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Push button for inverter pre charge switch? Momentary switch ok??

I have a 24v EG4 battery hooked up to a 24v Aeliussine 2000 watt Pure Sine Wave Inverter. The EG4 BMS is so sensitive that even with this "small", 2,000w, inverter it "sees" a dead short and shuts down every time I attempt to connect the inverter. My push-button, momentary switch with 40 ohm resistor, by-pass pre-charge circuit solves the problem as long as the inverter's own on/off switch is set to "off". Sig Solar (EG4 seller) suggests trying a 125A DC breaker as an inverter connect/disconnect switch because it fast snaps into full connect to minimize arcing... instead of the big, slow & sparky, rotary "Kill" switch that I've been using. Breaker arrives tomorrow. I'll see if it allows me to ditch the momentary pre-charge resistor circuit.
I will be pleasantly surprised if the breaker makes a difference. The BMS in the battery is shutting down due to thinking the surge is a short circuit.... not because there is an ark. The breaker is not going to reduce the surge in any way.

BTW: the EG4 has a sequence that will precharge for you, but it is one of those things where there are several buttons you must push in sequence so it is a bit of a PITA. If you are not aware of it you should look it up. (I don't remember the sequence off hand)
 
I will be pleasantly surprised if the breaker makes a difference. The BMS in the battery is shutting down due to thinking the surge is a short circuit.... not because there is an ark. The breaker is not going to reduce the surge in any way.

BTW: the EG4 has a sequence that will precharge for you, but it is one of those things where there are several buttons you must push in sequence so it is a bit of a PITA. If you are not aware of it you should look it up. (I don't remember the sequence off hand)
Well... new battery-to-inverter breaker has been installed and tested.

Suntree 63A DC Breaker 1.jpgSuntree 63A DC Breaker 2.jpg
Sadly it would appear that you are indeed correct... no change... BMS still instantly shuts down. Well, it was worth a try.

My 20 second 40 ohm resister pre-charge by-pass circuit continues to work great. I just wish you could figure out a way around the requirement to have the 2,000 w inverter physically shut OFF while pre-charging the caps. Once changed the main box switch is flipped on and all is good. If I have the inverter already running and try the pre-charge trick I still get the BMS shut down. The problem is that the inverter is in the belly of the beast (RV) and not easily flipped on or off from inside.
 
My 20 second 40 ohm resister pre-charge by-pass circuit continues to work great. I just wish you could figure out a way around the requirement to have the 2,000 w inverter physically shut OFF while pre-charging the caps. Once changed the main box switch is flipped on and all is good. If I have the inverter already running and try the pre-charge trick I still get the BMS shut down. The problem is that the inverter is in the belly of the beast (RV) and not easily flipped on or off from inside.

Have you tried turning on the main switch while the precharge button is still pressed? On some inverters, the resistor lets enough current through for the inverter to turn on.... but it immediately goes into a low-voltage error state. At that point, if the main switch is turned on *before* the precharge is disconnected, the BMS does not generally freak out. Even if the inverter does not turn on, the capacitors may be charged enough to reduce the surge to an acceptable level.

If this does not work for you, you could consider using a smaller resistor in series with the pushbutton. Even a 15ohm resistor would only let 4A or less through, but it might be enough to make things work.
 
Good ideas... I tried leaving the main inverter physical switch "On" while doing the pre-charge however, as others have noted, the inverter status screen started switching on and off... apparently my 40 ohm resistor wasn't allowing quite enough power through to do a full start.

Unfortunately it's still not easy to physically flip the inverter box switch on or off from inside (upstairs) the RV.

No, I don't have a remote switch for this model but that would still require that the master inverter switch be flipped to "on" which leaves me with an always "on" parasitic power drain for 110v devices that I seldom use.

Thanks for your suggestions. Keep 'em coming !!
- Dan
 
I tried leaving the main inverter physical switch "On" while doing the pre-charge however, as others have noted, the inverter status screen started switching on and off...apparently my 40 ohm resistor wasn't allowing quite enough power through to do a full start.
That tells me the inverter is close to being turned on. I would seriously consider trying it with a high wattage 15 ohm resistor.
 
