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Old 07-01-2022, 09:40 PM   #1
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Precharging inverters

Recently I became aware why in some cases you want to precharge an inverter. A precharge prevents the spark like when the battery cable is attached.


I'm hoping someone who has real knowledge about this subject can shed more light.


Some questions I have include:


1- At what size of inverter is precharge advisable? I've heard about 2000 watt.


2- Any reason a properly sized power resistor won't do a suitable job?


3- Do you have details how you may have installed aprecharge device?
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Old 07-01-2022, 10:48 PM   #2
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I’m not familiar with the term “pre-charging” an inverter. I assume that it means charging the house battery bank up with an external battery charger before connecting them to the inverter? If that’s what you mean then my thoughts are that since I understand that the spark you get when you first attach the 12V battery cables to the inverter is a result of the very large capacitors in parallel with the battery input inside the inverter drawing a very large current for a fraction of a second as they charge up and that the spark is harmless.
Having said all that though, I am sure that charging the batts up before hooking them to the inverter would not cause any harm either.
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Old 07-01-2022, 11:04 PM   #3
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The term "inverter precharge" deals with charging up the inverter's capacitors prior to applying main power. It's a way reduce that large initial load when powering on an inverter. Some lithium BMS may shut down when hit with that large of a load.
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Old 07-01-2022, 11:23 PM   #4
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Oh, I understand now. What would you use to charge the caps? Lead acid batt?
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Old 07-02-2022, 06:33 AM   #5
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A 100 ohm 2 watt resistor will work and will precharge the inverter caps at .1 amps. The time to charge will depend on the value of the caps in the inverter.
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Old 07-02-2022, 06:39 AM   #6
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You can also use a light bulb in series, just like a resistor to pre charge.
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Old 07-02-2022, 07:02 AM   #7
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A 100/150w incandescent bulb makes an excellent Capacitor all alone. Used one 60 years ago to keep a dying refrigerator going for several days until a new one was in place.
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Old 07-02-2022, 07:07 AM   #8
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I use a 50w 25ohm resister to precharge a 5kw 12v inverter.

https://www.amazon.com/dp/B07SPC8CKK...p_mob_ap_share
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Old 07-02-2022, 07:20 AM   #9
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What good is all this precharge worry when there are obviously going to be unattended outages and restarts?
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Old 07-02-2022, 08:23 AM   #10
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Restarts are not the issue.
The issue is the physical connection from inverter to battery creates a spark that degrades the connection.
This adds more resistance,heat reducing performance.
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Old 07-02-2022, 08:29 AM   #11
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If that's the case then final connection should be done away from batteries and explosive gases.
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Old 07-02-2022, 08:32 AM   #12
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Quote:
Originally Posted by TandW View Post
If that's the case then final connection should be done away from batteries and explosive gases.
that isn't the real problem although with lead acid yes it can be a real problem.

It's the issue with the BMS in the lifepo4's

It doesn't matter if you're a football field away well you get my point I hope..
That sudden spike
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Old 07-02-2022, 10:01 AM   #13
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Quote:
Originally Posted by wolfgolden View Post
I use a 50w 25ohm resister to precharge a 5kw 12v inverter.

https://www.amazon.com/dp/B07SPC8CKK...p_mob_ap_share



Ok you understand what I'm thinking here. I was planning on using a 25 watt 30 Ohm resister for a Xantrex 2000 watt inverter. Any thoughts on that?


How did/would you connect every thing? I want to use relay(s) so the control switches are elsewhere from the main power switching.
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Old 07-02-2022, 10:39 AM   #14
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Modern electronics can literally be made to walk, talk, and blink their big blue eyes at you. It is not a matter of what installation procedures should you use on all of a given appliance. It is a mater of what you should do for your specific appliance.

Each design has its own requirements. The manufacturers installation and operation instructions are the best source for how to do it.

Some make and model inverters/converter/chargers use a large capacitor connected to the 12 volt DC supply/output. These are large enough to draw extremely high current for very short times.

A system of charging them slowly can reduce arc and high pulse current damage to connectors, switches, and electronic devices.

Other designs use other techniques. They have their specific requirements.

See installation and maintenance instructions for your device for the best practice.
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