Why did my wire melt ?

Thread Starter

M3D0

Joined Oct 8, 2020
49
I know I did something stupid, but I still can't figure it out, and that led to electrocuting myself and melting down my jumpers (wires). So I was using my multimeter to measure the voltage of a battery (lithium-ion 12V,3A), and everything went smoothly until I tried to measure the current...so I did change the position of the multimeter lead (the red one) so I can measure the current and I rotated this wheel to the 200mA option, I didn't see a reading (but I smelled something burning), so I changed to the 20mA option and then there was a spark and I got electrocuted and the jumper wire which is connected to the left terminal got melted. I want to know why did this happen, and how I can avoid it. Thanks in advance.

Note:
A picture of the setup is attached to this post.
melted ground wire.jpeg
 

crutschow

Joined Mar 14, 2008
25,670
Sounds like you were measuring the short circuit current of the battery by connecting the ammeter directly across the battery, which can generate a very high current (and melt some wires).

So what current were you trying to measure?
 

Thread Starter

M3D0

Joined Oct 8, 2020
49
Sounds like you were measuring the short circuit current of the battery by connecting the ammeter directly across the battery, which can generate a very high current (and melt some wires).

So what current were you trying to measure?
I was measuring the current across the battery, is that the right way to measure the battery current ?
 

crutschow

Joined Mar 14, 2008
25,670
I was measuring the current across the battery, is that the right way to measure the battery current ?
Only if you want to measure its short-circuit current of the battery.

It you want to measure the battery load current, then you put the meter is series with the load.
 

Audioguru again

Joined Oct 21, 2019
2,078
You shorted the battery with your amp-meter. The battery in your other thread is rated for a capacity (current vs time) of 3Ah, not 3A. 3Ah is 3 amps for 1 hour. Its maximum allowed output current should be listed on its datasheet and it might be 30A or more for a short time. The 0.28Ah Lithium batteries for my radio controlled airplanes have a maximum allowed output current that is 30 x 0.28A= 8.4A.
 

RIKRIK

Joined Oct 11, 2019
107
This made me chuckle, Dont worry i blow things up from time to time. I use 18650s for my eciggerete, they can supply up to 25+ amps short circuit.

You need a resistor to measure current, a quick google should show you how. Volt meter wise. Unscrew it and you can change the internal fuses.
 

BobTPH

Joined Jun 5, 2013
2,593
Basically, there is no safe way to measure the current a battery can provide. Doing so requires a short circuit, which will (not may) damage the battery. And, even if you do so, you have no idea how long it can supply that current.

Batteries are rated by capacity, and by maximum current .

The capacity is the current that will exhaust the battery in 1 hour. Actually, it is more like 1/20 times the current it can provide for 20 hours. This number is called C. Typically, a battery cannot supply 1C for 1 hour, because it was measured tv over a much longer Interval.

Some batteries are rated for the maximum current they can provide for a short time. This is usually expressed as a multiple of C. For example, a typical battery for drones is often rated at 30C, which means it can provide that current for 2 minutes at most.

Bob
 
Usually what your interested in is ESR or Effective Series Resistance. You can impress a small AC voltage across the battery and measure the AC current and AC voltage. Dividing will give you ESR. Now you can get the voltage of the battery and divide by the ESR to get the short circuit current, indirectly.

This is a specialized measurement.
 

Tonyr1084

Joined Sep 24, 2015
5,249
My neighbor often has problems with his car batteries. They're always being drained somehow. Usually because he leaves a door open or an interior light on. Always calling me to come check his battery - I decided for Christmas a good gift would be a multi-meter. Obviously I got him a cheap one from Radio Shack. Was an analog meter. So when he tried to measure the battery he set it to measure current and promptly blew out the internal fuse. Complained that the new meter I gave him was faulty I checked it. Found the blown fuse and kindly replaced it for him. He then demonstrated how to check voltage with the same settings as before, and blew the fuse again.

Back in our youth our fathers often referred to the house voltage as "Current". I don't know if anyone else here has experienced this old way of talking about the house being 120 current, but apparently his father and mine had the same mind set.

