Battery not charging when using multimeter to measure

[FreshmanNewbie]

I'm using this eval board of this battery charger IC

I just started to connect a 3.7V battery, with the programmed charge current of 1A. When I connect the eval board module directly to the battery the red LED glows and the PG LED glows. All is working fine.

Now I want to view the charge current through a series multimeter on the VBAT output, so I inserted a series multimeter on the output and tried to measure the charge current.

When I did this, the charge current displayed on the multimeter is 650mA and then keeps decreasing slowly. I tried to maintain short leads to measure the current. The red LED does not glow, indicating that charging is not happening. The green LED is glowing.

What might be the reason for this behaviour? Can someone provide an idea?

 

[KlausST]

Hi,

Please show a photo of all your wiring.

Klaus

 

[FvM]

Presuming the meter works correctly, you are seeing how meter voltage drop causes reduction of charging current in charger CV mode. Just normal operation.

Alternative explanation, meter is defective, current input broken, displayed 650 mA isn't real, actually no charging takes place.

To overcome voltage drop problem, you'd use a meter with very low shunt resistance, e.g. 10 or 20A range. Or connect the meter in input rather than output branch. You'll measure Icharge + charger quiescent current, probably 10 or 20 mA higher than actual charging current.

 

[FreshmanNewbie]

Presuming the meter works correctly, you are seeing how meter voltage drop causes reduction of charging current in charger CV mode. Just normal operation.

Alternative explanation, meter is defective, current input broken, displayed 650 mA isn't real, actually no charging takes place.

To overcome voltage drop problem, you'd use a meter with very low shunt resistance, e.g. 10 or 20A range. Or connect the meter in input rather than output branch. You'll measure Icharge + charger quiescent current, probably 10 or 20 mA higher than actual charging current.

Meter's voltage drop? Current measurement is being done. So, no voltage should get dropped right? Even if it is dropped, very small amount is dropped right? Could you please explain on this point a bit more.

--- Updated Mar 7, 2023 ---

Presuming the meter works correctly, you are seeing how meter voltage drop causes reduction of charging current in charger CV mode. Just normal operation.

Alternative explanation, meter is defective, current input broken, displayed 650 mA isn't real, actually no charging takes place.

To overcome voltage drop problem, you'd use a meter with very low shunt resistance, e.g. 10 or 20A range. Or connect the meter in input rather than output branch. You'll measure Icharge + charger quiescent current, probably 10 or 20 mA higher than actual charging current.

When I connect the current meter at the input, all is fine. I measuring the right currents. But why am I not able to measure when I place the current meter at the output? Even in the 10A current range, I am not able to get the proper values.

 

[FvM]

Even if it is dropped, very small amount is dropped right?

You are just guessing, isn't it. Many meters have 200 mV or more voltage drop. Not "very small" related to battery charging characteristic.

 

[dick_freebird]

If you are measuring charger output with a shorting load (like a multimeter in current mode with its measurement shunt) then you see its sort circuit current limit and at a low voltage which will not charge s battery.

Modern battery management ICs monitor voltage and current, many of them temperature and some sort of state-of-charge as well, and current / voltage given to a load that "doesn't smell like legit battery cell" may well not resemble normal operation.

You might get more useful info with a second DMM reading voltage at the same time, so you know where you really are relative to battery-charging-normal.

 

[crutschow]

Use the multimeter 10-20A current range for minimum voltage drop.

 

[stenzer]

Hi,

as mentioned, depending on your current range setting a different burden is used, causing a different voltage drop. E.g. the Fluke 175 states a typical voltage drop of 2 mV/mA and 37 mV/A for the 400 mA and 10 A range, respectively [1]. So, depending on your used DMM and available current ranges, you have to expect a certain voltage drop across the DMM's burden.

[1] https://assets.fluke.com/manuals/175_____umeng0000.pdf

BR