Measuring CURRENT on Li Ion rechargeables

[gilly]

Recently got myself an older model Fluke multimeter. I've been having a good time getting acquainted with measuring all of my different cells. I am perplexed by my inability to measure current with my 18650 and 14500 AW rechargeable Li Ion cells. Everything else gives me a voltage and current except them - I can only get a voltage from them... Could anyone give me some clues??
:shrug:

 

[SemiMan]

Recently got myself an older model Fluke multimeter. I've been having a good time getting acquainted with measuring all of my different cells. I am perplexed by my inability to measure current with my 18650 and 14500 AW rechargeable Li Ion cells. Everything else gives me a voltage and current except them - I can only get a voltage from them... Could anyone give me some clues??
:shrug:

Please tell me you are not taking the two probe leads, with the meter sets to amps, and placing it across the terminals. If you are, you are likely tripping the self resetting fuse in the battery.

 

[gilly]

Yes, I did as you feared! Could you please explain what is happening? Have I damaged the cells in some way? I am a complete novice in this area and would very much appreciate any advice or information you could provide. Is there a way to measure current in the li-ion cells?

 

[FredM]

I don't think you fully understand what current is and why you would want to measure it.

 

[gilly]

I don't think you fully understand what current is and why you would want to measure it.

I have admitted to you I am a novice - could you provide some information other than your comment?

I am measuring current to compare my cells to see which are at a higher energy state. This works well for me using the AA Nimh and the 123 lithium cells. Obviously, from earlier comments the self-protection circuit in the AW li ion rechargeables prohibits me from doing what I can with the other cells.

I enjoy this forum because I learn a great deal on it. I am hoping to learn more about this topic now - if you care to contribute I thank you very much...

 

[FredM]

I was not trying to be rude. I was telling you the core of the problem is your understanding of current.

Just read what is current and you can understand better what you are reading with yout multi-meter.

http://en.wikipedia.org/wiki/Electric_current

 

[LuxLuthor]

It is perfectly fine to start with where you are. We all did...so consider it a good question to ask. I did exactly the same thing when I first got a DMM. I also made this post to try and explain some of these concepts in a more simple manner.

Basically the best analogy to understanding voltage and current is to compare it to a garden hose. The voltage is like the driving water pressure which can be measured in psi. The current is like the water flowing. If you close the water valve, there is no (current) flow, but the driving potential water pressure (voltage) is still there.

With a battery there is no (practical) current flowing unless something like a light bulb is put between positive and negative ends, and wire connection is made....known as the electrical circuit being "closed." So you doing a measurement of current on both battery terminals is not the way to do it.

 

[gilly]

Thanks for the explanation and the reference to the thread - I have bookmarked it! One question though:

Three cells, all Nimh:

#1: old, worn-out 2100 mAh Kodak - last charged a month ago

#2 newer, 2600 mAh Tenergy - last charged two weeks ago

#3 newest, 2000 mAh Eneloop - last charged four days ago

Measuring their current with the multimeter, I get:

3.62 for #1

5.69 for #2

9.12 for #3

For a pure comparative reference, this seems to work - I know that cell #1 is very weak. I know cell #2 is better but not great, and I know that #3 is very strong.

For a quick reference as to what cells are weak, strong, etc. is it not feasible to do what I am doing? I have recorded values for all cells just off the charger as a baseline and compare them as they self-discharge, etc.

Thanks for your help!!

 

[FredM]

Are those in amps? If so thats not good as it is really high and not good for the battery.

No its not really a good way to measure how much they have left, a better way is to measure voltage, but actually that is not always perfect either. No multi-meter can really tell you how much energy is left in the battery.

 

[LuxLuthor]

Gilly, when you measure current with a DMM, it is making a closed circuit during the measurement....so in effect, what you are doing is akin to touching ends of a piece of wire to both battery terminals. You are subjecting the battery to a "dead short."

When you change the leads to measure current like I showed in my linked thread, in effect, it is turning your DMM into a piece of wire with a 10Amp fuse inline. The fuse will break the connection (aka: "open circuit") if more than 10 amps flow through it.

Bottom line, however logical it may seem to do what you are doing...it does not represent anything remotely useful about the battery state of charge, battery capacity, or anything else....and it is damaging the battery.

 

[gilly]

Fred - yes, those readings are in amps. Gentlemen - thanks for your information! I will cease doing this to my poor cells and follow your advice. My rationale for doing this was my little $10 Maha battery tester. One side of the tester goes on the + terminal and the other on the - terminal and you are given a 'reading' on the capacity. I figured that the DMM would work the same way except would actually give me exact numbers! I will stick with reading voltage......

Much appreciated.:thumbsup: