Pre-charge resistor seems crazy

[JJJJ]

Steve_S said:

Has it occurred to anyone to just run a wire to a resistor to push-button (momentary) switch connected to the inverter.
Some have a fused resistor just left in place, while others use a timered relay.

I like the simplicity of this idea. I had picked up a 25 ohm earlier and could include this with a "big red" button. I guess the question would be how long to keep the button depressed, lacking a timer....

 

[timselectric]

In my opinion
If the 25 cent resistor was necessary it would be built into the inverter.

 

[WindWizard]

In my opinion
If the 25 cent resistor was necessary it would be built into the inverter.

It is not necessary for the Inverter but it is necessary for the disconnect switch that feeds the Inverter. What happens is the contacts will become welded together or damaged due to the arc and high current rush.

 

[timselectric]

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

 

[WindWizard]

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

That is certainly true. If you leave it on all of the time then there is never an issue. But the first time that you turn it on if you have a disconnect switch to do that then you will potentially damage your disconnect switch due to the current rush charging the capacitor. If you took a wire and connected across the disconnect switch to charge the capacitor and did not get to startled when the wire welded itself to the battery post then you could get away without the precharge resistor.

 

[timselectric]

I'm not saying that you shouldn't pre charge.
If it makes you feel comfortable, by all means do it.
But, properly rated disconnects are spring loaded.
Anderson plugs have self wiping contacts.
At 60vdc, it's not that big of an arc.

 

[chrisski]

I'm not saying that you shouldn't pre charge.
If it makes you feel comfortable, by all means do it.
But, properly rated disconnects are spring loaded.
Anderson plugs have self wiping contacts.
At 60vdc, it's not that big of an arc.

I disagree with that. It is quite an arc and enough to scorch the metal With a 3000 watt inverter, a 35 amp 24 volt AC to DC charger, and a 70 amp 24 volt to 12 volt DC to DC converter. Will has a video where it was more extreme than I have and took a chunk off a battery terminal.

I did not have this scorching with a 2000 watt inverter and a 12 volt 10 amp AC to DC charger.

Are you not seeing sparks flying and metal scorching when you power up similar. equipment?

 

[WindWizard]

I'm not saying that you shouldn't pre charge.
If it makes you feel comfortable, by all means do it.
But, properly rated disconnects are spring loaded.
Anderson plugs have self wiping contacts.
At 60vdc, it's not that big of an arc.

Not that big of an Arc? I wish that I would have saved my 200 amp disconnect switch that I ruined. I took it apart and the contacts had some nice big pits in them. If I still had it I would post the pictures. It totally destroyed my contacts. When I was a bench tech back in the 80's I use to trouble shoot new circuit boards. Sometimes the boards would have interlayer shorts on them which were difficult to determine exactly where the short on the board was. We use to charge up a capacitor to 5volts and then place it where the short path was. Most times it would fix the short by removing the whatever was causing it.

Why risk damaging your expensive disconnect whether spring loaded or not? Just take a few seconds to precharge your monster capacitor. I ended up ruining mine and had to replace it.

 

[HRTKD]

My Blue Sea 9003e switch is rated at 1200 amps for 30 seconds and 350 amps continuous. I have a 12 volt system with a Victron Multiplus 12/3000. While I'll be turning the inverter off, the Multiplus will remain connected to the system unless I'm doing maintenance. I don't think a pre-charge is necessary in my system.

 

[WindWizard]

I am not sure of what the current surge is for charging the input Capacitor on an Inverter but I suspect that it far exceeds 1200 amps. Looking at your Blue Sea switch it cost over $50. Just my thoughts on the matter sense I ruined mine. Granted mine was not near the quality of your disconnect switch. Why risk it? If all it takes is a few moments of time to charge the capacitor. Why risk the potential of damaging a switch which costs over $50. I have ruined so many things just by thinking that hey its not going to be that bad when in fact it ended up being worse. An ounce of prevention is worth a pound of cure.

