Idea to safely charge inverter capacitors

Lionking45 · Jan 23, 2022

Posting this to solicit feedback on any flaws in my logic

My idea is to create a switched parallel circuit to the circuit breaker on the negative leg from the battery to the inverter. There is a lamp and a 20 ohm resistor in this parallel circuit. This parallel circuit is initially open. The main negative breaker is open. The positive breaker side is closed I will then close the switch to bypass the breaker and allow current to pass through but the current is limited by the resistance in the resistor and the lamp. As the voltage in the capacitor builds up, the lamp will turn off as the voltage differences between the battery and the inverter capacitors equalize and go to zero. I will then confirm the voltage through a meter and only then close the negative breaker. I will open the switch on the parallel circuit before I close the breaker.

Thanks for any input.

Ariel

Smokin · Jan 23, 2022

The resistor goes on the positive breaker in the Jack Rickard design. Yeah Baby, Blow it Up...PoooooF

Ampster · Jan 23, 2022

MichaelK said:
Stick with Magnum, Outback, Schneider, and Victron, and this problem is a non-issue.

I have an Outback Skybox and have noticed only a little spark if I start with the battery. In most cases I turn on the AC input first. Perhaps there is some form of internal precharge circuit in my Skybox?

meetyg · Jan 25, 2022

I'm planning an automatic pre-charge for my 1500w inverter (for a DIY solar generator):
Parts:
- 12v time delay relay
- 12v Lifepo4 battery
- 12v 1500w inverter
- Anderson connectors
- 5-10w Ceramic resistor
- DC circuit breaker (MCB)
- Wiring

Basically, the inverter and battery will be disconnected until I need to use them.
When I connect the Anderson plug from battery to inverter, the 12v relay will also be powered. When the relay is powered, it will connect the input and output side of the DC MCB using the resistor, for a pre-defined time (say 5-10 seconds).
During this time a built-in LED lights up in the relay stating that it's connected. When the light turns off (relay delay time has passed, and relay opens) then I know that the inverter is pre-charged and I can turn on the MCB.

Does this help you?

Ampster · Jan 25, 2022

meetyg said:
Does this help you?

Thanks. That may help other readers. I think my Outback Skybox has some form of precharge circuit.

OzSolar · Feb 13, 2022

MichaelK said:
A different solution would be to buy a quality inverter instead of the cheapo imported junk being hyped here. Stick with Magnum, Outback, Schneider, and Victron, and this problem is a non-issue.

My 5000 watt Victron Quattro requires pre-charging. Well actually I think it's more accurate to say that my LiFePO4 batteries require that my inverter be pre-charged or their internal BMS will activate.

WindWizard · Mar 16, 2022

OzSolar said:
My 5000 watt Victron Quattro requires pre-charging. Well actually I think it's more accurate to say the my LiFePO4 batteries require that my inverter be pre-charged or their internal BMS will activate.

My Magnum 4400 Watt Inverter trips the BMS without a precharge resistor.

Will Prowse · Mar 16, 2022

MichaelK said:
A different solution would be to buy a quality inverter instead of the cheapo imported junk being hyped here. Stick with Magnum, Outback, Schneider, and Victron, and this problem is a non-issue.

You still need to use a pre-charge resistor with these inverters.

Can you please provide evidence of pre-charge circuits on all of these inverters that you listed? I am checking the manuals and am not seeing a single instance where it is mentioned.

If you are making a claim without evidence, your post will be removed.

Will Prowse · Mar 16, 2022

Ampster said:
Thanks. That may help other readers. I think my Outback Skybox has some form of precharge circuit.

I am reading the Skybox manuals right now and cannot find this being mentioned. Can you please provide proof of this precharge circuitry?

Will Prowse · Mar 16, 2022

Lionking45 said:
Posting this to solicit feedback on any flaws in my logic

My idea is to create a switched parallel circuit to the circuit breaker on the negative leg from the battery to the inverter. There is a lamp and a 20 ohm resistor in this parallel circuit. This parallel circuit is initially open. The main negative breaker is open. The positive breaker side is closed I will then close the switch to bypass the breaker and allow current to pass through but the current is limited by the resistance in the resistor and the lamp. As the voltage in the capacitor builds up, the lamp will turn off as the voltage differences between the battery and the inverter capacitors equalize and go to zero. I will then confirm the voltage through a meter and only then close the negative breaker. I will open the switch on the parallel circuit before I close the breaker.