1) You said: "Have you tried turning on the main switch while the precharge button is still pressed?"

Actually, that's exactly how I do it... only I'm talking about the main "kill" switch in the main line from the battery to the inverter. I just make sure to wait 20 seconds and then rotate the kill switch to "on" while still holding down the momentary by-pass resistor switch.

Or... were you suggesting that I flip the inverter's own master switch (on the box) while still pressing the momentary resistor by-pass switch? if so, I'd say I'd probably still see the inverter go through it's gasping attempt at a start.

That tells me the inverter is close to being turned on. I would seriously consider trying it with a high wattage 15 ohm resistor.
2) I'm actually using two wirewound, aluminum case, 25 Watt / 20 Ohm 5% resistors in series... making my 40 ohms total. I assume that's going to still be 25 watts? So, now, knowing this, what size resistor would you suggest I try next?

3) As for the use of the built-in EG4 pre-charge system... I've got that same "not convenient" problem with the battery that I have with the inverter. The momentary pre-charge by-pass resistor circuit and the main power "kill" switch are both up in the RV with me but flipping on the physical inverter box switch or the main on-the-box battery breaker are not easily reachable for a quick, coordinated, flip.

4) I'm still trying to figure out how to avoid having to leave the inverter, on-the-box, master switch on all the time. I'd rather not deal with the resultant parasitic power loss during the 99.9% of my RV time when I'm NOT in need of a 120v power source.
 
1) You said: "Have you tried turning on the main switch while the precharge button is still pressed?"

Actually, that's exactly how I do it... only I'm talking about the main "kill" switch in the main line from the battery to the inverter. I just make sure to wait 20 seconds and then rotate the kill switch to "on" while still holding down the momentary by-pass resistor switch.

Or... were you suggesting that I flip the inverter's own master switch (on the box) while still pressing the momentary resistor by-pass switch? if so, I'd say I'd probably still see the inverter go through it's gasping attempt at a start.
No..... you are doing what I was suggesting.


2) I'm actually using two wirewound, aluminum case, 25 Watt / 20 Ohm 5% resistors in series... making my 40 ohms total. I assume that's going to still be 25 watts?
The two 25W resistors in series are equivalent to a single 40ohm 50W resistor.

So, now, knowing this, what size resistor would you suggest I try next?
As a quick test, try using just one of the 20ohm resisters. However, don't press the button for very long or your 25W resistor might get too hot.

P=V^2/R = 58*58/20 = 168W (Worst Case). In reality, there will be a significant voltage drop across the inverter so the voltage across the resistor will be a lot less. Since the power is a factor of the voltage squared, a decent drop in the voltage will make a huge drop in power on the resistor.

If you find that 20ohms works, measure the current while the button is pressed. Then you can pick what wattage resistor you will need. (P = I^2 * R.)

My 20 second 40 ohm resister pre-charge by-pass circuit continues to work great.

Even at 40ohms, the capacitors are going to charge up a *lot* faster than 20 Seconds. Just a couple of seconds will do the job. 5 seconds if you want to be absolutely sure.
 
Correct. But that is good? No, that is bad! If you short a charge resistor across your breaker or relay or whatever the inverter will not have to be recharged. That is true. It will still have B+ at the input when you disconnect the batteries. (so you didn't disconnect the batteries). The inverter is still on. Your wife plugs in hairdryer. Resistor goes POP! (you can hear it).

There is more than leakage currents as the caps have to be discharged by some means. The electrical Gods have ruled that if you disconnect power from any "box" ALL potential energy must be removed from the box. If you have power coming in from another source running through relay contacts or something you will kill me if I open the box to work on it and I thought I had the power off. Large capacitors SHOULD have a means of discharging themselves when the power is removed. You can't leave lethal energy in the box for someone to remove and work on it. So if you want to keep the capacitors on when you disconnect the batteries you only need to overcome the discharge rate of the caps ( RC time) probably megaohm. Use a correct 100 ohm 5-1000watt or more charge resistor then sit back and wait for the pop. What you are trying to do is use a megaohm charge resistor shorted across your relay or breaker to remain in the circuit to overcome the rc time of discharge they have for the caps. They are all different and it doesn't matter. If you leave the charge resistor in there and If you have no output on the inverter ,At some point in time the cap will charge up to B+. Your inverter is on. You must remove the charge resistor in order to remove the B+ from the inverter to turn it off. On my homemade amplifier I couldn't turn it on because the 30 amp breaker on the wall would trip everytime. I have garage full of old school parts as I am strickly analog person. I had to put a NO/NC delay on make relay (Old allen bradley) in there with 100 ohm 7 watt precharge because that is what came out of the drawer when I reached in there. I apply power through nc contacts through 100 ohm to caps. After delay NC contacts opens and NO contacts close applying full power to the caps. Now that I think about it I used like a 300 ohm first and the breaker didn't trip on the wall but every light in the house went dim to black so my wife came running out to the garage wondering what I was blowing up THIS TIME so I soldered another one in parallel on it and was done. 150 ohms. 30 years ago.
Thanks for the reminder... lol
I had forgotten the electron dance... although with capacitors there is no time to count them !
 
Should all inverters be precharged or is it only suggested for a minimum established wattage?
Example, would a 1500 or 2000 watt inverter need a precharge.
As a data point, I have a 24V 2000W inverter. When setting up my system originally I screwed up and attempted to reconnect a wire to one of my LiFePO₄ batteries while all of the wiring to the inverter was in place and my cutoff switch to the inverter was mistakenly in the On position (dumb). When I touched the wire lug to the battery terminal there was a huge spark. It took a small chunk out of the wire lug and scared the crap out of me.

So yes, even a 2000W inverter needs to be precharged.
 
Please define "high wattage 15 ohm resistor"?
I can't put a definitive number on it, but I can put some boundaries on it.

If we assume the battery voltage can be as high as 60V (worst case) then the absolute most current would be 60V/15ohms= 4A.

4A through a 15 ohm resitor will burn 4A^2 X 15ohms = 4x4x15=240W. However, that is an extreme worst-case assuming the inverter presents as a dead short for an extended period of time. In reality, the resistor will never see the full 60V. In fact, it will see considerably less than 60V.

We could model the inverter like this:
1653799752579.png

The bleed-off resistor is probably going to be in the order of 1k Ohm or more, so we can ignore that.

When the push button is pressed there will be an initial huge surge as the capacitors charge and in less than a second, it will be over. This will be so fast that we do not have to consider it in the wattage. After the capacitors charge, the 15ohm resistor and the effective resistance of the control circuit will form a voltage divider. Part of the 60V will be over the 15Ohm resistor and part of it will be over the control circuitry.

Now we have to make some assumptions.

Let's assume the inverter can operate on 1A when it is in the low voltage disconnect state. (None of the power circuitry is engaged, only the control circuitry) That would mean the precharge resister would see 1A x 15ohms = 15V. That means it would only dissipate 15V*1A=15W.

Notice also that if 15V is dropped across the precharge resistor, there is 45V across the control circuitry. That is certainly in the range of the voltage for the control circuit to wake up.

So, if I had to guess, a 25W resistor would work. If I wanted to be safe, I would buy a 50W resistor.

Notice that the whole analysis is based on an assumption that the control circuit can operate on 1A or less. This is not unreasonable but this is a SWAG nonetheless.

Having done the above analysis, I am more convinced that you should try the 20ohm 25W resistor that you already have and when you do, measure the current. There is a good chance the 20ohm resistor will work and will never get hot.
 
Wow !! That's an awesome answer to a very short question !!!
Thanks for all of that effort. Big help !!

So, by dropping my system to only one of my 20ohm 25W resistors I'm hoping that you're thinking that this will allow me to keep the inverters own "box" switch ON at all times? Since I only occasional have a need for 120v appliances I was trying to avoid that parasitic background loss if at all possible but so far the system just wouldn't start with an "always on" inverter box. That would be great since manually turning on the inverter from inside my trailer would NOT be convenient.

I only hope that the fact that my battery and inverter are running with 24volts, instead of the 48volts in your diagram, wouldn't change your calculations and conclusions too much?
 
I only hope that the fact that my battery and inverter are running with 24volts, instead of the 48volts in your diagram, wouldn't change your calculations and conclusions too much?
Oh.... If it is a 24V system *definitely* try the 20 ohm resistor and if that doesn't work try a 15 ohm. I have had good luck with 15ohm 25W on a few different 24V inverters.

