Pre-charge resistor seems crazy

[Pyrofx]

This is only an issue, if you turn it off and on many many times.

Twice for me

 

[Roswell Bob]

Anyone in this forum have an idea what the surge current is to charge the Input capacitor on an Inverter?

Yes. They are all over the place. I have done a few simulations. The largest contributor would be the battery chemistry. A large LiFePO4 call array would have a small impedance leading to large current pulses. Likewise a large inverter with lots of capacitance will pull a nice pulse. Wiring impedance will temper the event. If you use big fat wire and mount the inverter directly next to the cells then you will get larger pulses. Tape the leads together to minimize inductance and you will get larger pulses. Below is a Growatt 6kW inverter on an EVE 280Ah array with fairly tight wiring. Current is about 5000A, with event over in about 0.001S. Ringing is due to wiring inductance. This current pulse could damage contactors, blow fuses and smoke a lesser BMS. Second set of plots is with a 0.1ohm NTC thermistor to limit the pulse. I posted a schematic somewhere with a very simple circuit using one contactor and the thermistor. If you are up and running without any problems without inrush limiter then you are probably good. I have heard of others smoking contactors, BMS, and clearing fuses so if you are putting something together then consider the cost of a good contactor and any issues you may have without inrush limiter.

 

[Roswell Bob]

Twice for me

What happened on second turn-on?

 

[timselectric]

I don’t connect under load And I see sparks. What size is the inverter, chargers and any converters you have on this line?

Many different types of equipment, over the years.
Currently, a 48v 5kw inverter.
The largest spark that I have seen, was probably a 6kw 12v portable inverter. (Probably the worst idea that ever came out of China. lol)

 

[12VoltInstalls]

Nope
I don't connect things under full load.

Sure you do: the caps when empty are more than a full load.

 

[chrisski]

Many different types of equipment, over the years.
Currently, a 48v 5kw inverter.

Thanks. Odd how some equipment needs it more than others.

 

[cinergi]

My dual 5k inverters plus the SCCs and DC-DC converter (I've proven that connecting any one of these, NO LOAD/OFF, causes a spark) on a 48vdc system (54v real) creates a violent spark when I connect the bus (to which all that equipment is connected) to the battery. I've blown 2 300-amp Class T fuses with the inrush. I precharge now with an A, B, A+B battery switch such that A is precharge and B is direct connect. As long as I turn the switch from Off to A+B or B via clockwise movement (which forces me to hit A first -- precharge), I'm fine.

 

[Rustic]

Inverter Disconnect Switch with Precharge

To get the paper, click on the orange button at the top of this page. This circuit is designed as a disconnect switch that allows the user to pre-charge the inverter capacitors before turning the switch completely on. 26 June 2020 update: At...

diysolarforum.com

 

[Pyrofx]

What happened on second turn-on?

Welded the contactor closed

 

[Roswell Bob]

Welded the contactor closed

yep

 

[WindWizard]

yep

Same thing that happened on my 200 amp shut off switch connected to my Magnum Inverter. I went to shut it off and it would not shut off. I replaced it and took it apart and the contacts were welded together. I have a resistor now. I put it in parallel with the shut off switch with a push button in series.

It gets expensive destroying things.

 

[12VoltInstalls]

It gets expensive destroying things

Yes. It does.
…unrelated but that’s why I’ll never use another modified sine inverter.

 

[memilanuk]

Well, all this focus is on the wrong thing. I need a precharge circuit because the BMSs in my 12V SOK 206Ah LFPs sense an overcurrent condition when I turn on power to the Victron MPII. They always go into protect mode and then I have to get them to wake up.

Seems odd that you would have issues *every* time you disconnect and reconnect. I have a similar system - two SOK 206 ah batteries in parallel, feeding a Victron Lynx distributor and a Victron MultiPlus 2k Compact. The *first* time, with everything completely dead/discharged, yes I needed to pre-charge. Subsequent times (had to do some wrangling taking the batteries in and out), there was no issue. Capacitors, as a general rule, don't magically discharge to zero when they are disconnected / out of service. Over time - hours, days, or even weeks, yes. Short term, no. Not unless something else is really wrong.

