Simple guide to using a DMM for measurements

[HKJ]

I want to check the open voltage of my battery charger, and I think this is what you call it when its plugged in and no battery installed. [Sorry, not much electrical knowledge]
My charger is labled at 4.2V output. Do I simply plug it in and attach each DMM probe [auto range set to AC V?] to the pos. and neg. sides of one channel?
When I did the above I get a reading of .027V?? on the auto range setting.

Does this make sence?

Evert thing except the AC V part makes sense, try using DC V.

But the open voltage does not say much about anything, the charger is electronically controlled and the open voltage might be above or below 4.2 volt, depending on how the electronic does the controlling.

 

[loquutis79]

That makes more sence now. Told you I know little about electrical stuff :], but willing to learn.

OK, now I get 4.2 from both channels.

Just trying to learn as much as I can, and like to re produce some of the tests I see here on the site. While waiting for a Pila charger for my 18650's I am using an Ultrafire WF139 [don't throw anything at me!], and as there are so many negitive posts I want to check things out where I can.


Thanks for the supper fast replies from you both. Now I can tell the wife that its not just me who spends weekend mornings reading about flashlight stuff.

 

[loquutis79]

So what I did next was to check voltage during testing. I did the tin foil at both ends of the 18650 while in the charger and attached the DMM leads to each side.
As I knew the batteries were fully charged just last night, minus a bit of playing on turbo after that, I knew that it should not take long to light up the green lights on the WF139.
So I watched for the few minutes until the light changed and what I saw was the following:
1st -red light = charging. 4.21V-4.24V during this stage.
2nd-green light = full charge. DMM jumped right to 4.59V
I left it on only for a couple of minutes and then pulled the plug.
Results = hurry up with the Pila charger order??
Or did I miss something here also. It that an viable test?

 

[HKJ]

2nd-green light = full charge. DMM jumped right to 4.59V
I left it on only for a couple of minutes and then pulled the plug.
Results = hurry up with the Pila charger order??
Or did I miss something here also. It that an viable test?

Do not worry to much about the voltage, that was because the over charge protection in the battery tripped. The charger would have stopped soon enough if it had not tripped. The "problem" with the WF-139 is that it measures the voltage with the current turned off and that means it goes above 4.2 volt with the current on (The current on/off is to fast to reliable measure with a DMM).

I had the same happen when I tested the WF-139.

 

[rjking]

Hi HKJ, can you recommend a Fluke DMM mainly to test and check batterries voltages and Mah?

 

[uk_caver]

A DMM isn't any use for measuring mAh (battery capacity), unless you use it to make repeated measurements of cell voltage as a cell or battery is discharged at a known current.

 

[HKJ]

Hi HKJ, can you recommend a Fluke DMM mainly to test and check batterries voltages and Mah?

Measuring mAh is not something you can just do with a DMM, a hobby charger is much better for it.

I often use a Fluke 179 DMM for various tasks, but for just checking batteries it is way to expensive, a Fluke 115 would do for that.
The 115 has a 6000 scale, i.e. you will get battery voltage with 3 decimal digits, but the DMM is missing the low current ranges and cannot be used to measure low modes current draw and standby current with.

 

[rjking]

A DMM isn't any use for measuring mAh (battery capacity), unless you use it to make repeated measurements of cell voltage as a cell or battery is discharged at a known current.

Measuring mAh is not something you can just do with a DMM, a hobby charger is much better for it.

I often use a Fluke 179 DMM for various tasks, but for just checking batteries it is way to expensive, a Fluke 115 would do for that.
The 115 has a 6000 scale, i.e. you will get battery voltage with 3 decimal digits, but the DMM is missing the low current ranges and cannot be used to measure low modes current draw and standby current with.

Oh okay. So the only way I can make sure that i'm putting 2 x CR 123 or 2 x 18650 with similar capacity level is by checking on their voltage then, is that correct? Man, that Fluke 115 is way too expensive. Thought, there's something half the cost.

 

[HKJ]

Oh okay. So the only way I can make sure that i'm putting 2 x CR 123 or 2 x 18650 with similar capacity level is by checking on their voltage then, is that correct?

Yes.

Man, that Fluke 115 is way too expensive. Thought, there's something half the cost.

Fluke is rather expensive.
For checking batteries just about any cheap DMM can be used, but many cheap DMM's shows a wrong value when the batteries are running low.
If you want something more than a cheap DMM, then a good DMM for the price is UNI-T UT61E.

You can also get a "cheap" Fluke (17B), but it is only sold in China (You can find it on Ebay). It looks like a decent DMM.

 

[staffyman]

So, I've read your guide, but I'm none the wiser.:duh2:

Nothing at all to do with the guide, it's me - I need to read things over and over again before it sinks in.

