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My Push Button Pre-Charger Install for the SW 4024 Inverter

Some folks have described using light bulbs to provide the needed pre-charge resistance... just wait until the bulb dims and stop pressing the momentary switch. Easy. So if instead you use the wire wound resistor method how do you know when to let up on the switch?... what acts as the dimming bulb indicator of a fully charged capacitor bank?
5 seconds is enough for my system. If you want to get specfic, find out what the capacitance of your system is and learn a little about the RC time constant. Google search will put you on the right track.
 
Hello again. This is a reposting from another thread that was dealing with a similar "inverter pre-charge" inrush-caused BMS faulting problem. Since I started problem solving within this thread and my final solution specifically involved the momentary pre-charge switch it seemed fair to share my success with those of you still monitoring this thread... Thanks for all of your earlier suggestions that lead to this solution... finally!!!
-----------------------------------
Fixed At Last...

To recap...
I'm running a EG4 24v (no screen) battery with a Aeliussine 2000 watt 24v Pure Sine Wave Inverter. I have been trying everything under the sun to start this inverter without the BMS faulting... NO success. I've tried every suggestion tossed out from SS... including installing their recommended, expensive, "true Fast Snap" breaker instead of the heavy duty (cheap & slow) car battery disconnect switch I've been using (see red switch handles in photo below). Nothing worked. I've tried numerous different pre-charge resistors to slow the capacitor inrush. All failed.

UNTIL I tried a 50w / 25ohm wire wound resistor from Amazon... $9 for two (I used only one).

Inverter Switch 1.jpg



It WORKED... every time!! Keep in mind I've been working on this since March !!!
As others have suggested you just have to set up a parallel line from the battery to the inverter that includes this resistor AND a momentary button switch (see photo). The wire can be small gauge (12awg in my case) since current is reduced by the resistor and only runs for a few seconds.

My procedure is as follows...
1) the main line from the 24v EG4 battery to my 12v RV breaker panel is on... via a Victron 70A 24v-to-12v step-down converter.
2) the 50A Epever solar controller and panels (600w in my case) are all connected (not enough capacity to use the "Solar Start" EG4 method others have suggested).
3) the inverter (on-box) switch is on. I'm not using the remote switch.
4) I hold down the momentary "pre-charge" button which causes the inverter to begin beeping and it's status screen to begin flashing due to a low-voltage state (capacitor inrush is stealing everything the resistors are allowing through).
5) after about 3 or 4 beeps (about 5 seconds) the inverter goes silent and I can see that the status screen has also stopped flashing, indicating a non-error state.
6) now, WHILE STILL HOLDING DOWN THE BUTTON, I flip the master inverter switch (red handle & keyed hole at center in photo). The red-handled switch on the right (photo) is just an example of what this type of switch looks like in back and with red handle inserted.
7) only now do I release the momentary pre-charge button.
8) the inverter screen stays on... the EG4 BMS software shows NO FAULTING... and I... at LONG last... am a very Happy Camper !!!
 
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congratulations on this success, and thanks for documenting it here for others to refer!

those capacitors inside the inverters ensure stable operation during active use, yet create headache when initially connecting power, being so ready to take on energy when "empty"

good work with the resistor pre-charge dance!

happy camping! ☀️
 
Here was my very simple wiring diagram for ordering parts and getting it all in my head

View attachment 42825
First, thanks for creating this thread as I have the pre-charge capacitor/BMS issue with my system. This thread has a lot of posts regarding the resistor but no one has asked about the voltage of the pushbutton. I have a Schneider SW 4048 inverter but I'm concerned about using a button rated for 12v/24v since my system is 48v. A search of Amazon doesn't include a 48V dc version of your pushbutton. Any thoughts?
 
Some folks have described using light bulbs to provide the needed pre-charge resistance... just wait until the bulb dims and stop pressing the momentary switch. Easy. So if instead you use the wire wound resistor method how do you know when to let up on the switch?... what acts as the dimming bulb indicator of a fully charged capacitor bank?
It does more than that, Dexter put up this post and video.
 
