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Inverter pre-charge resistor size?

Blackmoses

Blackmoses

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Unfortunately, :cry: I ordered a 20 ohm 100w resistor before discovering the invaluable guid in resources about how to design a pre-charge inverter switch by the wonderful user @FilterGuy
My current pre-charge resistor
link to it
Will it work? Is it enough To pre-charge the inverter without a spark?

My setup is
Two 48v 200A batteries in parallel
Growatt SPF5000es
Thanks


NB. I would like to thank Will and everyone in this forum. It’s a treasure of knowledge. ?
 
20R at 48V is about 2.5A or therabouts, I'd suggest that will be just fine, give it a suitably rated switch and you're good to go. You're just trying to avoid that massive (almost infinite) current splat when you first connect the discharged inverter.

The Seplos 48V BMS has a 51R 10W pre-charge resistor for about 1A pre-charge.

EDIT if you actually have two as the Amazon link shows, just connect them in series for 40R :)
 
Last edited:
crossy said:
20R at 48V is about 2.5A or therabouts, I'd suggest that will be just fine, give it a suitably rated switch and you're good to go. You're just trying to avoid that massive (almost infinite) current splat when you first connect the discharged inverter.

The Seplos 48V BMS has a 51R 10W pre-charge resistor for about 1A pre-charge.

EDIT if you actually have two as the Amazon link shows, just connect them in series for 40R :)
Click to expand...
Wow I didn’t think of connecting them in series. ? Thanks ?
 
Hi, of how many watts resistor needs to be for a 6kw/12kw inverter?
 
The voltage of the inverter is more important than the wattage. I assume you have a 48v inverter.

I like to keep the precharge current down to a couple amps so if we assume the max battery voltage is 60v, a 30 ohm resistor is about right. However,. You could do a lot smaller or larger and still be fine.
 
FilterGuy said:
The voltage of the inverter is more important than the wattage. I assume you have a 48v inverter.

I like to keep the precharge current down to a couple amps so if we assume the max battery voltage is 60v, a 30 ohm resistor is about right. However,. You could do a lot smaller or larger and still be fine.
Click to expand...
So I can use those smaller 1/4w resistor? Its a 24v inverter.
 
As voltage rises during precharge, current drops quite a bit.
Your 2.5 amps 20r at 48V drops down to less than 1 amp before capaicitor is even charge to 30V.
Math: (48v-30v)/20R=.9A
For what it's worth, I went with 25-ohm for my 36V battery and it barely gets the job done. Using 25r, the capacitors only charge about 75% because lower current never overcomes parasitic current draw of Inverter electronics.
 
pullin_gs said:
For what it's worth, I went with 25-ohm for my 36V battery and it barely gets the job done. Using 25r, the capacitors only charge about 75% because lower current never overcomes parasitic current draw of Inverter electronics.
Click to expand...
Is that with the inverter on or off
 
It hasn't been mentioned exactly. Those 5 inch long 300W - 500W 120V replacement lamps found in some flood lights are just a couple ohms and the ones I have tried take solder on the end caps very easily.
 
For a 12v system, would a 5ohm resistor be just right, or what is the most ideal size? I've got a 100ohm - 100w resistor lying around, can I use it?
 
On every system I've ever built, I used a 10w 10ohm resistor, doing it the old fashioned way. Even for dual 48v Victron Quattro 10k's, it wasn't too low of a resistance, and works good on 12v systems too.

The voltage is only ~52v for a few fractions of a second. Half precharged means it's only 26v across the resistor, and you get there in a second or so
 
chess-equality said:
For a 12v system, would a 5ohm resistor be just right, or what is the most ideal size? I've got a 100ohm - 100w resistor lying around, can I use it?
Click to expand...

No reason not to use the 100 ohm, it will just take longer to pre-charge.

If you stick an LED in series with 4k7 or so across your pre-charge resistor and hold the button until the light goes out you are within a volt or so of fully pre-charged.
 
chess-equality said:
What's a "4k7"?
Click to expand...

4.7 kilo ohms it's written like that to avoid confusion when the "." vanishes in the murk, a 1/4W would be fine there :)

Other examples
5M1 = 5.1 mega ohms,
10k0 = 10.0 kilo ohms (could be written as just 10k)
1R0 = 1.0 ohm (could be written as just 1R)
0R5 = 0.5 ohms (you wouldn't write that as R5 in case it got confused with the component designation R5)
 
Last edited:
Generally not required to pre-charge the caps, but that will depend on the battery in question. If using one of our battery partners, most options there are adeqaute to not require a pre-charge.

Best,
Sol-Ark
Click to expand...


I have the 15k that has battery breakers and three battery banks I will be hooking up. So still need to “pre-charge” before final connection to batteries and throwing the breakers?
 
1663083325675.png


You have to test what the olms are cold because the watt spec is for lit but a lightbulb works as resistor.
 
They are pretty dang low when cold, definitely check first. They are a resistor, but more specifically a thermistor, and a PTC thermistor. Just some search terms if anyone wants to dig into that.
 
The battery BMS is shutting down battery power when I close the battery isolate switch.
It has been suggested that the my inverter capacitors are sending too much inrush of current to the BMS & that a pre-charge resistor should solve this problem.
What size pre-charge resistor would be recommended for my system?
Where is the best location to connect the resistor?
Will this resistor limit inverter output?
My set up...
Battleborn 100AH 12v Lithium battery with built in BMS.
2200W inverter 91% efficient (I know it is oversized for 1 battery).
2/0 multi-stranded cables connect the inverter to the battery & switch.
Blue Sea Systems 9003e battery isolate switch connected to +ve battery side.
250 Amp main fuse between isolate switch & inverter.
500 Amp shunt (with battery monitor gauge) on -ve battery side.
Let me know if you need more specs/info.
 
crossy said:
20R at 48V is about 2.5A or therabouts, I'd suggest that will be just fine, give it a suitably rated switch and you're good to go. You're just trying to avoid that massive (almost infinite) current splat when you first connect the discharged inverter.

The Seplos 48V BMS has a 51R 10W pre-charge resistor for about 1A pre-charge.

EDIT if you actually have two as the Amazon link shows, just connect them in series for 40R :)
Click to expand...
You mean that the SEPLOS 48 V BMS comes with an onboard switch or resistor so I would not get a spark while connecting the battery cables from the battery to the inverver ?

Thanks .
 
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