I'm not very savvy with electronics either but if I read it a few times and keep referring back to it when needed then I'm sure I'll be ok.

My multimeter came with a little booklet but only gives specs and isn't a guide to how to use it.

I've only ever used it to measure that voltage is present but now I can refer to your guide, I'll be able to use it correctly.

Many thanks.:thumbsup:

 

[Planz]

"To get a better idea about how it will work in a flashlight, I need a load on the battery, here I am using a 1ohm resistor. The battery still maintains voltage enough to drive a flashlight. The resistor must be at least 3 watt, but a resistor rated for a higher wattage (i.e. 10 watt) is much better, because it will stay cool.

Next battery is a LiIon 18650 battery, this requires the 20V range, the battery has 4.17 volt. For a LiIon this means that it just about fully charged. The voltages for a LiIon must be measured without load and are: Fully charged 4.2V, empty 3.6V
When getting a new LiIon charger it can be a good idea to check the voltage, on the cells, when they are removed from the charger, they must not be above 4.3V, it is much better if they are at 4.2V."

Hi HKJ,
Could I ask why a LiIon must be measured without load whereas an Alkaline or NiMH needs to be measured with load in order to gauge the energy level of the battery? What happens if you measure a LiIon with load?
Thanks.

 

[HKJ]

Hi HKJ,
Could I ask why a LiIon must be measured without load whereas an Alkaline or NiMH needs to be measured with load in order to gauge the energy level of the battery? What happens if you measure a LiIon with load?

You can measure LiIon with load, but the tables listing how much energy is remaining will not be correct.
Measuring Alkaline or NiMH without load will show them as nearly full, even if they are close to empty.

 

[Planz]

Got it. Thanks!

 

[OneBigDay]

Wanted to say thanks for this guide and let HKJ know some of us are using it.

A few years ago this was the first guide I saw to show me how to check voltage on my cells. Little did I know how frequently I would be doing this the more I got into things. I have also diagnosed two bad McClicky switches with the tailcap part of the guide; one of them just today. Good stuff.

 

[RI Chevy]

I agree wholeheartedly! It is a great resource, and Henrik is a great member! Thank you. :thumbsup:

 

[kosPap]

A few years ago this was the first guide I saw to show me how to check voltage on my cells. Little did I know how frequently I would be doing this the more I got into things. I have also diagnosed two bad McClicky switches with the tailcap part of the guide; one of them just today. Good stuff.

I am in the process of monitoring battery aging by measuring their Internal Resistance.
Another thumb up for HKJ

 

[HKJ]

I am in the process of monitoring battery aging by measuring their Internal Resistance.

Did you see my writing about that: http://lygte-info.dk/info/Internal impedance UK.html

 

[kosPap]

Yes i did...And if i hadn't , i would never sit down and do it.....
It has opened my eyes on how my 18650s age and helped me avoid a 4x battery combiantion that one 18650 had increased resistance (some batteries, rarely used, but with 1 year difference in production lot)

BTW I am using the "first" method, with a resistor that matches real life current draw at max output

 

[applevalleyjoe]

Just the info that I was looking for...thanks a lot.

 

[IonicBond]

I often use a Fluke 179 DMM for various tasks, but for just checking batteries it is way to expensive, a Fluke 115 would do for that. The 115 has a 6000 scale, i.e. you will get battery voltage with 3 decimal digits, but the DMM is missing the low current ranges and cannot be used to measure low modes current draw and standby current with.

I'll vouch for the Fluke 115 as being the least expensive multimeter you can trust out of the box. Or perhaps the 114 if you don't do any sort of current measuring with it. While the 115 doesn't have a milliamp range, it will measure below 1A, but the cutoff seems to be about 100ma. So it is not ideal for testing very low currents like you mentioned, and if you test led's, may not have enough voltage to light them brightly. It is after all an an electrician's meter, not an electronics oriented one. For that kind of use my 87V is brought out.

Most importantly, the min-max function, which also beeps audibly at changes of 1mv, has a very important function when working with li-ion. When a cell that is under discharge or charge, when you hit the steep charge/discharge knees, the meter starts to beep very fast - important for safety in case something goes wrong. You can also use it to know if you have suddenly dropped a cell, or if a cell that seems normal just goes berserk. With the audible beeps, you can catch this activity before it goes to the extremes. The beeps on the 11x series are a bit lower than say an 87V, as the piezo beeper is much smaller.

Also not mentioned about using cheap meters is their measuring speed, and some tendency for over and undershoot. The Flukes are plenty fast and are bang-on without nary an under or overshoot.

For me, working on li-ion without a quality meter is just asking for trouble. I also vote for HKJ's wonderful work to be a sticky!