18650 voltage but no continuity

[mickyd]

Troubleshooting a 2P5S Default battery pack. One of the 5 parallel groups has voltage at 4.4V and I wanted to discharge it down to 4.2. I don't know how the voltage got up so high on that single pair but it is what it is. The odd thing is even though it measures voltage, there is no continuity. All the other pairs have continuity. (That's naturally when reversing the leads on the multimeter negative to positive and positive to negative) When hooked up to a lipo balance charger in discharge mode, the display shows connection break as soon as the test starts. Any clues for the issue may be?

 

[amberwolf]

Not sure what you mean by "continuity". Do you mean that when you attempt to measure the connection resistance between cell groups?

If so, you should be careful, as if you try to measure resistance in a "live" circuit (like a battery), if there is a broken connection and you measure across it, the multimeter is now the current path, and voltage not designed for can end up across the meter inputs (which can damage the meter).

If the problem is a broken connection between groups, then the pack won't operate--no current will flow.

If the problem is a broken balance connection *to* a group, then the pack will operate, and current will flow, but no monitoring of that group is possible, and no balancing of it.

 

[PaulD]

I would guess that group of cells got overcharged, and the CID (current interrupt device) got triggered in both cells. A voltage may still be present at the terminals due to a high resistance connection inside the cell(s) if the CID doesn't completely disconnect.

 

[mickyd]

Troubleshooting a 2P5S Default battery pack. One of the 5 parallel groups has voltage at 4.4V and I wanted to discharge it down to 4.2. I don't know how the voltage got up so high on that single pair but it is what it is. The odd thing is even though it measures voltage, there is no continuity. All the other pairs have continuity. (That's naturally when reversing the leads on the multimeter negative to positive and positive to negative) When hooked up to

Troubleshooting a 2P5S Default battery pack. One of the 5 parallel groups has voltage at 4.4V and I wanted to discharge it down to 4.2. I don't know how the voltage got up so high on that single pair but it is what it is. The odd thing is even though it measures voltage, there is no continuity. All the other pairs have continuity. (That's naturally when reversing the leads on the multimeter negative to positive and positive to negative) When hooked up to a lipo balance charger in discharge mode, the display shows connection break as soon as the test starts. Any clues for the issue may be?

a lipo balance charger in discharge mode, the display shows connection break as soon as the test starts. Any clues for the issue may be?

Not sure what you mean by "continuity". Do you mean that when you attempt to measure the connection resistance between cell groups?

If so, you should be careful, as if you try to measure resistance in a "live" circuit (like a battery), if there is a broken connection and you measure across it, the multimeter is now the current path, and voltage not designed for can end up across the meter inputs (which can damage the meter).

If the problem is a broken connection between groups, then the pack won't operate--no current will flow.

If the problem is a broken balance connection *to* a group, then the pack will operate, and current will flow, but no monitoring of that group is possible, and no balancing of it.

Deeper look found the the voltage was -4.4V with no continuity. Negative voltage was weird.

The continuity test was to see if there was electrical connection between the pos neg battery ends since trying to charge the pair directly indicated 'connection break' as of the nickel strip wasn't welded. Continuity checks for a completed (closed) circuit and is a common electrical test you can read up on. On an 18650 cell, if it's not shorted, touching neg multimeter lead to positive battery end and positive lead to negative end should cause the meter to beep.

I tore off the nickel strip from the series pair and measured the cell voltage directly. 0 volts so the cells must have shorted somehow. No clue why they measured -4.4V when connected but that mute now. Soooo, replacing the two bad cells and should be good to go.

 

[amberwolf]

The continuity test was to see if there was electrical connection between the pos neg battery ends since trying to charge the pair directly indicated 'connection break' as of the nickel strip wasn't welded. Continuity checks for a completed (closed) circuit and is a common electrical test you can read up on. On an 18650 cell, if it's not shorted, touching neg multimeter lead to positive battery end and positive lead to negative end should cause the meter to beep.

Oh, I well know what a continuity test itself is --what I was asking was *exactly* how you were doing the test.

You can't do a continuity test on a voltage source. You can only do it on a purely resistive connection, because the test itself puts a voltage across the resistance, and measures the current flow. (or in certain meters, it places a current source in the resistive circuit, and measures the voltage produced).

If you try to do a continuity test on a voltage (or current) source, the meter will not show a valid result, and it can also damage the meter if the voltage or current generated exceeds the ability of the parts inside to handle it (fusing should protect a good meter, but many cheaper ones don't have fuses or don't have the correct ones to properly protect them against this specific incorrect usage).

So you can do a continuity test on connections *between* cells, but you cannot validly do one *on* a cell with any voltage at all.

I tore off the nickel strip from the series pair and measured the cell voltage directly. 0 volts so the cells must have shorted somehow.

More likely they weren't shorted, but instead internally open, as PaulD described. If they were shorted, a continuity test on them would have shown no resistance, and would have read a zero (perfect continuity), not open (no continuity).

No clue why they measured -4.4V when connected but that mute now.

That would depend on how the meter was connected, what it was set to, it's internal resistance, and what the voltages on the other cells were on either side, and exactly how they failed internally and why they failed. A high resistance connection inside them from an open CID, with negative voltage caused by overdischarging those cells*** could show a negative voltage, if hte meter's internal resistance was very high (it should be) and it's set to DCV.

*** if the cells had significantly less capacity than the rest of the cells, and any "BMS" or LVC in the pack or device using them didn't shut off discharge current, they'd keep discharging even after being totally empty because current is still flowing thru them from the not-yet-empty cells, and the votlage on these empty ones will be driven negative.

The same thing can happen anytime there is no individual cell level sensing in a device, and the cells are not matched in characteristics (capacity, internal resistance, etc), so that they discharge to empty at different times.

 

[Benjamin Yang]

It sounds like you're dealing with a potential issue in one of the 5 parallel groups within your 2P5S (2 parallel, 5 series) lithium-ion battery pack. The voltage on one of the parallel groups is higher than it should be, and there is no continuity within that group.