 

[John Frum]

I am not sure of what the current surge is for charging the input Capacitor on an Inverter but I suspect that it far exceeds 1200 amps. Looking at your Blue Sea switch it cost over $50. Just my thoughts on the matter sense I ruined mine. Granted mine was not near the quality of your disconnect switch. Why risk it? If all it takes is a few moments of time to charge the capacitor. Why risk the potential of damaging a switch which costs over $50. I have ruined so many things just by thinking that hey its not going to be that bad when in fact it ended up being worse. An ounce of prevention is worth a pound of cure.

I think you would have to look at all the caps in the inverter.

 

[HRTKD]

I am not sure of what the current surge is for charging the input Capacitor on an Inverter but I suspect that it far exceeds 1200 amps. Looking at your Blue Sea switch it cost over $50. Just my thoughts on the matter sense I ruined mine. Granted mine was not near the quality of your disconnect switch. Why risk it? If all it takes is a few moments of time to charge the capacitor. Why risk the potential of damaging a switch which costs over $50. I have ruined so many things just by thinking that hey its not going to be that bad when in fact it ended up being worse. An ounce of prevention is worth a pound of cure.

I doubt that the surge to charge the resistors is that high. If it was, my BMS would have gone nuts. There are no stored warnings/errors in the BMS. The shunt hasn't registered anything out of the ordinary.

 

[WindWizard]

I doubt that the surge to charge the resistors is that high. If it was, my BMS would have gone nuts. There are no stored warnings/errors in the BMS. The shunt hasn't registered anything out of the ordinary.

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

 

[corn18]

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. I use the simple red momentary push button with 6 ohm, 25W resistor. Hold it for 1 sec and turn on the main power. Easy peasy.

 

[featherlite]

One of my last guests left skid marks in the toilet. He's on the permanent ban list.

LOL! I like the way you think.

 

[Hedges]

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

Yes, my estimate is 20,000A

That comes from the internal resistance (and cell voltage) of a single string of 280 Ah cells for one LiFePO4 battery. Possibly its short-circuit current is actually less (I think someone did publish a bit of measured data), but that's what I have to go on.

Connect several banks in parallel, and multiply that figure.

Capacitors in an inverter present a dead short, so closing the switch is like a "bonded short" on output of battery. Wire resistance limits current a little, but cell IR dominates. Of course, capacitor charges up in a short time so no wires burn up. But BMS FETs may take a hit, also switch/relay contacts.

Capacitors in an inverter will be about 10,000 to 100,000 uF, 0.01 to 0.1 Farad.
A Farad is an ampere-second per volt. 0.1 F charged 1V is 0.1A for 1 second; charged to 50V it is 5.0A for 1 second. 5000A for a millisecond.
It might peak around 10,000A for an instant, taper down to zero in a fraction of a second.

That is enough to burn contacts. Relays tend to close, bounce open, close again. During the time open, it melts the contact point, then the molten points fuse when they touch again. Next time it opens (if you're lucky), and tears out a bit of metal.

 

[WindWizard]

I doubt that the surge to charge the resistors is that high. If it was, my BMS would have gone nuts. There are no stored warnings/errors in the BMS. The shunt hasn't registered anything out of the ordinary.

Anyway with 20,000 amps keep an eye on your $50 Bluesky. If it starts getting hot or it fails to shut off then you will know that you need to replace it.

 

[WindWizard]

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. I use the simple red momentary push button with 6 ohm, 25W resistor. Hold it for 1 sec and turn on the main power. Easy peasy.

View attachment 87545

This is a very nice setup that you have. Thanks for sharing

 

[timselectric]

I disagree with that. It is quite an arc and enough to scorch the metal With a 3000 watt inverter, a 35 amp 24 volt AC to DC charger, and a 70 amp 24 volt to 12 volt DC to DC converter. Will has a video where it was more extreme than I have and took a chunk off a battery terminal.

I did not have this scorching with a 2000 watt inverter and a 12 volt 10 amp AC to DC charger.

Are you not seeing sparks flying and metal scorching when you power up similar. equipment?

Nope
I don't connect things under full load.

 

[chrisski]

Nope
I don't connect things under full load.

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