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show and Tell - Spark retardant switch when connecting a battery

Jim Burrow

Solar Enthusiast
Joined
Jun 27, 2020
Messages
516
We all know that if you do not put at least a 50-ohm resistor between the controller input and the positive side of your battery, for a moment, you will get a big spark and scare you to death. The resistor prevents that initial spark from the capacitor in the controller/inverter from discharging to fast.

In my setup, I have a circuit breaker between the positive terminal of the battery and the battery positive input on the controller. – see the image!

So, what I do every time I want to shut down my controller (I’m still in the testing phase), I always pop the circuit breaker, so the battery does not have a small discharge current while everything is off. – In theory!

But before I reset the circuit breaker in order to connect the battery to the controller, and turn on the controller, I have added a 50-ohm resistor in series with a push-button switch, (like a doorbell) across the breaker terminals.

To prevent a large spark when I connect the battery right away, I push the button switch for a moment to discharge that capacitor then I can go ahead and reset the breaker with no spark.

The image shows the push button, 50-ohm resistor, and the battery circuit breaker wired along with the 2424Lv -MDS controller/inverter by MPP

1603573447568.png
 
To prevent a large spark when I connect the battery right away, I push the button switch for a moment to discharge that capacitor then I can go ahead and reset the breaker with no spark.
Inverter capacitors will most likely have no energy in them when delivered. When one connects voltage to the capacitor one is charging the capacitor, not discharging it. When connected for the first time there is a surge of current going into the capacitor and that is the reason one will see sparks. This is why a resistor is recommended to pre-charge the capacitors. The resistor is supposed to be connected in series between battery positive and the inverter positive. Or between battery negative and inverter negative. I have seen it takes around 25 seconds to pre-charge the inverter capacitor.

If voltage is disconnected from the capacitor it will immediately begin to lose energy on it's own. So when voltage is applied to the capacitor it will be taking a charge. The surge will be dependent on the length of time the capacitor was disconnected from the battery. The only way to discharge a capacitor is to short it, apply a load to it, or let it discharge on it's own with no voltage applied. If shorting the capacitor one must be careful depending on the ratings of the capacitor.
 
As a retired Electronic Engineer, I have never seen a discharge capacitor spark when you apply a voltage to it that's because there is no energy in it to go anywhere. But I have always seen capacitors spark when charged and you touch them to the ground.

I do not know what is in the controller that makes it spark, but everyone says it's a capacitor. So I'm going with that. I know with my controller when I connect the positive post to the controller battery input ports, I get a very big spark. Similar to touching the positive post of a car battery to the negative post by accident. Like jumping the battery on another car to start it and you screwed up.

However, with respect to my controller and battery, whenever I touch a 50-ohm resistor between the controller positive terminal and the positive terminal of my battery, for less than 10 seconds, I never ever get a spark. So I might be doing something wrong, but it works for me.

According to MPP support, the big spark doesn't hurt their controller.
 
Yes, the resistor is acting to charge the capacitor slower instead of the huge current rush when a solid contact closes. All inverters have a fairly large capacitor and most do recommend a pre charge before connecting the battery. On my Schneider, I use a large 8 ohm resistor. It makes a tiny spark, and then the inverter actually powers up. The fan even cycles and the display comes up before I turn on the disconnect switch. My 8 ohm one is a bit overkill, but I have it for amplifier load testing. The 50 ohm will charge up a bit slower and might not get it fully charged, but as you have seen, it eliminates the spark as it brings the capacitors up at a reduced current. At 48 volts difference (56 volts at the battery), my 8 ohm resistor pulls over 6 amps. Your 50 ohm would be about 1 amp. So that is not bad. Without using a pre charge resistor, you can easily pull well over 200 amps and I have seen it pop expensive fuses. On larger battery banks and inverters with big caps, it can burn off a bit of molten metal and send it flying. Think about the sparks from an arc welder.
 
Can't say it's ever worried me. Not like it's going to kill you.

As a Kid I used to get those Disposable cameras with the flash and charge them up and short the capacitor to make a bang. Used to parallel them Till shorting them blew the legs off the caps and then I soldered them to heavier copper wire I stripped the insulation off which would be akin to bus bars and wired that to a Knife switch I made up with a wooden ruler for a handle. The Contactor was a 4" nail and the earth was a small piece of 1/2 Steel plate. Used to sound like gunshots. You could often feel the splatter coming off as well sometimes. I took about an inch off that nail in the end and almost drilled a hole half way through the plate.

I had 4 boards and about 20 caps on that thing. Wonder I didn't kill myself but geez it was fun especially showing it to friends and getting them to close the switch telling them a light would come on.

Don't really see the problem with this, You know it's normal, you know it's coming, it's a bit of fun to me!
Makes me smile every time! :0)
Well that's true. I wanted to add that statement as a caution thing.

I had fun too when I was a kid with those things. I liked hooking them up with reverse polarity and watch them pop and self destruct with paper hanging out of them. It was fun for sure. It's when I didn't know it was coming it scared the crap out of me. Yup, had a few accidents playing around with circuits.
 
@Jim Burrow There is nothing wrong with your design. My point was the purpose of your design is to charge the capacitor, not to discharge it. Maybe I am misunderstanding something but that's what you have been saying and that is simply not true.
 
