My Push Button Pre-Charger Install for the SW 4024 Inverter

[Hedges]

Wait - Is that mine our yours?! How did you take a picture inside my e-Panel?! ?

Remotely enabling the camera in your smartphone. We're listening to you talk to yourself, too.

 

[MurphyGuy]

I find that breaker in Sunny Island isolates its capacitors from the battery. So I think a precharge resistor is desired when charging those caps.
Unless you muck around inside you can't put a precharge resistor around its breaker.
If you have an external switch with precharge resistor, you can open that switch, close SI breaker, apply precharge circuit, close switch.

I don't think that is accurate. While the SI's breaker isolates something, the DC cables will still have battery voltage at them even if the battery is disconnected. This is voltage bleeding out from the SI's capacitors and it takes a good hour to drain down below 10 volts.

I put my meter on it once with the SI breaker off and the battery's main breaker off, and the SI still had 54 volts at the DC input terminals.

When I start up my system from cold, I flip the main battery breaker on, then turn on the BMS. The BMS will activate a precharge relay before engaging the main 400 amp kilovac contactor.. The precharge relay sends bat voltage to the Sunny Island and the capacitors charge slow. Then, once the contactor engages, I flip on the SI breaker and the system starts up.

When I first built the system, I had incorrectly assumed that the SI's breaker would isolate it like a breaker is supposed to.. But it doesn't do that. Using a precharge relay with the SI's breaker in the ON position, will result in a failed start.. The green LCD panel will illuminate, but it will be blank and then it will go dark.

Had an interesting discussion with the engineers at SMA who said to stop thinking of it as a circuit breaker and start viewing it as just a control system on-off switch. In other words, the SI breaker on the front panel doesn't do squat to isolate the inverters from anything.

My system works flawlessly now.. leave the SI main breaker(s) off until the precharge sequence has completed and the kilovac main contactor has engaged to send full power to the inverters. When the SI's breaker is turned on, there is no noise, no slamming, no massive current flow.. just a smooth transition from a dark screen to a "ready" state.

 

[DIDDLYV]

I will be wiring a resistor pre-charge circuit to both my Victron Multiplus II and my 2 epever MPPT Solar Charge Controller Tracer4215BN,

 

[Horsefly]

I will be wiring a resistor pre-charge circuit to both my Victron Multiplus II and my 2 epever MPPT Solar Charge Controller Tracer4215BN,

I'm a bit confused. First, I don't think I've ever heard anyone claim that a pre-charge is needed on a SCC. Second, do you have two separate feeds from your battery, one to the Inverter and one to the charge controller? I'm making an assumption that you are doing a DIY battery and have a BMS. Is it that you have a separate port BMS (instead of common port BMS)?

Anyway, others can chime in here but a separate pre-charge for the SCC does not seem needed. The purpose of the pre-charge is to charge up the large capacitors on the front-end of an inverter, which can look like a short circuit to the batteries for a small fraction of a second. Doing the pre-charge allows the capacitors to charge up gently before you connect the battery. SCC's don't have that large capacitor on the battery side.

 

[FilterGuy]

First, I don't think I've ever heard anyone claim that a pre-charge is needed on a SCC.

I have noticed small sparks when hooking up an SCC, but I have never felt they were large enough to worry about.

 

[upnorthandpersonal]

The reason you need a pre-charge resistor for an inverter is because it has huge capacitors on the input - it's called a DC-Link. This is used to handle the effects of load fluctuations, and usually consists of a few very large electrolytic capacitors in the order of a couple thousand uF each. Because a discharged capacitor acts essentially as a short circuit when applying a voltage, you draw huge currents that can cause very large sparks, burn contacts, and potentially destroy your equipment. You don't have something like that on a solar charge controller, at least not nearly at the same magnitude.

 

[FilterGuy]

You don't have something like that on a solar charge controller, at least not nearly at the same magnitude.

Correct....

 

[DerpsyDoodler]

I have noticed small sparks when hooking up an SCC, but I have never felt they were large enough to worry about.

Agreed. Regardless, my precharge circuit precharges the positive bus bar. As long as the scc breaker is on, its capacitors will fill. My battery monitor also lights up during precharge (as I would expect with any low power consumer).

 

[DIDDLYV]

egnI'm a bit confused. First, I don't think I've ever heard anyone claim that a pre-charge is needed on a SCC. Second, do you have two separate feeds from your battery, one to the Inverter and one to the charge controller? I'm making an assumption that you are doing a DIY battery and have a BMS. Is it that you have a separate port BMS (instead of common port BMS)?

Anyway, others can chime in here but a separate pre-charge for the SCC does not seem needed. The purpose of the pre-charge is to charge up the large capacitors on the front-end of an inverter, which can look like a short circuit to the batteries for a small fraction of a second. Doing the pre-charge allows the capacitors to charge up gently before you connect the battery. SCC's don't have that large capacitor on the battery side.