What inverter do you have?
 
Time constant calculator:
The power dissipation of the resistor time will be very short based on 15 Ohms, 10,000uF, 48VDC.
The Voltage on the cap after 1st time constant will be about 63% of 48V after 0.15 second = 30V.
It takes about 5 tc for the cap to be fully charged.
You can estimate (using the calculator) the size of the capacitor bank by timing how long it will take the cap to charge up to 48V (it takes 5 tc) with that 15 Ohms resistor (control circuit is in the off mode).
 
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What inverter do you have?
Very famous brand... I'm sure you've heard of it... "The Cheap Co" ! Seems to work well enough for my, very infrequent, 120v needs. The only remaining problem appears to be the USB-connected remote control panel... it shows a nice battery voltage status screen but the On/Off switch does not appear to work... no reaction to a button press. I have submitted a support ticket but since I bought it over a year ago I wouldn't blame them if they balked at replacing the remote. I was one of those affected by the failed Lishen cells Group Buy so never owned a 24v LFP battery long enough to test the new inverter. No more warranty.

 
Very famous brand... I'm sure you've heard of it... "The Cheap Co" ! Seems to work well enough for my, very infrequent, 120v needs. The only remaining problem appears to be the USB-connected remote control panel... it shows a nice battery voltage status screen but the On/Off switch does not appear to work... no reaction to a button press. I have submitted a support ticket but since I bought it over a year ago I wouldn't blame them if they balked at replacing the remote. I was one of those affected by the failed Lishen cells Group Buy so never owned a 24v LFP battery long enough to test the new inverter. No more warranty.

Keep us posted on whether a smaller resistor solves the problem. I am optimistic that if the 20ohm resistor does not work, the 15ohm will.
 
Keep us posted on whether a smaller resistor solves the problem. I am optimistic that if the 20ohm resistor does not work, the 15ohm will.
Will do.
I'm on the road for a few days but it will be first on my list when I get back. Thanks again for your help!
 
Keep us posted on whether a smaller resistor solves the problem. I am optimistic that if the 20ohm resistor does not work, the 15ohm will.
OK... I'm back. I took the trailer out for a shake down cruise with the new battery, etc.

1) In the past, every time I created a fake "dead short" by flipping on the in-line power to the 2,000 watt inverter the BMS would shut things down instantly when it detected the big inverter capacitor power in-flow and then, about 30 seconds later, would reboot the battery and return everything to normal. Inverter still off of course.

2) Now, on the road, for some unknown reason the battery stopped rebooting... it would just sit there until I manually reset the breaker and rebooted it. The error messages in the BMS software didn't change either.

3) Oddly, my double resistor (20ohm, 25W in series) pre-charge by-pass circuit totally failed me on the trip... worked every time while parked at home! Go figure!

4) Got home, removed one of the resistors, and it worked like a charm! I leave the manual on/off switch on the inverter box in the "on" position. When I start the 5-second pre-charge the screen on the inverter lights up and then I release the pre-charge momentary switch and flip the main in-line power switch the moment the box makes it's first error beep.

Bingo! No fake short and everything keeps on running. I don't think I'll even bother trying the new 25 watt, 15 ohm resistor I bought!

Mind you now... remember that everything worked great the LAST time I was doing my testing in the garage too. Well, I have another shake down cruise on Tuesday... let's see if the single resistor solution continues to work !!

Thanks again for your help! Looks like I MIGHT just have coffee this time !!
 
Bad news... something must have changed. Nothing I try now seems to work. Is there really no hope that SS will come up with a solution to this problem? I mean who uses LiFePO4 batteries without also using an inverter? I do realize that for those using these EG4 batteries in an Off Grid setting the battery and the inverter seldom get turned off... so this isn't a problem for them... but some of us are using these items in an RV setting and the inverter is usually OFF. There MUST be some way to turn them on? Getting desperate here.
 
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 photo below). Nothing worked... including everything you've read in the above posts and other threads. 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. 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 (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 and I... at LONG last... am a very Happy Camper !!!
 
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