After teaching him the difference between voltage and current, and fixing his meter one last time I've never seen him use the meter again. Last year (before CV19) he had a dead battery again. Asked for help. So took my meter over and tested his battery VOLTAGE - I stressed to him. Asked him where his meter is "I dunno." Some people never learn.

Not saying you can't learn, just bringing up a point that may be stuck in your head. Here's the point we're making: With the meter set to voltage you touch both leads of the battery and get a voltage reading. Voltage is akin to "Electric Pressure". With the meter set properly to current you interrupt the flow of electrons (let's not get lost on that term) from the battery (most often) positive lead and place the meter in between the positive battery terminal and the rest of the circuit. Current (akin to how many electrons flow through the wire) is measured by passing the battery voltage through the meter to the load. If you try to measure current from battery positive to negative you'll blow the fuse.

You say the fuse is good because every time you tap the leads you get a spark. So I'm wondering if your meter has a fuse in it or if it has something else - or if it even has a fuse at all. Can you give us a diagram of how you're testing? And have you physically checked the fuse either visually or electrically?
 

Thread Starter

M3D0

Joined Oct 8, 2020
49
My neighbor often has problems with his car batteries. They're always being drained somehow. Usually because he leaves a door open or an interior light on. Always calling me to come check his battery - I decided for Christmas a good gift would be a multi-meter. Obviously I got him a cheap one from Radio Shack. Was an analog meter. So when he tried to measure the battery he set it to measure current and promptly blew out the internal fuse. Complained that the new meter I gave him was faulty I checked it. Found the blown fuse and kindly replaced it for him. He then demonstrated how to check voltage with the same settings as before, and blew the fuse again.

Back in our youth our fathers often referred to the house voltage as "Current". I don't know if anyone else here has experienced this old way of talking about the house being 120 current, but apparently his father and mine had the same mind set.

After teaching him the difference between voltage and current, and fixing his meter one last time I've never seen him use the meter again. Last year (before CV19) he had a dead battery again. Asked for help. So took my meter over and tested his battery VOLTAGE - I stressed to him. Asked him where his meter is "I dunno." Some people never learn.

Not saying you can't learn, just bringing up a point that may be stuck in your head. Here's the point we're making: With the meter set to voltage you touch both leads of the battery and get a voltage reading. Voltage is akin to "Electric Pressure". With the meter set properly to current you interrupt the flow of electrons (let's not get lost on that term) from the battery (most often) positive lead and place the meter in between the positive battery terminal and the rest of the circuit. Current (akin to how many electrons flow through the wire) is measured by passing the battery voltage through the meter to the load. If you try to measure current from battery positive to negative you'll blow the fuse.

You say the fuse is good because every time you tap the leads you get a spark. So I'm wondering if your meter has a fuse in it or if it has something else - or if it even has a fuse at all. Can you give us a diagram of how you're testing? And have you physically checked the fuse either visually or electrically?
I really enjoyed the story you said as it shows that not every one keen to learn from their mistakes or simply they just run away from the problem as they think that this is the solution somehow. I made an exact diagram of my setup that's attached to this reply. I still didn't check the fuse, but I'll do now. Thank you!
A disaster_bb.png
 

Audioguru again

Joined Oct 21, 2019
2,078
Yes, you shorted the battery with your meter set to measure current. You are lucky that the battery, the breadboard and the meter did not explode and hurt you. You show how to measure the battery voltage, not its current.

Do you understand that 3Ah is the length of time that the battery can produce a low current and is nowhere near the maximum current the battery can produce?
 

Papabravo

Joined Feb 24, 2006
14,690
My favorite piece of engineering doggerel

Twinkle, twinkle
Little star,
Power's equal
I squared R.


The wire will always burn whenever there is too much I and too little R.
 

SgtWookie

Joined Jul 17, 2007
22,220
Attempting to measure current across a voltage source is a very common mistake that people make; even old hands might occasionally make that mistake. Welcome to the charred component club; I'll bet you won't forget that lesson.

Voltage is measured in parallel with the source, or on a component.

Current is measured in series with the load.

A more safe way of measuring current is indirectly, by using a precision resistor in series with the load, and measuring the voltage drop across the resistor.

If the resistor measures 1 Ohm, then a voltage drop of 1v across the resistor is 1A current through the resistor. I=E/R
 
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