Overkill BMS connecting Inverter = short circuit! Help!!

[SolarHawaii]

Yesterday randomly my inverter turned off. Wouldn't turn back on so I disconnected held button down and then waited 30 minutes hoping it would reset. When I went to reconnect, I used an old resistor on the negative terminal of the inverter and there was a huge spark. Melting the metal on terminal which i them had to grind down, this happened a couple times then i opened inverter and found the fuses blown and replaced them. So I assumed it was the BMS balance cables maybe twisted or just not securly fastened. So I disassembled the entire system and rewired everything best I could, its unfinished obviously but need to figure out how to fix this short circuit first. Frustrating is an understatement, going on 13 hours on this fricken project. BMS is reading fully charged, and I'm out of ideas while my freezer slowly melts.

 

[Hedges]

You used a resistor - for precharge? And there was a huge spark when resistor connected? Or after attempted precharge you connected inverter cable and got a huge spark?

If there was a short, precharge would never actually pull voltage up. Depending on resistor value and wattage, it might get too hot. DMM across resistor or across inverter would show the voltage.

One way to blow fuses is to connect inverter reverse polarity.
Assuming it is already wired correctly, only one wire was disconnected and reconnected, shorted transistors in inverter is a possibility.

Could it be your precharge didn't function due to something open-circuit, so when you connected battery and inverter the capacitor inrush blew the fuses?

Nothing seems to explain "inverter randomly turned off". If inverter had failed with shorted transistors, fuse would have blown then, so no spark expected when connected later.

Have to debug each piece. Start with measuring voltage and polarity which will appear on inverter when connected. Then connect through precharge resistor and watch voltage rise.

 

[SolarHawaii]

Used 25 watt resistor between The negative terminal on the inverter for 10 seconds. Then when I went to connect the negative battery terminal to the inverter, it sparked. Inverter randomly turned off during the day when I was running a decent amount of power but definitely not over 1200 WATT when batteries were fully charged.

wired correctly

This last attempt it sparked and said short circuit in BMS app. And I needed to reset the BMS not the fuses in the inverter.

Nothing reads on inverter because short circuit is tripped when I try to connect

 

[SolarHawaii]

The only thing not wired is ground wires because I wanted to test it out and tidy it up before I put it back in its home. Could that cause the short?

 

[Hedges]

25 watt, how many ohms? (30 ohms, the photo shows)

Sounds like voltage did not pull up in the 10 seconds. Maybe due to a fault, maybe due to load.
If capacitors were discharged when battery connected, it would spark, maybe blow fuses.

Put volt meter on inverter input and watch while precharging. watch after disconnecting resistor, before connecting cable. maybe it discharges rapidly. Possibly inverter powers up through resistor but then discharges. That would depend on load and resistor value.

This doesn't explain random turn-off. If you get that again, put volt meter on DC side, see if it is dropping, maybe due to poor contact.

Swapping parts to test can be useful. 8 cells, 24V nominal, shows 27V. Have any 24V loads you can power? A 1500W 120V space heater would draw about 2.8A. Small load to test battery & BMS.

A couple 12V car batteries in series could be used to test inverter.

 

[SolarHawaii]

I hooked up 24 V to 12 Volt converter and then to a 12 volt inverter and WE GOT POWER AGAIN! Now the question is what's wrong with 24v inverter? This is second one in the last 6 months. But I got 2 now so should be able to swap parts if I knew what I was doing inside this beast ?

 

[SolarHawaii]

I hooked up 24 V to 12 Volt converter and then to a 12 volt inverter and WE GOT POWER AGAIN! Now the question is what's wrong with 24v inverter? This is second one in the last 6 months. But I got 2 now so should be able to swap parts if I knew what I was doing inside this beast ?

 

[Hedges]

Cheap inverter? Excess loads? Heat?

Hawaiian humidity?
You might be better off with a sealed inverter if that is the problem.
Or at least one with conformal coat on PCB.
Depends on failure mode.

Try a couple car batteries for 24V, double-check inverter that way.

 

[Kifo]

Hello i am going through something similar when i try connecting my battery to inverter i get SCP on battery 100ah ,and it get the fault how can i solve that issue ,i am thinking my inverter messed it doesn't have no fault when i remove battery connection to inverter and turn on battery ,together with bms it get back normal but as soon as i try connecting negative wire back to battery getting spark and short circuiting again.

 

[Hedges]

Make sure polarity isn't reversed.

Look up "precharge" basically a resistor to limit current charging capacitors in inverter. That is needed for many lithium battery & inverter circuits.

Try a lead-acid battery to debug the issue. Also serves to precharge.

 

[Kifo]

Polarity isnt reverse just thinking maybe inverter messed up too .don't know just when trying to connect back wires to battery the SCP comes up right away .

 

[Hedges]

Connecting a capacitor to output of battery looks like a short circuit. If SCP = Short Circuit Protection, no surprise there.
Connect a resistor in series (or an incandescent light bulb. Wait for glow to go out, or voltage seen with DMM to drop near zero. Then complete the circuit. That is called "Precharge".

 

[Kifo]

Make sure polarity isn't reversed.

Look up "precharge" basically a resistor to limit current charging capacitors in inverter. That is needed for many lithium battery & inverter circuits.

Try a lead-acid battery to debug the issue. Also serves to precharge.

Connecting a capacitor to output of battery looks like a short circuit. If SCP = Short Circuit Protection, no surprise there.
Connect a resistor in series (or an incandescent light bulb. Wait for glow to go out, or voltage seen with DMM to drop near zero. Then complete the circuit. That is called "Precharge".

ok will try thank you.

 

[Kifo]

ok will try thank you.

do i need a 48v device to that the bulb would'nt be ?

 

[Hedges]

A 120V light bulb is probably fine for precharging 48V circuit.
A 12V car headlight or taillight might work or might burn out.

120V 100W is about 140 ohms when hot, maybe 14 ohms cold.

You could also use a toaster or other heating appliance. Not one with a fan or electronics! Only a heating element.

 

[Kifo]

A 120V light bulb is probably fine for precharging 48V circuit.
A 12V car headlight or taillight might work or might burn out.

120V 100W is about 140 ohms when hot, maybe 14 ohms cold.

ok np thanks.

 

[Quattrohead]

It has to be a bulb that can handle the battery voltage because if there is a short in the inverter the bulb will see the full battery voltage and you would not believe how bright a 12 volt bulb will get at 48 volts before it probably blows up ?

 

[Kifo]

It has to be a bulb that can handle the battery voltage because if there is a short in the inverter the bulb will see the full battery voltage and you would not believe how bright a 12 volt bulb will get at 48 volts before it probably blows up ?

noted thank you.

 

[Kifo]

noted thank you.

Funny thing all this happen while using my water heater which pulls like 2.5kw ,the inverter was using only battery at the time with battery remaing 48% which i believe should have caused the battery to shut down eventually getting that SCP.

 

[Hedges]

That would be low voltage protection. But 2.5kW would be 2 hours on a 100 Ah battery, so it can operate for a while.
50A draw should be OK.