The plan is the batteries connected to a shunt on the negative side then to the negative bus bar, the positive lead to a 250 amp fuse on each battery then to the positive busbar. Switches to control 12 volt connection to the Multiplus and a resistor pre charge circuit. I want breakers in the SCC wiring also with a precharge circuit. I believe I have seen Will precharge both his inverters and his SCCs when he hooks them up. Resistors and 12 guage wire are pretty cheap so even if overkill it will not break the bank. The bms is common port

 

[curiouscarbon]

Just hopping in to say I think its pretty cool and thoughtful to precharge the SCC Capacitance in addition to Inverter Capacitance

I’m trying to build a system that lasts a long time, and personally think managing inrush currents responsibly is a good way to ensure long operation.

Look what you all have done to me, now I’m feeling curious to set up a hall effect current sensor on a wire and log the data while connecting it from battery to victron bluesolar SCC (that’s the type i have available on hand)! ???

In order to say that it’s unnecessary or overly cautious I need the data log from the above paragraph experiment. Haven’t done that yet..

 

[DerpsyDoodler]

Just hopping in to say I think its pretty cool and thoughtful to precharge the SCC Capacitance in addition to Inverter Capacitance

I’m trying to build a system that lasts a long time, and personally think managing inrush currents responsibly is a good way to ensure long operation.

Look what you all have done to me, now I’m feeling curious to set up a hall effect current sensor on a wire and log the data while connecting it from battery to victron bluesolar SCC (that’s the type i have available on hand)! ???

In order to say that it’s unnecessary or overly cautious I need the data log from the above paragraph experiment. Haven’t done that yet..

The test would only be viable for the specific SCC that you tested, right? The spark that i experienced when I first connected and tested my panels/mppt months ago wasn't too extreme. I installed through breakers for the installation in the RV.

 

[Hedges]

Inverter has large amount of capacitance on battery input to smooth out current surges from generating AC. I don't think they are big enough to smooth out 60 Hz, but rather the higher frequency used to either PWM synthesize 60 Hz (low frequency design) or boost voltage (high frequency design.)

MPPT charge controller is a buck converter, with inductor feeding batter side, capacitors on PV side, and a transistor switch to PWM regulate current draw from PV. No need for much capacitor on battery side, since current delivery is pretty much constant. There is probably a small capacitor filtering supply to its electronics.

So I don't think precharge matters for MPPT, just for inverter. Besides, you probably connect MPPT once and leave it connected, but you might disconnect inverter more often to turn it off.

As for the input capacitors of MPPT, no need for precharge there because PV array Isc is the most they will get. That isn't much higher than operating current.

The one time I would see need for precharge (inrush limiting) is if either using a battery to feed the MPPT, or rectified AC from grid or generator. That's not normal use of an MPPT SCC, but people have connected a hybrid's PV input to an EV battery. I would consider connecting a generator to PV input rather than AC input (where it would have had to be the grid-forming source.) There are inrush-limit components used in some AC power supplies which could be used. Compared to inrush from battery bank, higher voltage lower current, and not trying to supply the motor-starting surge that an inverter draws.

 

[curiouscarbon]

So I don't think precharge matters for MPPT, just for inverter. Besides, you probably connect MPPT once and leave it connected, but you might disconnect inverter more often to turn it off.

Totally agree. Have not encountered an SCC that made a big enough spark to concern me. Inverters, yes.

For SCC rated 50-100A my interest in evaluating inrush goes up. But that’s because I’m very curious. My SCC is rated for 30A and I have no plans to intentionally precharge it unless an inrush test shows an alarmingly high amperage which I don’t expect and might not even check because there are other parts of my system that need design attention?

 

[curiouscarbon]

The test would only be viable for the specific SCC that you tested, right?

Yes absolutely.

The ultimate complex solution to me is a precharge resistor circuit that monitors voltage on the Bus Positive side and triggers contactor when bus voltage stable above programmed threshold for programmed duration. If fail then don’t connect. Thermometer on resistor maybe, to enable safety.

Most precharge circuits seem to be dumb timers. Which I think is fine if designed appropriately. But I do think it would be cool if the precharge circuit could be more intelligent, maybe even logging inrush current on each use and intelligently indicating whether to continue with system boot up.

 

[Hedges]

But I do think it would be cool if the precharge circuit could be more intelligent, maybe even logging inrush current on each use and intelligently indicating whether to continue with system boot up.

Simple enough to do, if the FET or relay disconnect has communication with the load.
Inrush limiter IC with "power good" output signal is common in electronics design.
But I've seen problems with it, when the loads downstream weren't wired to that "power good" signal, instead watched supply voltage to make their own decision. Lots of FETs in the inrush circuit got burned up.

Future inverters designed for lithium batteries ought to integrate the disconnect function (service to BMS) and inrush limiting. Or, BMS should do inrush limiting and provide signal for inverter. They ought to be on speaking terms anyway, to best manage the battery.

