also be pumping out up to 100 Ampres!

Is this a possible picaxe project. Obviously I will be needing to step down the power very drastically before anything else. How is the best way of doing this?

I am still figuring out Ohmes Law and other technical calculations, so would be very glad of some patient and informative support.

I guess you are asking yourself, 'why doesn't he start with something easy???'. Well, I am needing this tester in rather a hurry. Also, I have been experimenting with the basics of Picaxe and want something big to chew.

Many thanks and look forward to hearing from those who know where I need to be heading.
 

Buzby

Senior Member
Internal resistance can be determined my measuring the battery open circuit voltage and then the voltage on load. ( See : http://en.wikipedia.org/wiki/Internal_resistance and http://data.energizer.com/PDFs/BatteryIR.pdf )

Not difficult calculations, but if you are just starting to master Ohms Law I don't think this is a good project to get your sums wrong on.

100's of Amps implies very low internal resistance, so you will need to push a lot of Amps through a small resistance. A good recipe for smoke and fire.

Do some theory on paper first, using the Wikipedia info above, then you will have some idea as to what the voltage measurements will be.

Then we can see about interfacing to a PICAXE.

Cheers,

Buzby
 

geoff07

Senior Member
You will need to measure simultaneously the voltage and the current at several levels of current, and then solve the equations. The internal resistance is a function of the voltage drop when on load.

At the resistance and current levels you will be dealing with, your biggest problem will be precision mesurement. If the batteries are on load at the time as you imply, then intercepting the current to measure it will be tricky. No doubt a Picaxe could do the measurements and the calculations for display. But I suspect that sensing will be more of a challenge.

It does seem a bit of a stretch for a novice. Commercial units generally measure the capacity rather than internal resistance, and they do it off load. Good luck!
 

eclectic

Moderator
"Well, I am needing this tester in rather a hurry"


What is it?

And should you really be doing it,
if you don't know what you're doing?

(See posts 2 and 3).
 

Roman505

Member
Newone, you say "up to 100 Amperes". Is the load specified or variable? If it were fixed then you can skip all of the current measurement risks and simply monitor battery voltage on and off load.

Why do you want to do this, by the way? What is your purpose to be served by knowing the internal resistance?
 

Dippy

Moderator
"Why do you want to do this, by the way? ..."

Apparently, from post 1
"This will mean that I am able to monitor the status of the guts of the battery."

So, does this mean Newone is trying determine 'state of charge' or 'condition of battery'?


"rather a hurry" sounds like a school/college project which has to be in by next Friday :)


If Newone can clarify this then people can zoom in with some ideas. I would suggest, while waiting for the answer to be served, that a little research is done. There are loads of pages to give you ideas. Buzby has given you a starter.

Once you have distilled your idea and say precisely what you are trying to do then people can ensure their suggestions are relevant.

In theory it is quite easy but there may be some 'extras' in your requirements that complicate the answer. Best give the whole picture and a picture may help here.
 
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