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DIY Inrush Current Limiter solution needed

GVSolar

GVSolar

Solar Enthusiast
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Soon I'll be disconnecting my 740AH 24v L16 FLA bank and connecting 600AH of LFP (3 of the pre-assembled BYD units from TechDirect) to my Conext SW4024 inverter. I am concerned about tripping the BMS on the LFP batteries - and would like a good method of pre-charging the capacitors in the SW4024. Or possibly some sort of time limit that the SW capacitors would remain charged for the transfer? Any advice appreciated. Not savvy with board level electronics - so keep it simple, if you would. Thanks in advance.
 
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I like option 1 better than option 2 because even if someone is very fast with the rotary switch, the capacitors will get some pre-charge.

Option 1 has the potential downside is that the user could leave the rotary switch in position 1 and turn the inverter on. At that point the inverter could go into an indeterminate state that is probably not good for it.

Option 2 has the downside that someone could forget to press the pre-charge button first.

Both options have the potential problem of someone trying to connect the inverter while it is on. It is unclear what would happen in this case.
 
I have played with ideas of how to automate this to eliminate the downside but have not figured something out that I feel is workable and worth it.
(Given enough circuitry, it can be automated..... but at some point the added complexity is just not worth it)
 
I had seen somewhere that a residual charge in the inverter capacitors is held for a certain amount of time... any thoughts on that?
Thx.
 
Yes. The capacitors bleed off slowly..... What slowly means is going to vary from one inverter to another. If you are just connecting it once you can manually put the resister in line and then *quickly* switch over to solid wire. (Most inverters have tight spaces for the connection so I would hook the big wire to the inverter first, and then do the resister thing at the other end of the big wire where it connects to a Fuse or battery.
 
One thing that has always bugged me is that if the BMS kicks off for a while, the Capacitors will discharge and when the BMS kicks back on, there will be no pre-charge..... this is one of the reasons I would like to find a clean way to automate this.
 
BTW: Some BMS's have functionality built in to help with this. The one I am familiar with (from reading) is the Chargery BMS. It has an accessory that will allow you to first kick on a relay with a resistor in the circuit and a timed amount later kick on a relay for a circuit without the resistor.
The accessory works on 12 volts so it does not work very well with 24 V or larger systems.
 
It is only 20 Amps so it can't be the path for the main current. I might work for a pre-charge circuit, but I am not sure there are any advantages over a resister and switch.
 
An old thread, but still relevant.
I have some ideas to automate this, using a time-delay relay and a DC contactor.

But first, I would like to know, if there are any consequences/problems with leaving the resistor in circuit all the time ?

Let's say I have a DC breaker from the battery to inverter. What if I just leave a resistor connected between the input and output terminals of this breaker? Maybe with an inline fuse for added safety...

This way, when the breaker is OFF, when battery is connected, the resistor bypasses the breaker and pre-charges the inverter.
Then we can turn on the breaker (after waiting a few seconds). The current will follow the path of least resistance, and most of the current will flow through the breaker.

What do you think?
 
meetyg said:
An old thread, but still relevant.
I have some ideas to automate this, using a time-delay relay and a DC contactor.

But first, I would like to know, if there are any consequences/problems with leaving the resistor in circuit all the time ?

Let's say I have a DC breaker from the battery to inverter. What if I just leave a resistor connected between the input and output terminals of this breaker? Maybe with an inline fuse for added safety...

This way, when the breaker is OFF, when battery is connected, the resistor bypasses the breaker and pre-charges the inverter.
Then we can turn on the breaker (after waiting a few seconds). The current will follow the path of least resistance, and most of the current will flow through the breaker.

What do you think?
Click to expand...
A few potential things to watch for.

1) turning off the breaker will probably not turn off the inverter. If the inverter is left on it will likely go into a low voltage error state. This is not a killer issue, but be aware.

2) Find out how much current is being drawn when only the resister is powering the inverter and make sure the resister wattage is large enough.

3) Even if the inverter is turned off, there will be a small leakage current that will drain down the batteries over time.
 
FilterGuy said:
A few potential things to watch for.

1) turning off the breaker will probably not turn off the inverter. If the inverter is left on it will likely go into a low voltage error state. This is not a killer issue, but be aware.

2) Find out how much current is being drawn when only the resister is powering the inverter and make sure the resister wattage is large enough.