Thanks for any input.

Ariel

This design looks good. I do not see a problem with adding a resistor in parallel to a circuit breaker unless there is a safety issue that I am not realizing. You could have an inline fuse on the battery side of the resistor.

On server rack BMS, the pre-charge resistor circuit is connected in parallel to the bank of charge/discharge FETs. Making a DIY precharge circuit seems easier to add on the circuit breaker as you plan to do.

Brett V · Mar 16, 2022

MichaelK said:
A different solution would be to buy a quality inverter instead of the cheapo imported junk being hyped here. Stick with Magnum, Outback, Schneider, and Victron, and this problem is a non-issue.

Not everyone has the resources to purchase high end gear. Your reply wasn't helpful at all.

Zil · Mar 16, 2022

A solution looking for a problem? I notice the OP has a 125 amp breaker.
I have a 2000W PSW samlex 12 volt inverter. A moderate priced device that has served well for close to 4 years.
I generally use the Inverter on/off switch to rest the inverter. That doesn't disconnect the battery from the capacitors.
I do have a blue Sea M series battery switch to disconnect my battery bank. It is used often, but not daily. I have not experienced any problem switching the battery bank off and then back on weeks later. When I first connected the new inverter there was a spark, a very small, very quick, spark when the negative cable made contact at the negative BusBar. That was before I installed the battery switch.
The manual mentions the capacitor charging but doesn't make an issue. It just instructs to make the final connection away from the inverter. Actually it states to insert the fuse last to insure "the sparking is limited to the fuse area".
At least on lower wattage inverters, lower dc amperes, this inrush spark is not the problem many think it will be. Certainly not during my hands on experience.
If you do have a resistor bypassing a breaker or fuse, you must have a fuse to protect that smaller wire.

Ampster · Mar 16, 2022

Will Prowse said:
Can you please provide proof of this precharge circuitry?

I don't have any proof. It was speculation. What I have noticed is the spark does not seem to occur if I boot up the skybox with AC first before powering the DC breaker.

Texican · Mar 16, 2022

Can someone briefly explain the issue here ?
I am building a dual LV6548 inverter system using 6 eg4 batteries in the signature solar rack

if I turn on all the batts and then turn on the inverters is there a problem if I doint do this precharge procedure ??
thanks

Will Prowse · Mar 16, 2022

Texican said:
Can someone briefly explain the issue here ?
I am building a dual LV6548 inverter system using 6 eg4 batteries in the signature solar rack

if I turn on all the batts and then turn on the inverters is there a problem if I doint do this precharge procedure ??
thanks

John Frum · Mar 16, 2022

Texican said:
Can someone briefly explain the issue here ?
I am building a dual LV6548 inverter system using 6 eg4 batteries in the signature solar rack

if I turn on all the batts and then turn on the inverters is there a problem if I doint do this precharge procedure ??
thanks

Since you have 6x eg4 batteries the problem is divided by 6.
I suggest you turn on all the batteries before you turn on the inverters.
Also the eg4 batteries have a priming circuit if I'm not mistaken.

Zil · Mar 16, 2022

Will Prowse said:

One of Will's better videos. I am glad he covered eye protection. I wish he had showed the Spark. That battery terminal is not really "totally destroyed". Puffery is permitted.

Texican · Mar 16, 2022

smoothJoey said:
Since you have 6x eg4 batteries the problem is divided by 6.
I suggest you turn on all the batteries before you turn on the inverters.
Also the eg4 batteries have a priming circuit if I'm not mistaken.

Thanks Joey, good to go I guess then

toms · Mar 16, 2022

Will Prowse said:
If you are making a claim without evidence, your post will be removed.

Crikey! If you did that half the forum would be removed.

A quality inverter doesn’t need a precharge. A BMS/circuit breaker/contactor may need one no matter the inverter though

GLC · Mar 16, 2022

Wills video explains it very well. Using his example, it always works.

Idea to safely charge inverter capacitors

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