Simple guide to using a DMM for measurements

[Zeva]

could you please elaborate about how to estimate run time? say i have a flashlight that is drawing 1 amp but i am using 2x RCR 123 or 1x 18650 how would you guess run time? thanks a bunch!

 

[HKJ]

could you please elaborate about how to estimate run time? say i have a flashlight that is drawing 1 amp but i am using 2x RCR 123 or 1x 18650 how would you guess run time? thanks a bunch!

You need to measure the current at the tailcap and it will probably not be the same with 2xRCR123 or 1x18650.

The you divide the mAh with the measured current, but if you have the current in amp you need to convert the mAh to Ah first. Luckily it is very easy, just move the point 3 places, i.e. 2400 mAh is 2.4 Ah and will give an estimated runtime of 2.4/1 -> 2.4h hours.
But that is only a rough estimate, one reason is that the mAh specifications on LiIon batteries are usual very optimistic (I have done some measurements on 18650 batteries). Other problems includes meters that can not measured amps correct, due to pulsing current and that current draw will vary between full and empty batteries.

 

[Zeva]

Ohhh isee! one more question..> if my flashlight uses 2x RCR-123 and one RCR-123 is rated at 880 MAH would i double that? or would it be 880 MAH? soo this light is drawing about .6 so i assume thats 880/.6 = 1466.6666? or 2933.333? and i asume thats 1.46 hrs or 2.93 hrs? thanks a bunch !

 

[HKJ]

Ohhh isee! one more question..> if my flashlight uses 2x RCR-123 and one RCR-123 is rated at 880 MAH would i double that? or would it be 880 MAH? soo this light is drawing about .6 so i assume thats 880/.6 = 1466.6666? or 2933.333? and i asume thats 1.46 hrs or 2.93 hrs? thanks a bunch !

They will be in series, that gives double the volt and the same mAh, i.e. the calculation would be 0.880/0.6 -> 1.46
But that result would be very optimistic, the 880 mAh is probably more like 600 mAh, i.e. 1 hour would be more realistic.

 

[Zeva]

Thanks for the info! my DX DM doesnt seem that bad!

 

[ampdude]

I have two less expensive older DMM's of different brands which don't have auto-ranging and they both measure approximately the same, I haven't found any difference between the two. I wouldn't mind owning a Fluke, but I haven't found the need for a more expensive meter.

 

[kosPap]

just want ot ask....

circuit & conection resistance will show up under load. is there a problem measuring it (ie wire from +batt to +board) will the flashlight/circuit is running?

 

[HKJ]

just want ot ask....

circuit & conection resistance will show up under load. is there a problem measuring it (ie wire from +batt to +board) will the flashlight/circuit is running?

Do you want to measure resistance in wires and switches?
This is a very low resistance and requires a special technique to measure and is not covered in this guide. If there is a need for it I can add it to the guide.

 

[Mr Happy]

Also never use the resistance setting of a multimeter on a live circuit with current flowing through it.

 

[kosPap]

Also never use the resistance setting of a multimeter on a live circuit with current flowing through it.

That was exactly what I thought it would be and made me ask...

Do you want to measure resistance in wires and switches?
This is a very low resistance and requires a special technique to measure and is not covered in this guide. If there is a need for it I can add it to the guide.

let me guess ...parallel metering with a know value resistor?

 

[HKJ]

let me guess ...parallel metering with a know value resistor?

No, use a known current, measure voltage drop. This way you can measure 0.01 ohm with less than 10% tolerance (in many cases a lot better than that).

 

[kosPap]

right....the skinny imlemation of Ohm law....
thanks

 

[kosPap]

Just a question, even if it turns out to be nonsense...

Can I have a DMM with all 3 leads connected while measuring (Current and Voltage alternatively)

My purpose is to monitor the Li-Ion charger with one DMM only. So if I connect the elads correctly will it matter is for example, current is flowing still through the 10A socket while the Dial is turned to voltage metering?

Thanks, kostas

 

[HKJ]

Just a question, even if it turns out to be nonsense...

Can I have a DMM with all 3 leads connected while measuring (Current and Voltage alternatively)

My purpose is to monitor the Li-Ion charger with one DMM only. So if I connect the elads correctly will it matter is for example, current is flowing still through the 10A socket while the Dial is turned to voltage metering?

Thanks, kostas

NO, some of the better meters will warn about incorrect probe connection.

With that said, it might work on some meters, but you will have to test it.

 

[Mr Happy]

I think generally it will work (the 10 A socket is just a low resistance shunt internally), but it is risky. If you forget which lead is which and accidentally connect the current probe to the wrong place you will blow the fuse in the meter (if there is one). Also when you turn the dial between voltage and current settings you might pass through other functions like resistance that do not like having a voltage across the probes.

 

[45/70]

......If you forget which lead is which and accidentally connect the current probe to the wrong place you will blow the fuse in the meter.

I just thought I'd point out that most relatively inexpensive DMM's anyway, do not incorporate a fuse in the 10A circuit, only the lessor ranges.

Dave

 

[HKJ]

My purpose is to monitor the Li-Ion charger with one DMM only. So if I connect the elads correctly will it matter is for example, current is flowing still through the 10A socket while the Dial is turned to voltage metering?

A safe way to do it is to add a 0.1 ohm resistor in series with the battery, the switch the meter between the resistor and the battery.
With some chargers you might even be able to fit the resistor inside the charger, add a switch and two wires out to the meter.

 

[kosPap]

hmmm thanks....

 

[Zero_Enigma]

Just found this guide. Thank you OP. I've not checked everything out but being a visual learner the pictures greatly allowed me to learn faster. I can see some questions I had about how to test a light bulb answered there as with how to put the battery under simulted load via resistor so you know if the battery has enough power to drive the LED.

I like the guide so far even thou I just skimmed it. Dropping a comment of thanks and also so I subscribe to this thread to find it later. Found it off a tagline of some other user here. So far it seems like it was geared with the flashaholic/batteriholic which is what I like with many scenrios of testing... I'll check out more later when I have time. :twothumbs DMM newbie here that only uses it just to test batteries. :sigh:

 

[RAGE CAGE]

excellent resource- thank you!