3) Even if the inverter is turned off, there will be a small leakage current that will drain down the batteries over time.
Click to expand...
Thanks.
I think I will end up going with an automated solution (for a new mobile solar generator I'm building).
I wanted to avoid adding a DC contactor, but I guess it's inevitable...
 
My plan has been option 2 even before I saw it here. S2 is a bluesea 6006 and s1 a momentary push button. Haven't looked in the resistor drawer yet although one may be included with the inverter. 125 amp fuse at each of 4 batteries and 300 amp at s2. S2 is attached to the positive bus bar.
 
How about a contactor across the resistor that closes when the voltage across the inverter reaches some high fraction of the applied voltage? With the proper selection of contactor coil voltage and resistor it could be just the contactor and resistor, but I’d probably use some kind of comparator or time delay.
 
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meetyg said:
An old thread, but still relevant.
I have some ideas to automate this, using a time-delay relay and a DC contactor.

But first, I would like to know, if there are any consequences/problems with leaving the resistor in circuit all the time ?

Let's say I have a DC breaker from the battery to inverter. What if I just leave a resistor connected between the input and output terminals of this breaker? Maybe with an inline fuse for added safety...

This way, when the breaker is OFF, when battery is connected, the resistor bypasses the breaker and pre-charges the inverter.
Then we can turn on the breaker (after waiting a few seconds). The current will follow the path of least resistance, and most of the current will flow through the breaker.

What do you think?
Click to expand...
This old starting post is what I did. Initial tests show it works, but I have not done extended max load testing of the inverter it is attached to.
 
My idea for automation is this (sorry no diagram yet):
Time delay relay, with NC port connected to resistor, going to inverter positive. COM port connected to 24v from battery breaker positive.
NO port connected to 125A rated DC contactor coil (that connects from breaker positive to inverter).

The time delay relay will wait for around 5 seconds since powered, and only then turn relay ON, in effect closing the DC contactor.
Meanwhile, during those 5 seconds of "OFF" time, the inverter will pre-charge.

Components that I am looking at to accomplish this:


and of course a relevant resistor.

The only issue with this specific time delay relay is, that while input voltage is up to 30v (suitable for 24v Lifepo4 battery), it's trigger signal is only up to 24v (which is basically the voltage of an almost empty 24v Lifepo4).
But, I noticed that this time delay relay also has a 5v pad on the back of the PCB (probably via an on-board voltage regulator), so I might use it to trigger the signal port when it receives power.

I hope I explained myself well. I will try to create a diagram when I get to doing so.

In my previous solar generator build (12v Lifepo4) , I also used a different time delay relay, but no contactor. As soon as I hear the relay turning off, I close another breaker going to inverter, so it's semiautomatic I guess.

But now my Dad also wants one, so I'm building a new one for him (with 24v Lifepo4) , and I want to make it fool proof. That's why I'm looking for automation.
Basically, when he needs to use the solar generator, he will just turn on the DC breaker and then after around 5 seconds, the inverter can be powered ON.
 
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meetyg said:
But now my Dad also wants one
Click to expand...
Not to add Feature Creep, but if it's for someone else, I'd want to (after another time delay? It's only another $1.29 plus a relay for the inverter power switch) have the inverter fire up _after_ the power's on (and shut down when power's removed. In fact, I'd wire up a 12V supply using another Ali Express board to run the whole thing, and have a much simpler low-power toggle switch for On/Off functionality for the inverter.

And not to back up too far, but what's the quiescent current draw of the inverter when it's switched off?
 
wpns said:
Not to add Feature Creep, but if it's for someone else, I'd want to (after another time delay? It's only another $1.29 plus a relay for the inverter power switch) have the inverter fire up _after_ the power's on (and shut down when power's removed. In fact, I'd wire up a 12V supply using another Ali Express board to run the whole thing, and have a much simpler low-power toggle switch for On/Off functionality for the inverter.

And not to back up too far, but what's the quiescent current draw of the inverter when it's switched off?
Click to expand...
I'm trying to keep my solution simple ("KISS" principle). Adding more components adds complexity and reduces reliability (more possible points of failure).

Anyways, what's interesting is that when I connected the inverter (FCHAO 1800W 24v) to my PSU, after an initial charge, the amps showed 0 when inverter was off. I guess that the capacitors were fully charged and therfore no more current flowed.
When I turned on the inverter (no AC loads), power draw was around 7w.
 
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