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

[MrM1]

Up until Last week I had never heard of pre-charging a larger inverter before connecting the battery cables. I just lived with the spark. But I was watching a "Will" video about a DIY LFP build and saw that he used a resistor before connecting the Positive Terminal. This peaked my interest, and after poking around on the forum and asking a few questions, I decided it was time to put one on my system. I guess all manor of possible nasty things can happen and the little spark can be damaging to connections, DC breakers, and / or capacitors hidden deep within the inverter.

So I rounded up all the necessary parts and a few tools (like a cone shaped step drill bit) and it all arrived today. It was a fun project and works perfectly. The push button is very robust and about the size of a quarter. It is rated for 12v 60am or 24v 30amp according the seller on Amazon. I put in a Gold anodized aluminum 100w 50 OHM resistor and used 12awg stranded wire. Yeah most of that was overkill, but I wanted it to be robust. Very happy with the out come.

Please Let me know if I did anything wrong or what could have been done better. As always, its a work in progress.

Here is the Push Start Button and Resistor.
The squares on the mat are 1 inch.



Here are the Links
Fastronix Heavy Duty Push Button Momentary Start Switch
LM YN 100 Watt 50 Ohm 5% Wirewound Resistor
Here is the Step Bit that makes the holes. I had never seen one of these before. I LOVE this tool
Titanium Step Drill Bit

Here is the Schneider Conext SW 4024 Before Install
Here it is in its native habitat surrounded by the Midnite Soar E-Panel and the Classic 150 SCC




The Hole for the Push Button Came out Nice




Wiring it all up was pretty straight forward
I came off the back of the 250 amp breaker on the battery side
Thru the Push button and on to the resistor
Connected it at the main Positive (+) Inverter Input





And here is how it all came Together.
I plan to place a label and instructions when I go to my office and get my label maker tomorrow.



Hope this helps anybody. Like I said, I had never heard of a Pre-Charger or Pre-Charging, but it sure makes sense, especially with hi-dollar gear. Thanks to this Forum and to all of you who answered my questions when I "hijacked" other threads.

MIke

 

[MrM1]

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

 

[GVSolar]

Nice.

 

[Winterhawk1]

You have added a soft start for your inverter. Congrats! It not only looks nice but I do believe it will wonders on the sudden inrush on the inverter.

 

[MrM1]

I mostly posted this for the parts list. I like the push button I found. It is really well made. So far so good.

 

[jtaon001]

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

View attachment 42825

I was wondering. I just purchased a 20 Ah battery with an 1800 watt inverter. I don't like the scary spark haha. Can I connect the battery positive to a 3 pin toggle switch with one toggle option being a line with a 50 watt 25 ohm resistor and the other toggle being the positive battery going straight to inverter positive

 

[MrM1]

You can, but you do not what to leave it in the "capacitor by-pass" position. I mean like you probably do not ever want to forget. This is why the push button is more desirable. When you push it, its on, when you release it, its off.

 

[jtaon001]

You can, but you do not what to leave it in the "capacitor by-pass" position. I mean like you probably do not ever want to forget. This is why the push button is more desirable. When you push it, its on, when you release it, its off.

Is that because it would eventually overheat?

Maybe I'll get one of the toggle switches where it only stays in position for the "on" position and I have to hold it down for the by-pass position.

Thank you!

 

[MurphyGuy]

Very nice.. Every inverter needs a precharger. At a minimum, it will wear out your components, and worst case it it can cause damage to have such a large inrush current.

 

[MrM1]

Is that because it would eventually overheat?

Maybe I'll get one of the toggle switches where it only stays in position for the "on" position and I have to hold it down for the by-pass position.

Thank you!

I would think he would be an issue the push button I used is linked in the original post

 

[Choco]

Newb Question.
What sort or resistor will suffice for 48v? So many available online. How do you calculate what you need?

 

[MrM1]

Any resistor will work. It needs to be somewhere in the 20 - 50 ohm range and probably 50+ watts. Heat sinks are nice for permanent install. The higher the resistance, the longer (slower) it will take to charge the inverter capacitors. That is the longer you'll have to leave it connected. A 50 ohm might take 15 to 30 seconds.

 

[Horsefly]

This does seem to be a good idea. I have that same inverter, so will consider it.

Just FYI, the current through the resister will never be more than the battery voltage divided by the resistance. So in your case @MrM1 you are going to have around 25V / 50 ohm, or around 0.5A. The watts for resistor are therefore 0.5 * 25 = 12.5 watts. Meaning, if you are going to go with a 50 ohm resistor you can go with a lower wattage rating. A resistor with less ohms would need to be capable of more watts. Obviously it's better to have a higher wattage rating on the resistor than needed.

 

[MrM1]

@Horsefly Thx... And I am not sure that a 50 ohm is too much. The inverter kinda comes on (lights up), but also clicks like it's not fully on. I might drop back to a 20 ohm.

 

[Horsefly]

@Horsefly Thx... And I am not sure that a 50 ohm is too much. The inverter kinda comes on (lights up), but also clicks like it's not fully on. I might drop back to a 20 ohm.

My gut says not to go lower than 50 ohms. You don't want the inverter to actually power up, just let the capacitors charge up somewhat slowly. Here slowly is still pretty fast, but not instantaneously.

 

[MrM1]

I found this suggestion from Mr @FilterGuy . I might change my 50 ohm on my 24v inverter... But maybe it doesn't matter?

 

[MrM1]

My gut says not to go lower than 50 ohms. You don't want the inverter to actually power up, just let the capacitors charge up somewhat slowly. Here slowly is still pretty fast, but not instantaneously.

OK... U posted as I was posting the pic

 

[FilterGuy]

My gut says not to go lower than 50 ohms. You don't want the inverter to actually power up, just let the capacitors charge up somewhat slowly. Here slowly is still pretty fast, but not instantaneously.

I have not found this to be a problem. I have played with this some and what happens is that the inverter powers up but immediately goes into a low voltage disconnect mode.

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]

My rec uses a 66 ohm setup. 11 sec delay. It has now welded 2 contactors. About 10 cycles and Bam. Welded.
Very disappointed.

 

[Horsefly]

I have not found this to be a problem. I have played with this some and what happens is that the inverter powers up but immediately goes into a low voltage disconnect mode.

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

See, that sounds to me like you are definitely giving it too much current. It it powers up enough that it does a low voltage disconnect, that is too much. You are only trying to charge up the capacitor(s) on the front-end, and that shouldn't take much current. So, I think a larger resistor would be better.