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I may have officially blown something up :(


Nuno
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I was switching from lead-acid to new lithium batteries, prepared everything well in advance and rehearsed all the connections I had to undo and do.

This morning, for some stupid reason, I had an epic failure for a few seconds, I pretty sure I short circuited the inverter battery IN terminals. It actually happened to me a couple of years back, at that time something blew up and smoke came out. This time, no noise, no smell.... but the inverter is pretty much dead.

When I connected the batteries and tested voltage it read 7V and went up 0.1 every 5 seconds or so... stabilizing at round 30. Turning on the solar panels made no difference.

I disconnected everything and with the multimeter I testes for continuity between the negative and positive terminals of Battery IN, and it beeped (which means there is a circuit). Same thing for - and + of AC out.

So, am I right that this is messed up and needs repair?

(Axpert Inverter)

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1 hour ago, Nuno said:

This time, no noise, no smell.... but the inverter is pretty much dead.

If you made a short circuit over the battery I would think there should of been sparks and some noise? 

 

1 hour ago, Nuno said:

When I connected the batteries and tested voltage it read 7V and went up 0.1 every 5 seconds or so... stabilizing at round 30.

Also test the voltage on the batteries with the fuses open between inverter and battery, what is battery voltage also 7V ?

That batteries might be too flat to switch the inverter on.

1 hour ago, Nuno said:

So, am I right that this is messed up and needs repair?

Hopefully not make sure the battery voltage is ok and reset the inverter, also reset the small button on the bottom of the inverter.

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Maybe a fuse blew.. Batteries (even a small 7Ah) can deliver an amazing amount of energy. It surprises me every time!

Any equipment with batteries needs to have a disconnect (typically a fuse) when the current is excessive and could start a fire etc.

Edited by Richard Mackay
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Hi guys

I may have not explained myself properly.

I think the short was without battery interference... so probably a capacitor?

the confusion was due to the terminals in the lithium batteries, I had connected the negatives, and then the positive cable got connected to ANOTHER negative terminal as well.

With the batteries disconected I get 50.1 in each. Once I get them plugged in to the inveter, it imediately drops to 6..7V like I said above.

I'm actually afraid to try it again in case I fry it even more 😛

You mean there is nothing strange in the multimeter reading continuity between the + and - terminals of battery In and also between + and - of AC out??

(also, how do I reset the inverter?the manual doesnt mention anything...)

Edited by Nuno
missed a question
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44 minutes ago, Nuno said:

I'm actually afraid to try it again in case I fry it even more

I would connect 1 pylon to the battery terminal, no other power connected and then press the red button on the pylon to make it switch its output on. If ist followed by an red alarm light on the Pylon, you can be sure something on the DC side is blown.

 

If by miracle  it does switch on with only the batteries connected, switch the AC on and check again....

A dead short between Plus and Minus is normally an indication that the DC side of the inverter is blown. 

@Coulomb , should be able to confirm my suspicion..

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51 minutes ago, Nuno said:

You mean there is nothing strange in the multimeter reading continuity between the + and - terminals of battery In and also between + and - of AC out??

Continuity does not sound right, I think this should be infinity. 

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44 minutes ago, Jaco de Jongh said:

I would connect 1 pylon to the battery terminal, no other power connected and then press the red button on the pylon to make it switch its output on. If ist followed by an red alarm light on the Pylon, you can be sure something on the DC side is blown.

 

If by miracle  it does switch on with only the batteries connected, switch the AC on and check again....

A dead short between Plus and Minus is normally an indication that the DC side of the inverter is blown. 

@Coulomb , should be able to confirm my suspicion..

Its not a Pylon, but very similar!

I tried connecting just one and turning the battery switch on, it went back to 6..7V and no sign of life in the inverter

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34 minutes ago, Gerrie said:

Continuity does not sound right, I think this should be infinity. 

sorry, not sure the right term in english... I mean there is conductivity between the two ends

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14 minutes ago, Nuno said:

Its not a Pylon, but very similar!