First, thanks for creating this thread as I have the pre-charge capacitor/BMS issue with my system. This thread has a lot of posts regarding the resistor but no one has asked about the voltage of the pushbutton. I have a Schneider SW 4048 inverter but I'm concerned about using a button rated for 12v/24v since my system is 48v. A search of Amazon doesn't include a 48V dc version of your pushbutton. Any thoughts?
My SW is a 24v. That's all I got.
 
First, thanks for creating this thread as I have the pre-charge capacitor/BMS issue with my system. This thread has a lot of posts regarding the resistor but no one has asked about the voltage of the pushbutton. I have a Schneider SW 4048 inverter but I'm concerned about using a button rated for 12v/24v since my system is 48v. A search of Amazon doesn't include a 48V dc version of your pushbutton. Any thoughts?
First, don't sue me if I'm wrong, but...
  1. The voltage across the switch is only the voltage difference between the battery positive and the positive side of the capacitor bank. Even if the capacitor is very large (say 100,000 microfarads), they will charge up very quickly. There's some math to explain all this: T = R x C. Where R is the resistance in ohms and C is the capacitance in farads. A capacitor is fully charged at 5T, but it is 50% charged at only 0.7T. So say you have a 50 ohm resistor, and the capacitor is 100,000 microfarads, or 0.1 farads. So T = 50 X 0.1 = 5 seconds. so for a 48V battery the voltage across the switch will be down to 24V in 0.7 X 5 = 3.5 seconds.
  2. The current running through the switch is pretty small as well. It will start out at 48V / 50 ohms = 0.96A, and by 3.5 seconds it will have dropped down to 24 / 50 = 0.48A.
I wouldn't be worried about using a switch rated for 24V on your 48V pre-charge.

Like @MrM1 I have a Schneider 4024, and I don't even know how big the capacitors are in it, but I have a pretty wimpy pre-charge push button switch (and I use 25 ohm resistors on both of my battery pre-charge circuits).

Edit to add: I used the 100k µF as an extreme example. I really don't think inverters would use this much!
 
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First, don't sue me if I'm wrong, but...
  1. The voltage across the switch is only the voltage difference between the battery positive and the positive side of the capacitor bank. Even if the capacitor is very large (say 100,000 microfarads), they will charge up very quickly. There's some math to explain all this: T = R x C. Where R is the resistance in ohms and C is the capacitance in farads. A capacitor is fully charged at 5T, but it is 50% charged at only 0.7T. So say you have a 50 ohm resistor, and the capacitor is 100,000 microfarads, or 0.1 farads. So T = 50 X 0.1 = 5 seconds. so for a 48V battery the voltage across the switch will be down to 24V in 0.7 X 5 = 3.5 seconds.
  2. The current running through the switch is pretty small as well. It will start out at 48V / 50 ohms = 0.96A, and by 3.5 seconds it will have dropped down to 24 / 50 = 0.48A.
I wouldn't be worried about using a switch rated for 24V on your 48V pre-charge.

Like @MrM1 I have a Schneider 4024, and I don't even know how big the capacitors are in it, but I have a pretty wimpy pre-charge push button switch (and I use 25 ohm resistors on both of my battery pre-charge circuits).

Edit to add: I used the 100k µF as an extreme example. I really don't think inverters would use this much!
Thanks, Horsefly, for you post above. Very helpful. I finally had the time to install my "soft start" switch, and it worked on the first try after the install, which was very satisfying--usually takes me two or more tries to get something right! However, with the help from you and others on the forum (especially the OP), my wife and I won't have to hold a resistor by hand to charge the inverter's capacitors any more every time we need to reconnect the battery. I've attached a pic of the finished install. Our switch is mounted at the bottom, and is a simple battery on/off switch rated for 48V. You can see it just below the Midnite Solar logo.
 

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