Purpose of a Pre Charge Resister?

[Collin ev/solar]

Hello,

Can someone explain to me the purpose of a pre charger resister? Why or why not have one? How do I know if my setup has one?

The battery I have is the big battery kong elite 48v.

Thanks!

 

[Ampster]

The purpose is to charge the capacitors in the inverter to reduce surge when full battery power is connected to avoid large DC spark which can damage switches or breakers. Sometimes they are built into inverters. I would look at inverter specs to find out. Sometimes the capacitors get charged from the AC on the grid if that is how a system is configured.

 

[LakeHouse]

Your inverter has big capacitors that need to be charged. This takes a small amount of energy, but without a pre-charge resistor it happens really fast which means a very high current; hundreds or thousands of amps.
That high current can cause battery BMS’s to trip because it looks like a short. A pre-charge resistor reduces the current and lets the capacitors charge in seconds instead of milliseconds and solves the problem.

 

[Collin ev/solar]

Great thank you. It is the inverter that has the pre charge resister not that battery, correct?

 

[pollenface]

I don't own a "precharge resistor"

Having 4 groups of 12v agm batteries I connect up the negative, then tap the postive on the 12v, then the positive on the 24v, then positive on the 36v then connect it straight up to the 48v with minimal sparking.

I've also used a 0-30v bench power supply before connecting to 48v battery in the past, which also works but one needs alternative power present to run the bench power supply.

 

[timselectric]

I have never seen an inverter with a built in pre-charge resistor.
But some batteries do have them built in.
The inverter doesn't care if you use one or not. It's the BMS that has issues with the capacitor surge.

 

[LakeHouse]

Great thank you. It is the inverter that has the pre charge resister not that battery, correct?

The battery wouldn't have something like that. Most inverters don't either.
It's not necessarily something you need to 'install' exactly, though that is a common way of doing it and it's what I did with my system. Like @pollenface notes, there are lots of ways to do it: Bench power supply, incremental voltage increases, a temporary connection through a resistor of some sort (@sunshine_eggo likes to recommend a #2 pencil), or a hard-wired wired pre-charge circuit.

 

[LakeHouse]

lol, for everyone's benefit... If @timselectric and I disagree, personally I'd go with what Tim said!

 

[timselectric]

It's also not needed if you have enough BMS capacity.
I'm not using a pre-charge resistor, and have no issues.
But my current BMS capacity is 1,100a.

 

[GXMnow]

Many of the "Server Rack" style battery units do have a pre charge circuit. If your battery has it, it will be clearly mentioned in the instructions.

On some of them, you have to put the BMS in a standby mode, turn on the breaker, then when you take the BMS out of standby, it will current limit to something like 10 amps for a second or so to allow the voltage to climb before it turns on the protection switch at full current. While others have a resistor across the BMS with a simple turn on delay. Turn on the breaker and the resistor carries the current for a second before the BMS turns on.

I have a large 8 ohm resistor I normally use for amplifier testing. Any time I have my system shut down, I just connect that resistor across my DC disconnect switch. It charges in under a full second and carries enough current to power up my XW-Pro inverter to standby. Once I see the display light up, I then turn on the disconnect switch.

For 12 volt systems, I use a car headlight bulb. I guess 4 of them in series would work on 48 volt stuff.

 

[meetyg]

I have never seen an inverter with a built in pre-charge resistor.
But some batteries do have them built in.
The inverter doesn't care if you use one or not. It's the BMS that has issues with the capacitor surge.

I think the inverter does care - won't that high inrush damage the capacitors, or at least shorten thier lifespan?

 

[Mattb4]

I noticed that the WZRELB split phase inverters have a built on pre-charge terminal. it is adjacent to the regular positive terminal . The instructions are you touch the pos cable to it for a few seconds before attempting to connect up.

 

[timselectric]

I think the inverter does care - won't that high inrush damage the capacitors, or at least shorten thier lifespan?

No
If it was a problem, they would add pre-charge circuitry.
It's only a problem for the types of batteries that have BMS's.
Pre-charging wasn't a thing until BMS's were.

 

[meetyg]

No
If it was a problem, they would add pre-charge circuitry.
It's only a problem for the types of batteries that have BMS's.
Pre-charging wasn't a thing until BMS's were.

I thought it wasn't a thing until Lifepo4 batteries came around, having low internal resistance and a high discharge capability.
Of course Lifepo4 and BMS go together.

 

[timselectric]

I thought it wasn't a thing until Lifepo4 batteries came around, having low internal resistance and a high discharge capability.
Of course Lifepo4 and BMS go together.

The only reason to pre-charge is to keep the BMS from shutting down.
BMS OCP is faster than the capacitor can charge.

 

[GXMnow]

The only reason to pre-charge is to keep the BMS from shutting down.
BMS OCP is faster than the capacitor can charge.

Not always.

It is very possible for the inrush to fry the MOS FETs. And in many cases, when they fail, they can fail in a short circuit. The BMS is a small computer that is running instructions. It has to measure the overload current and then react to it and then try to turn off the FETs. Each instruction takes time. Sure, a fast processor might do it in just a few milliseconds, but the FETs can be toast in a microsecond at 5,000 amps. They don't use super fast processors because they don't want the BMS using any more power than it has to to do it's job.

And also yes.. The huge inrush currents from a lithium battery bank can damage the filter capacitors as well. Even with lead acid batteries, most inverters recommend using a pre-charge resistor to avoid damage from the high inrush surge. It is also a good idea for charge controllers as well. When I first connected my Victron charge controller, it made a pretty healthy spark. Every time since, I have been using my resistor to be nice to it. It certainly didn't pull near the inrush that the inverter does, but it was enough that I feel the pre-charge is a good idea.

 

[timselectric]

Not always.

It is very possible for the inrush to fry the MOS FETs. And in many cases, when they fail, they can fail in a short circuit. The BMS is a small computer that is running instructions. It has to measure the overload current and then react to it and then try to turn off the FETs. Each instruction takes time. Sure, a fast processor might do it in just a few milliseconds, but the FETs can be toast in a microsecond at 5,000 amps. They don't use super fast processors because they don't want the BMS using any more power than it has to to do it's job.

And also yes.. The huge inrush currents from a lithium battery bank can damage the filter capacitors as well. Even with lead acid batteries, most inverters recommend using a pre-charge resistor to avoid damage from the high inrush surge. It is also a good idea for charge controllers as well. When I first connected my Victron charge controller, it made a pretty healthy spark. Every time since, I have been using my resistor to be nice to it. It certainly didn't pull near the inrush that the inverter does, but it was enough that I feel the pre-charge is a good idea.

You are definitely entitled to your own opinion.
If damage to the inverter from lack of pre-charge was an issue. Every single inverter would include pre-charge circuitry.
They don't, which proves that it isn't.