Pre-Charge Resistor Recommendation

How about a 20 ohm 100 watt power resistor?  https://www.mouser.com/ProductDetail/Vishay-Milwaukee/MCRL010020R00JHB00?qs=dgwYPV2%2FDPgRJb%2B%2BjM3lFQ%3D%3D

But I'm cheap, so I'd probably just use three automotive brake light bulbs in series.

Thanks for that. I guess that is also a option. I would like to also calculate the timing needed depending on the resistor used. I have been looking around and it looks like I need the inverter’s capacitance to calculate the timing it the pre-charge will take.

<a contenteditable="false" data-ipshover="" data-ipshover-target="/profile/2-sid-genetry-solar/?do=hovercard" data-mentionid="2" href="/profile/2-sid-genetry-solar/" rel="">@Sid Genetry Solar  or <a contenteditable="false" data-ipshover="" data-ipshover-target="/profile/1-sean-genetry-solar/?do=hovercard" data-mentionid="1" href="/profile/1-sean-genetry-solar/" rel="">@Sean Genetry Solarwhat would you guys recommend? My bms will disconnect the batteries if voltage gets out of range and reconnect once charged. Contactors are used to protect the batteries. 

2 hours ago, jony787 said:

Thanks for that. I guess that is also a option. I would like to also calculate the timing needed depending on the resistor used. I have been looking around and it looks like I need the inverter’s capacitance to calculate the timing it the pre-charge will take.

<a contenteditable="false" data-ipshover="" data-ipshover-target="/profile/2-sid-genetry-solar/?do=hovercard" data-mentionid="2" href="/profile/2-sid-genetry-solar/" rel="">@Sid Genetry Solar  or <a contenteditable="false" data-ipshover="" data-ipshover-target="/profile/1-sean-genetry-solar/?do=hovercard" data-mentionid="1" href="/profile/1-sean-genetry-solar/" rel="">@Sean Genetry Solarwhat would you guys recommend? My bms will disconnect the batteries if voltage gets out of range and reconnect once charged. Contactors are used to protect the batteries. 

I personally just use a 1500W space heater (or 700W heat gun--or a 300W floodlight--whatever's handy that uses a lot of power).  Very minimal spark, and you're guaranteed to get the unit charged up in under a second.  (If the resistor is too big, the inverter's quiescent startup loads will prevent it from actually charging up.)

Any resistance is better than a "splat" spark connect--which would be up to tens of thousands of amps inrush.  The resulting surge can blow other DC appliances out (I speak from experience!) if they don't have a sufficiently high voltage rating.

The current GS6 units have 40,000uF of capacitance.

2 hours ago, jony787 said:

My bms will disconnect the batteries if voltage gets out of range and reconnect once charged. Contactors are used to protect the batteries. 

My suggestion would be to configure your appliances/systems to not hit said limits.  If you live on the BMS, batteries (and equipment) won't be too happy.

In an ideal world, the BMS will never do anything.  Like the guard rails on a highway.  They're there for safety--but you shouldn't make a habit of driving on them.

Since I deal a lot with electric cars and stuff I have a lot of need to do these sort of calculations. The usual rule of thumb is to use 3RC as your precharge time. That is, take the resistance of your precharge resistor, multiply it by the capacitance of the thing you are precharging, and then multiply that by three. This is because you will charge about 2/3 of the remaining capacitance in 1RC time. 3RC thus charges you to (1-(1/3)^3) * 100 percent of the full voltage. In this case, that's about 96.4% which is close enough. Then you close your main contactors. Well, the easiest approach is to wire your precharge resistor in series with the positive all of the time. Then to precharge you close your negative contactor which completes the circuit. You do this for 3RC seconds then close the positive contactor. It probably has a resistance of only a few milliohms so the precharge is effectively an open circuit compared to the positive contactor and doesn't hardly pass any current through it. However, naturally, with this approach you must never open the positive contactor under load as the first move. This would be bad. Instead it's better to open the negative contactor first otherwise you'd be trying to power everything through a precharge resistor and that's no bueno. You can gain more safety by having a precharge relay as well.

For the above suggested 20 ohm resistor this would be 20R * 0.04F * 3 = 2.4 seconds of precharge. This might be too slow. 5 ohms instead would be a quarter the time or 0.6 seconds. It's best to have a surge rated resistor as 40v through 5 ohms is 320 watts surge. A suitably chunky 100 watt resistor would probably be OK. Usually surge rated resistors are rated to about 10x their constant rating for surges. So a surge rated 30 watt resistor may just barely be enough but 40 watt or greater (and surge rated) would be ideal.

I ended up using two 6 Ohm resistors in series for a total of 12 ohms and this is what I did to find out the precharge time.
I used the multimeter to measure voltage between positive side of inverter and positive side of batteries. 

With precharge contactor disconnected, voltage should measure close to battery voltage. Once precharge resistor engages, I waited until voltage went down. In my case, it was 0.650 volts and it took around 5 to 6 seconds. I setted my precharge timer to 10 seconds and so far Inverter and BMS are happy. Thanks for the help

I use a 1500w heater set to high as my resister 😄

So do I.  Did that at Sean's once and sent him for a loop 😉.  Very effective.