Don't really see the problem with this, You know it's normal, you know it's coming, it's a bit of fun to me!
Makes me smile every time! :0)

Only problem is if you are using a contactor between the batteries and inverter. Without a precharge circuit the contactor is likely to get welded in the closed position.

I’ve seen this happen several times (on “professional” installations)
 
Am also concerned about this since my Reliable 2500W inverter is rarely connected to my golf cart batteries. Has lots of time for the caps to discharge. On the initial connection I used a 60W incandescent bulb as a resistor and was amazed it would light for a few seconds.

However I now use Anderson connectors and there is no convenient method to connect the bulb. Maybe I will install an external bulb socket with alligator clips for the temporary connection.
 
Yes, the resistor is acting to charge the capacitor slower instead of the huge current rush when a solid contact closes. All inverters have a fairly large capacitor and most do recommend a pre charge before connecting the battery. On my Schneider, I use a large 8 ohm resistor. It makes a tiny spark, and then the inverter actually powers up. The fan even cycles and the display comes up before I turn on the disconnect switch. My 8 ohm one is a bit overkill, but I have it for amplifier load testing. The 50 ohm will charge up a bit slower and might not get it fully charged, but as you have seen, it eliminates the spark as it brings the capacitors up at a reduced current. At 48 volts difference (56 volts at the battery), my 8 ohm resistor pulls over 6 amps. Your 50 ohm would be about 1 amp. So that is not bad. Without using a pre charge resistor, you can easily pull well over 200 amps and I have seen it pop expensive fuses. On larger battery banks and inverters with big caps, it can burn off a bit of molten metal and send it flying. Think about the sparks from an arc welder.
Thanks for explaining it better than me. You're 100% correct. Anyone ever reading a manual written by an engineer knows how confusing it can be. I'm no exception, Although what I write makes clear sense to me can be confusing to others.

Thanks again for making my point clear to everyone.
 
@Jim Burrow There is nothing wrong with your design. My point was the purpose of your design is to charge the capacitor, not to discharge it. Maybe I am misunderstanding something but that's what you have been saying and that is simply not true.
As a retired engineer, I'm no exception with it comes to writing. What is clear to me can be very confusing to others.
 
Am also concerned about this since my Reliable 2500W inverter is rarely connected to my golf cart batteries. Has lots of time for the caps to discharge. On the initial connection I used a 60W incandescent bulb as a resistor and was amazed it would light for a few seconds.

However I now use Anderson connectors and there is no convenient method to connect the bulb. Maybe I will install an external bulb socket with alligator clips for the temporary connection.
That will work. To get fancy, you can add a switch in series with the light bulb. Think of it as a light switch in your house.
 
Only problem is if you are using a contactor between the batteries and inverter. Without a precharge circuit the contactor is likely to get welded in the closed position.

I’ve seen this happen several times (on “professional” installations)
I'm doing what you see in Wells videos, expect I made the resistor permanent, and make the connection momentarily with a push-button switch in series with the resistor.
 
I bought some Controllers to turn lights on at dark a while back. Got the AC and DC voltages which looked identical.
Of course some time later I wired one up in a circuit and powered it up and Nothing then BLAM! The thing blew to pieces, just the wires sitting there and nothing else. For some reason it didn't scared me but I was in awe that the little devise just disappeared without a trace and I still had my eyesight and no shrapnel wounds.

Took a minute to Figre out, some Clown just put them all together and forgot some were 12V and some were 240. Got the right one replaced in and no more fire works.

Lots of Vids on YT of people blowing caps. Good job YT wasn't around when I was a kid although I would have probably been as big of a teacher for this sort of Mischief as I would have been a student.

I was pretty careful in my stupidity as I was pretty certain a mistake would kill me or make me very sorry for a long time if I lived.

The only time I got bit was after I had played with the thing the day before or whatever and it was not powered up. ( Luckily for me) .
I have read caps won't "recharge" or what ever the term is by themselves but I'm telling anyone who thinks that's its crap its not and I can build a demonstrating unit to prove it.

I had Discharged the caps and left them some time and touched the things and got a belt. After that happened a couple of times ( yeah , I'm a slow Learner) I made sure to do the main discharge and a couple more for good measure.
Still got bit a day or so later. They will definitely come up with a residual charge after having no voltage.

After that I would take the rubber band off the "switch" holding it open so the contacts were permanently closed and the caps were dead shorted all the time and couldn't regenerate any Zap in them.

Lesson I have never forgot. whenever I open anything up now and see any caps of any decent voltage, First thing I do si get a screwdrive and short the things out. If I hear a noise or see a spar, I get an alligator clip lead and leave it there while I'm working on the board.
Especialy careful of this when I scrap old solar inverters for parts. They have some great caps in those things!
Although I have not experienced a shock from a discharged capacitor, you are right, large capacitors can build back up a residual charge that can shock you if you get your figures across the ends of it.
 
The only time I got bit was after I had played with the thing the day before or whatever and it was not powered up. ( Luckily for me) .
I have read caps won't "recharge" or what ever the term is by themselves but I'm telling anyone who thinks that's its crap its not and I can build a demonstrating unit to prove it.

Yep, it's real: https://en.wikipedia.org/wiki/Dielectric_absorption ;)
 
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