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Old 03-21-2023, 12:04 AM   #1
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Precharging inverter capacitors

This might be common knowledge but I was not aware of this. I recently had the coach batteries out. When reconnecting the wires I had a large spark when connecting one of the wires. I stopped and checked everything with meters and verified my photos that everything was going on the correct location. The second time there was no spark. Everything worked fine and I forgot about the issue. Then watching a video on the victron multiplus I saw that on larger inverters you need to precharge the capacitors by using a resistor to limit the voltage surge. To do this you place a resister on the positive lead and touch the restor to the battery connection for a few seconds. Once that is done you remove the resister and can safely connect the lead without sparks. While our systems are not large, if this eliminates the sparking I am going to try this next time I have the batteries disconnected.
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Old 03-21-2023, 01:39 AM   #2
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Yes itís quite shocking (pun intended) the first time you experience that.
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Old 03-21-2023, 08:42 AM   #3
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A 12v tail light bulb or similar (non LED) will also work if a resistor isn't available.
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Old 03-22-2023, 11:34 AM   #4
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This "sparks" all kind of memories... Light bulb can be your friend as mentioned above.
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Old 03-22-2023, 03:35 PM   #5
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Hmmmm, never encountered that problem. My inverter is only 1000 watts. Maybe the spark was contained in my battery disconnect switch that I have on each battery. The cables are connected to an open post until you screw the switch knob in place. Would be hard to see a spark there.

Ah, the memories of charged capacitors. Not only are resistors used to slow the charge going into a capacitor but they are also used to discharge caps. Evil electronic techs would leave charged caps around for some unknowing fool to touch. Without a resistor to bleed off the cap, the charge would remain for a very long period of time.

I am very familiar with charged capacitors. When I worked in the field I serviced equipment in libraries that had caps charged to 320 volts DC that would stay there if you did not engage the safety interlock to bleed them off. Techs would often (usually only once) forget to verify they were bled off and then accidentally get across the terminals with a screwdriver. Instant very loud BANG, like a gunshot, in a very quiet library. Not so bad in the old days where the result was major embarrassment. With the world today, hearing that loud bang might cause people to hit the floor.


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Old 03-22-2023, 04:12 PM   #6
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My Sensata 2400 watt LF inverter shut down all 3-100 AH li-iron batteries BMS boards at once the first time I connected it. I only did that once!

Also great fun in electronics class many years ago charging high voltage capacitors and the "pop" of discharge melting screwdriver tips with sparks flying.
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Old 03-22-2023, 09:14 PM   #7
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you can see it when plugging in a switching power supply for a lap top....for example
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Old 03-22-2023, 09:31 PM   #8
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so... who remembers how to discharge the second anode of a CRT ? And which one is a better bang? or Which lead of a flyback transformer is the hottest?

I remember hooking up 250uf caps to an AC cheater cord draped under a classmates chair in electronics class... that was FUN!
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