That said, I am considering setting up a simple push-button system like yours for those times when (for whatever reason) the disconnect has been open longer than normal.

 

[corn18]

Seems odd that you would have issues *every* time you disconnect and reconnect. I have a similar system - two SOK 206 ah batteries in parallel, feeding a Victron Lynx distributor and a Victron MultiPlus 2k Compact. The *first* time, with everything completely dead/discharged, yes I needed to pre-charge. Subsequent times (had to do some wrangling taking the batteries in and out), there was no issue. Capacitors, as a general rule, don't magically discharge to zero when they are disconnected / out of service. Over time - hours, days, or even weeks, yes. Short term, no. Not unless something else is really wrong.

That said, I am considering setting up a simple push-button system like yours for those times when (for whatever reason) the disconnect has been open longer than normal.

Good observation. I don't have to precharge every time. But I do have to when I show up to the trailer in storage and it hasn't been powered on for a few weeks. After the first turn on, it's fine. Not sure what the length of time between off and on that requires precharge. It's real easy to push the precharge button. It is a PITA to wake up a BMS that has gone into protect mode.

 

[memilanuk]

Good observation. I don't have to precharge every time. But I do have to when I show up to the trailer in storage and it hasn't been powered on for a few weeks. After the first turn on, it's fine. Not sure what the length of time between off and on that requires precharge. It's real easy to push the precharge button. It is a PITA to wake up a BMS that has gone into protect mode.

Yah. If you have the disconnect open while the RV is in storage, I can absolutely see needing something like this. Mine parks alongside the house, and the system stays 'hot' pretty much all the time whether it's in the driveway or out camping.

 

[RCinFLA]

Anyone in this forum have an idea what the surge current is to charge the Input capacitor on an Inverter?

It has a lot to do with your particular setup. HF inverters have lower DC input capacitance than LF inverters. This is due to LF PWM switching freq is in 6 kHz-8 kHz range where HF inverter is 20-25 kHz switching.

The total resistance to battery has a great effect. Battery internal resistance, BMS resistance, battery cables gauge and length, connectors, breaker, shunt all adds to series resistance that reduces peak surge current. Also, the ESR (quality) of capacitors matter.

At 55v, a relatively low 20 mill-ohms net series resistance with 30,000 uF low freq inverter caps will peak at 2500 amps dropping to 20 amps in 2.7 msecs.

At 55v, a more typical 50 milli-ohm net series resistance with 7,500 uF high freq inverter caps will peak at 1000 amps dropping to 20 amps in 1.4 msecs.

 

[suprajayaz]

Good observation. I don't have to precharge every time. But I do have to when I show up to the trailer in storage and it hasn't been powered on for a few weeks. After the first turn on, it's fine. Not sure what the length of time between off and on that requires precharge. It's real easy to push the precharge button. It is a PITA to wake up a BMS that has gone into protect mode.

Our SOK 206ah battery shut down after shutting down entire system (2020 Magnum Energy 2.8k inverter) for just a day and had to remove and wake it up using portable solar panel with PWM controller. Glad to find your post as I need to do the same thing. Like simple push button precharger. Link to components?

 

[yaq]

Excusez ma question : mais pourquoi ne pas simplement mettre une résistance en parallèle sur les contacts du relais, l'oublier et insérer un interrupteur en série si vous souhaitez isoler complètement la batterie ?

 

[Zwy]

Excusez ma question : mais pourquoi ne pas simplement mettre une résistance en parallèle sur les contacts du relais, l'oublier et insérer un interrupteur en série si vous souhaitez isoler complètement la batterie ?

Translated: Excuse my question: but why not just put a resistor in parallel on the relay contacts, forget about it and insert a switch in series if you want to completely isolate the battery?

 

[yaq]

Oui, il suffit de ponctuer le contact non avec possibilité de gestion manuelle par un interrupteur série sur le pont.