 

[DerpsyDoodler]

Simple enough to do, if the FET or relay disconnect has communication with the load.
Inrush limiter IC with "power good" output signal is common in electronics design.
But I've seen problems with it, when the loads downstream weren't wired to that "power good" signal, instead watched supply voltage to make their own decision. Lots of FETs in the inrush circuit got burned up.

Future inverters designed for lithium batteries ought to integrate the disconnect function (service to BMS) and inrush limiting. Or, BMS should do inrush limiting and provide signal for inverter. They ought to be on speaking terms anyway, to best manage the battery.

Samlex evo inverters have this integration. samlex has taken some effort to facilitate and incorporate the use of lifepo4 with the evos (not the precharge part, though).

 

[curiouscarbon]

ADS1115 16-Bit ADC

ADS1115 16-Bit ADC - 4 Channel with Programmable Gain Amplifier

For microcontrollers without an analog-to-digital converter or when you want a higher-precision ADC, the ADS1115 provides 16-bit precision at 860 samples/second over I2C. The chip can be ...

www.adafruit.com

15 usd

ACS711EX Current Sensor Carrier -31A to +31A

Pololu - ACS711EX Current Sensor Carrier -31A to +31A

This board is a simple carrier of Allegro’s ±31 A ACS711 Hall effect-based linear current sensor with overcurrent fault output, which offers a low-resistance (~0.6 mΩ) current path and electrical isolation up to 100 V. This version accepts a bidirectional current input with a magnitude up to 31...

www.pololu.com

4 usd

and a generic wire wound resistor that looks like this: (be sure to select resistor ohm value carefully based on system voltage of course)


these are the components i’m using to experiment with current monitoring precharge circuit. samd21 microcontroller. ina219 to monitor bus voltage for situations with VBUS<26V

 

[Horsefly]

ADS1115 16-Bit ADC
View attachment 59327

ADS1115 16-Bit ADC - 4 Channel with Programmable Gain Amplifier

For microcontrollers without an analog-to-digital converter or when you want a higher-precision ADC, the ADS1115 provides 16-bit precision at 860 samples/second over I2C. The chip can be ...

www.adafruit.com

15 usd

ACS711EX Current Sensor Carrier -31A to +31A
View attachment 59328

Pololu - ACS711EX Current Sensor Carrier -31A to +31A

This board is a simple carrier of Allegro’s ±31 A ACS711 Hall effect-based linear current sensor with overcurrent fault output, which offers a low-resistance (~0.6 mΩ) current path and electrical isolation up to 100 V. This version accepts a bidirectional current input with a magnitude up to 31...

www.pololu.com

4 usd

and a generic wire wound resistor that looks like this: (be sure to select resistor ohm value carefully based on system voltage of course)
View attachment 59330

these are the components i’m using to experiment with current monitoring precharge circuit. samd21 microcontroller. ina219 to monitor bus voltage for situations with VBUS<26V

Cool idea.

I'm not sure the little ADC board is needed if you can round up an ESP8266 or ESP32 board, both of which already have an ADC. The current sensor is another animal. If you want current sensing, that looks like a good solution.

Personally, I think we (on this board) have kind of over-played the whole "pre-charge" thing. I certainly don't know the whole population of inverters, but I think many - maybe most - inverters would be pretty hard to harm with the inrush to their front-end capacitors. I am very convinced that SCC's don't need to mess with this.

Having said that, I put a pre-charge circuit into my new LFP box, just because I could. Rube Goldberg was a genius!

 

[DerpsyDoodler]

Cool idea.

I'm not sure the little ADC board is needed if you can round up an ESP8266 or ESP32 board, both of which already have an ADC. The current sensor is another animal. If you want current sensing, that looks like a good solution.

Personally, I think we (on this board) have kind of over-played the whole "pre-charge" thing. I certainly don't know the whole population of inverters, but I think many - maybe most - inverters would be pretty hard to harm with the inrush to their front-end capacitors. I am very convinced that SCC's don't need to mess with this.

Having said that, I put a pre-charge circuit into my new LFP box, just because I could. Rube Goldberg was a genius!

That kind of inrush might not harm the inverter, but it might blow fuses. class-t fuses @ $40 a piece make a $20 precharge circuit worth it.

 

[FilterGuy]

That kind of inrush might not harm the inverter, but it might blow fuses. class-t fuses @ $40 a piece make a $20 precharge circuit worth it.

I agree....

I have seen the posts that say the inrush can hurt the capacitors. I have wondered if that is true, but have never researched it personally so I take it at face value that it can.

However, I can easily believe the huge inrush current can be problematic with other parts of the circuit (like Class-T fuses). If nothing more, we have seen videos where Will triggered the over-current protection of the BMS and then had to reset the BMS. That is a PITA I would gladly avoid.
I figure using a simple pre-charge circuit keeps the current very low and the whole system is happier for it.