measuring max battery current with multimeter

[neazoi]

Hi my multimeter has a voltage and a mA/A plug.
I am trying to find out how much instantaneous current can a homemade battery supply.
How can i measure this with the digital multimeter? What procedure?

 

[jayanth.devarayanadurga]

Multimeter gives RMS value.

Im = Imsin(wt)

Irms = Im/sqrt(2) = 0.707 Im

Im = Irms * sqrt(2)

Irms is the value you get in multimeter.

 

[FvM]

A max current needs a voltage specification, can be short circuit current (Vbat = 0) or current at specific voltage.

Decide if the multimeter can stand the expected current range, then you can measure current in series with a load resistor. Otherwise measure voltage across the load and calculate current.

 

[BradtheRad]

The DMM's I've had contain a fuse which will blow if you try to measure more than 200 mA...
Except on the 10A range, the fuse is not in the circuit.

Therefore it's a good idea to set the meter on the 10A range to take the first current reading.

 

[vignesh ssvv]

connect your multimeter in series with a variable resistor and now keep multimeter in 10A range, when the supply to this ciruit is given, you will be able to measure the current drawn by the resistor, now by varying the resistance you can also find the maximum value of current that your battery could supply.

 

[pinout]

You state " instantaneous current", so are you referring to a peak capability in response to a pulse/transient in the load or the maximum current available?
Some meters have a 'peak hold' function if it is the former. For maximum current the previous post should work.

 

[neazoi]

No that is ok, maximum current should be enough.
At the beginning I thought that the meter could be connected in parallel to the battery terminals, when set to the current setting, but you refer to connect a resistor in series with the multimeter. That is ok.
Varying the value of this resistor, when should I find out that the battery can no longer supply current? Maybe I should also monitor the voltage of the battery terminals until I notice a significant voltage drop?

 

[jayanth.devarayanadurga]

Oops. You are measuring DC. Its value is same at all instance of time as it is constant and not varying like AC.

 

[alexan_e]

connect your multimeter in series with a variable resistor and now keep multimeter in 10A range, when the supply to this ciruit is given, you will be able to measure the current drawn by the resistor, now by varying the resistance you can also find the maximum value of current that your battery could supply.

I wouldn't suggest that unless you are actually using one of high wattage wire-wound variable resistors.
On the other hand you may want to create smoke effect so a standard pot would be fine for that

 

[neazoi]

I wouldn't suggest that unless you are actually using one of high wattage wire-wound variable resistors.
On the other hand you may want to create smoke effect so a standard pot would be fine for that

I thought so.
Maybe 1w resistors could be used instead, or these cheap 5w wirewounds to find a value close to the max current. When should I find out that the battery can no longer supply current? Maybe I should also monitor the voltage of the battery terminals until I notice a significant voltage drop?

 

[Venkadesh_M]

Hi use carbon rheostat for unknown current values first use the multimeter to get the max current and then change connections and make it voltmeter to measure the voltage which you allow to be the lowest voltage of given circuit and right now measure the current delivered by the battery, and I think 1W and and 5W resistors wont be enough what is your battery AH and v rating??

 

[BradtheRad]

I purchased several 1 ohm power resistors at Radio Shack.
Each rated 10W.
10% tolerance.

They have been very handy, in different combinations of series and parallel.

Also the 10 ohm value.

 

[neazoi]

Hi use carbon rheostat for unknown current values first use the multimeter to get the max current and then change connections and make it voltmeter to measure the voltage which you allow to be the lowest voltage of given circuit and right now measure the current delivered by the battery, and I think 1W and and 5W resistors wont be enough what is your battery AH and v rating??

This is just a little homemade chemical battery shown in this page qrp.gr/energy-harvesting about at the end. I would expect it to be less than 100mA

 

[pinout]

If you use the separate resistors option you can also get away with just using one meter reading the voltage and calculate the current through the known resistance combinations.
It will be interesting to see your results.

 

[Venkadesh_M]

for a small battery It will a have high internal resistance with 1 ohm and 2 ohm in series nothing before them, be ready and short it and find the current, and don forget to put the multimeter rating where it wont reach...