Do I need a pre-charge resistor for a 300 watt inverter?

[bob1]

Hello I am using a 300 watt inverter, I am sure it would be considered small to most of the people here that are running >5000 watt systems. However even with such a low rating, I am getting sparks when I connect it to the battery.

I did some searching and it looks like Will says you should have one for "large" inverters.

How large is large and what specs do I need on the resistor?

 

[curiouscarbon]

hello!

if it is a 300 W inverter and 12V, then i suggest to not worry about using a resistor to precharge.

for my 300 W giandel 12V inverter, it makes a tiny spark, so i tap the side of the ring terminal so the flat part is happy.

if you really want to do it, a 1 ohm 25-100watt resistor should be workable with 12V

good luck ?

 

[bob1]

Is there a way to calculate the watt and resistance of the resistor that is needed mathematically?

 

[JoeHam]

This will do :

Amazon.com: YiePhiot 2PCS 50W 25 Ohm Resistor Aluminum Case Wirewound Chassis Mounted Compatible with Ring Doorbell, Nest Hello Doorbell, SkyBell Doorbell Etc : Industrial & Scientific

Amazon.com: YiePhiot 2PCS 50W 25 Ohm Resistor Aluminum Case Wirewound Chassis Mounted Compatible with Ring Doorbell, Nest Hello Doorbell, SkyBell Doorbell Etc : Industrial & Scientific

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Or an incandescent light bulb, or even a pencil.

 

[Ellcon123]

Any 5 or 10W resistor around 100 ohm will be fine. You will soon learn how long you need to connect it for but can't imagine more than 10s for a 300W inverter. My 3000W inverter needs about 30s.

 

[Al D]

Is there a way to calculate the watt and resistance of the resistor that is needed mathematically?

You would need to know how big the capacitor in your inverter is.

 

[Ellcon123]

Don't need to over think it. Not connected long enough and you will still get a spark. Hold for 30s and you'll be fine.

 

[Bud Martin]

Also have inverter power ON/OFF switch in the OFF position when connecting the inverter to the battery.

 

[time2roll]

Is there a way to calculate the watt and resistance of the resistor that is needed mathematically?

Ohms law. Say you want 500mA current and have a 24v system. V / A = R. or 24V / .5A = 48 Ohm resistor.
Wattage is Amps x Volts. so 12 watts. Probably 5 watts is fine as by the time it starts to get warm the event is over. Some like more wattage rating so that the resistor is physically larger to hold in your hand.

 

[HRTKD]

I have a Victron 400 watt PSW inverter that I use periodically. Between uses it is disconnected from the battery, so each time the battery is reconnected, the capacitors need to charge. I have a circuit breaker between the battery and inverter. I flip the breaker on after connecting the battery and I don't see any spark. Is it there? Probably, internal to the breaker. It's a good quality breaker so I'm not worried about it. I don't have a pre-charge circuit, nor do I plan to implement one. I use this inverter with both lead acid and LiFePO4 batteries, but not both chemistries at the same time.

On my Victron Multiplus 12/3000, connected to my 560 Ah LiFePO4 battery bank, I don't have a pre-charge circuit there either. The original concern that brought about the pre-charge circuit was overloading the BMS with the brief, but high amp surge to charge the capacitors. With two batteries in parallel they have no problem satisfying the required surge amps. The quality and specifications of the BMS could also come into play. If I had a single battery, I might need the pre-charge circuit.

 

[bob1]

Also have inverter power ON/OFF switch in the OFF position when connecting the inverter to the battery.

I always have it off when I connect the battery. Today I just connected it and there was a huge 2 inch spark that startled me, yesterday it was barely half an inch. I'm worried about the terminals being melted or flying into my face.

Maybe I'll just leave the inverter connected for now even though it might drain the battery by a small amount, until I get a resistor.

I can't just use one of these tiny ones I have laying around right?

Ohms law. Say you want 500mA current and have a 24v system. V / A = R. or 24V / .5A = 48 Ohm resistor.
Wattage is Amps x Volts. so 12 watts. Probably 5 watts is fine as by the time it starts to get warm the event is over. Some like more wattage rating so that the resistor is physically larger to hold in your hand.

I was thinking if there was a way to estimate the farads of the capacitors based on the wattage of the inverter. Then using voltage, and arbitrary current picked to get the desired resistance without causing sparks (like 5 amps), and then another equation to figure out how long it would take to charge to make sure you don't need to hold it for more than a few seconds.

 

[time2roll]

Need to decode them. Couple megohms and it will be slow.

 

[Bud Martin]

I always have it off when I connect the battery. Today I just connected it and there was a huge 2 inch spark that startled me, yesterday it was barely half an inch. I'm worried about the terminals being melted or flying into my face.

Maybe I'll just leave the inverter connected for now even though it might drain the battery by a small amount, until I get a resistor.

I can't just use one of these tiny ones I have laying around right?

I was thinking if there was a way to estimate the farads of the capacitors based on the wattage of the inverter. Then using voltage, and arbitrary current picked to get the desired resistance without causing sparks (like 5 amps), and then another equation to figure out how long it would take to charge to make sure you don't need to hold it for more than a few seconds.

Number 4 from the bottom is 100 Ohms, you can try that (second one from the bottom is 47 Ohms). The capacitance of the capacitor will determine how long the cap will be charged up to the applied Voltage.
With 12V the max current limit will be 0.12A, so power dissipation will be 1.5W but will last about 1 seconds.
You can also try using 2K2 (5th one from the bottom (Red, Red, Red)) in parallel with 1K5 (3rd one from the bottom (Brown, Green, Red)), it will just take longer for the cap to charge to full.
The capacitor bank for the 300W may only have 4700uF caps.
It takes 5 TC (Time constant) for cap up to the applied Voltage), so you can estimate the capacitance by timing how long it takes for the Voltage at the DC input terminal to reach the applied Voltage.

Resistor-Capacitor (RC) Time Constant Calculator | DigiKey Electronics

Calculate resistor-capacitor (RC) time constant of a resistor-capacitor circuit by entering voltage, capacitance, and load resistance values.

www.digikey.com

 

[curiouscarbon]

those type will work but get quite warm if the power flowing exceeds a watt or two. (those look like 1/4 w type)

 

[time2roll]

Or put your voltmeter on the inverter terminals the first time. Watch the voltage rise. Then you will know if it is 2 seconds or 20 or something else.

 

[Al D]

those type will work but get quite warm if the power flowing exceeds a watt or two. (those look like 1/4 w type)

I've caught one of these on fire once. In my beginning years, I forgot to take the power rating into account. I think I sent a very momentary 1kW through the 1/4W resistor. Lesson learned and not forgotten. Hopefully OP won't make that mistake ?

 

[Ellcon123]

You just need the white ceramic wire wound resistor. 5 or 10W at 100 ohm will do the job easily.

 

[time2roll]

I think I sent a very momentary 1kW through the 1/4W resistor.

Must be a monster inverter to get that high feed through on a pre-charge

I think a 1/4 watt is fine for what is going here.

 

[Lt.Dan]

I've always figured under 2000-3000 watt is unnecessary. For 300 watt? I wouldn't even think twice.

 

[Al D]

Must be a monster inverter to get that high feed through on a pre-charge

I think a 1/4 watt is fine for what is going here.

Good to bring it back into perspective. I don't know that I would use a 1/4W resistor, but I doubt anything like 1kW will be seen via the pre-charge.