I tried connecting just one and turning the battery switch on, it went back to 6..7V and no sign of life in the inverter

Then I am afraid your inverter is indeed blown and in need of some repair.

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9 hours ago, Nuno said:

I mean there is conductivity between the two ends

There must be something wrong it should not have conductivity, I also have an Axpert and I tested between the battery terminals there should be no conductivity, it should be like a open circuit reading. (infinity).   

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14 hours ago, Nuno said:

I disconnected everything and with the multimeter I testes for continuity between the negative and positive terminals of Battery IN, and it beeped (which means there is a circuit). Same thing for - and + of AC out.

It's impossible to be sure from this, as multimeters vary with what they class as a "beepable" continuity. There are sizeable capacitors in the inverter across the battery terminals, and these WILL look like a short, but only briefly; the beep should go away as the capacitors charge to the ~3 V of the multimeter on ohms or diode setting. It might take of the order of 10 seconds for the beep to go away if all is well.

[ Edit: a much more definitive test is the multimeter in ohms range; usually if the inverter MOSFETs are blown and fail shorted, they will read very close to zero resistance, much less than the resistance of your multimeter leads. So if you read about 0.3 Ω, and that's what you measure with your multimeter leads shorted to each other, then that's proof that the inverter is damaged. ]

As for continuity between live and neutral (often misnamed as + and -; it's AC output), that's pretty bad. Again, there is capacitance there, but much smaller, or the order of a microfarad. So again it could register as a short initially, but very soon (less than a second I'd guess) it should read high resistance (no beep).

The ~7 V that you measure: is that at the battery terminals of the inverter?

Or of the battery?

When the battery is "switched on"?

And this same reading was over 50 VDC before the inverter was connected?

If you measure this 7 V at the inverter battery terminals, do you also measure the same thing at the battery terminals?

When you shorted the battery terminals, if the battery was off, then no great harm should be done. Even if there is a healty splat, that's just the capacitors in the inverter discharging. It's not good for them, but it's nowhere near fatal either. There is usually a ~200 A fuse inside the inverter, but it's rarely blown in my experience.

We need to distinguish between an inverter failure, blown fuse/thrown breaker, and something wrong with the battery.

Edited by Coulomb
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Thanks @Coulomb

You are right about the conductivity. I held it longer than 10seconds and after  a while the beep went off.

Measuring the ohms, i got 3.3 MΩ, at the battery in and 0.3 MΩ, at AC out (no battery connected)

About the batteries, these are lithium, so they have 2 + terminals, 2 - terminals, a small lcd and a switch to turn it on. When the battery is off, there is no voltage across the terminals (which was the case when I misconnected the cables by plugging both in the negative terminals) BUT, there is continuity between the 2 + terminals, or between the 2 - terminals. So I did close  the circuit of the battery cables, even though the battery was off. If this is not clear maybe I can take a picture of it :)

Edited by Nuno
clarity
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2 minutes ago, Nuno said:

If this is not clear maybe I can take a picture of it :)

That would be nice to understand what you mean. Its not clear. (Not to me at least...)

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34 minutes ago, Nuno said:

You are right about the conductivity. I held it longer than 10seconds and after  a while the beep went off.

Measuring the ohms, i got 3.3 MΩ, at the battery in and 0.3 MΩ, at AC out (no battery connected)

That's sounding much more hopeful for the inverter then. So now to test the fuse/breaker/switch and the battery.

Perhaps to be really sure, you could connect your old lead-acid battery modules to confirm that the inverter is working.

It's sounding like your new lithium battery isn't connected properly as yet.

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48 minutes ago, Coulomb said:

It's sounding like your new lithium battery isn't connected properly as yet.

Exactly. Or the lithium batteries are completely exhausted because when he connects them the voltage drops to 7V he said (due to charging capacitors?) and after that they slowly recover to 30V. Edit: when this is true the BMS should disconnect...

What is the battery voltage when it is not connected?

Edited by RikH
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1 hour ago, RikH said:

That would be nice to understand what you mean. Its not clear. (Not to me at least...)

here it goes. the first one is the right connection, the second one is highlighting the mistake. Again, the battery switch was OFF at the time, but the cables were connected to the inverter, thus closing the circuit.

20200501_163808.jpg

20200501_163808-2.jpg

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2 minutes ago, RikH said:

Exactly. Or the lithium batteries are completely exhausted because when he connects them the voltage drops to 7V he said (due to charging capacitors?) and after that they slowly recover to 30V. Edit: when this is true the BMS should disconnect...

What is the battery voltage when it is not connected?

the three of them read 50.4V when disconnected from the inverter

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9 minutes ago, Nuno said:

the three of them read 50.4V when disconnected from the inverter

Ok, that means your battery is not empty. Than I guess @Coulomb is right by stating your battery wiring is somewhere faulty. Now I understand how you short circuited the battery terminals on the inverter. Well there is hope for the inverter but sometimes hope is just deferred disappointment...

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All that happened is he accidentally connected the inverter terminals together and the capacitors on the DC bus gave a nice spark. This sort of thing should not (normally) blow anything up, though in rare cases I suppose it could blow a fuse (that would be rare, though it has a high discharge peak, the energy isn't really enough to melt a large fuse).

There is one thing I'm not quite sure of. Many inverters have reverse-polarity protection. There are two kinds. The one kind simply puts a large diode in reverse (or a FET) with a fuse, and if you connect it the wrong way round the diode/FET is pushed into conduction and the fuse blows. The other kind has a kind of inline switch (on smaller units this will be a diode), though I don't really know how common that is. So this mechanism might have been damaged. But, even as I write this, that sounds implausible: The multimeter test clearly shows the impedance slowly rising as the multimeter charges the internal capacitors.

So the inverter is probably fine. But I'm still confused about this going down to 7V when you connect the battery to it. I would expect everything to remain at 50V+ once you connect it all up, since a lithium battery (assuming it is at a good SOC) is rather a hard voltage source.

If you then disconnect the inverter from the battery and immediately measure the voltage across the inverter DC terminals, I would expect to see  voltage gradually dropping down from 50V and then asymptotically leveling out towards zero (so it might remain at around 10V for hours or even days...). The spark you see when you touch them together is this last bit of residual charge.

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Ok guys, mystery solved! Kind of .... :)

I was experimenting connecting just one battery at a time to check if maybe one of them was fault. Then I noticed something odd. Sometimes when I turned on the battery switch it went to 7V, other times it stayed off. I then tried the same thing with the 3 of them together and got the same result... no consistent behavior.

Then it ocurred to me that maybe the fact that I'm switching them on one by one is somehow confusing the inverter<->battery, and by safety it shuts down.

So I unplugged the inverter cables, turn them all on (getting 50.4V), and then plugged in the cables (getting the expected spark in the last connected terminal). The charge held up at 50.4. So I switched on the panels and finally the inverter came alive :D

My bad for not having a proper DC breaker, this whole thing might have been avoided!

Phew! thanks everyone

 

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29 minutes ago, Nuno said:

then plugged in the cables (getting the expected spark in the last connected terminal)

Ouch. That's shorting the battery to the internal capacitors. It would be better if you used three hands (!) and switched the three battery modules on at once. Then the internal MOSFETs could current limit, providing a rough kind of pre-charge service. But hopefully they battery modules won't have to be connected again for a long time.

Of course, I'm assuming that the battery modules have the usual MOSFETs for switching and current limiting. Any battery that has Kelvin·Watts per hour written on the front (KW/h) is not instilling confidence in me.

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14 hours ago, plonkster said:

the proper unit is kWh (small k)

And also, no slash (implying per, or divide). If anything, it should have a dot, preferably a middle dot ('·' as opposed to the period '.').

So: 2 correct (the 'W' and 'h') out of four given 🙄  When people are in a hurry on the internet, that's one thing, but on a commercial product...

At least it wasn't kelvin week henries. [ Edit: thanks to my colleague @weber for that excellent post on the correct use of units. ]

[ Edit 2: corrected capitalisation of units 😳 ]

Edited by Coulomb
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