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Should I change my inverter precharge circuit?

corn18

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I am going to install an MPII and was looking at my inverter precharge circuit. As it is wired now, the precharge applies power to all DC circuits when I push the button. Not sure if that is good or bad. I could add a cutoff switch on the invert + wire and put the precharge output on the inverter side of that. With the cutoff switch in the off position, that would only precharge the inverter and not power anything else. The procedure to power up would be:

1. Disconnect inverter
2. Turn on main DC breaker
3. Precharge inverter
4. Connect inverter

I could add the two position cutoff precharge circuit that is oft cited on here, but I don't like the failure mode of forgetting to switch it from Pos 1 to Pos 2.

With no cutoff on the AC out from the MPII, I am wondering if this will work. I never turn the MPII off so it will try to power the AC out when I am precharging.

Appreciate your thoughts.

DC wiring 290RL victron.jpg
 
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I don't like the failure mode of forgetting to switch it from Pos 1 to Pos 2.
See:


for a switch and precharge setup that avoids that problem.
 
See:


for a switch and precharge setup that avoids that problem.
I guess you missed my point that I don’t like that setup.
 
My point was the switch avoided the issue you mentioned. But maybe I misunderstood the issue.
 
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This may be a dumb question and I'm not asking to to be a smart-alec.

How do you know you need a pre-charge circuit? Do you know that the inrush current at inverter startup can exceed what the BMS in your two batteries can provide?

I'll be moving up to a Multiplus soon and these questions has been in the back of my mind. I helped a friend install the Multiplus II 120x2 recently and he has nothing special to handle the inrush. He does have four 100 Ah LiFePO4 batteries in parallel, so maybe that's enough to handle the inrush?
 
I still don't get how often an inverter actually needs to be disconnected from the battery. Mine stays connected for years at a time. A manual pre-charge is not a huge deal if you are removing a cable for service. I say skip the switch and pre-charge complication and use the power button.
 
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This may be a dumb question and I'm not asking to to be a smart-alec.

How do you know you need a pre-charge circuit? Do you know that the inrush current at inverter startup can exceed what the BMS in your two batteries can provide?

I'll be moving up to a Multiplus soon and these questions has been in the back of my mind. I helped a friend install the Multiplus II 120x2 recently and he has nothing special to handle the inrush. He does have four 100 Ah LiFePO4 batteries in parallel, so maybe that's enough to handle the inrush?
I needed a precharge circuit when I had a single SOK 206Ah battery hooked to the 1200W inverter. The BMS would go into protect mode when I flipped on the main breaker. Came here and everyone helped me figure out it was the inverter caps sucking too many amps too fast which caused the BMS to go into protect mode.

Then I added a second SOK 206Ah battery still with the single 1200W inverter. Now I could flip the main switch on and the BMSs would not go into protect.

But the precharge is there to protect both the batteries and the inverter caps. You can blow up the inverter caps with LiFePO4 batteries. So the precharge is still recommended to prevent that.

Your friend might be fine, but the consensus I got on here was to keep my precharge even though my setup works without one now. So it remains.
 
Yes I turn mine off with the remote power switch. And the 12v stays connected.
I don't want anything powered when I leave my trailer in storage where it spends 90% of its life. Don't see the need to keep the caps charged 24/7/365 when I can just add a simple circuit to precharge them.
 
Yes I turn mine off with the remote power switch. And the 12v stays connected.
Which, as far as I know, keeps the caps charged so the inrush is only when physically connecting the 12v input. I don't disconnect the 12v in our RV unless we won't be using it for an extended period
 
I needed a precharge circuit when I had a single SOK 206Ah battery hooked to the 1200W inverter. The BMS would go into protect mode when I flipped on the main breaker. Came here and everyone helped me figure out it was the inverter caps sucking too many amps too fast which caused the BMS to go into protect mode.

Then I added a second SOK 206Ah battery still with the single 1200W inverter. Now I could flip the main switch on and the BMSs would not go into protect.

But the precharge is there to protect both the batteries and the inverter caps. You can blow up the inverter caps with LiFePO4 batteries. So the precharge is still recommended to prevent that.

Your friend might be fine, but the consensus I got on here was to keep my precharge even though my setup works without one now. So it remains.

That's useful information and aligns with what I suspected. With a big enough BMS or enough numbers of BMS it's not a problem.

I may keep mine on all the time. I've been doing that with the 1000 watt MSW inverter. I run a battery charger to my side-by-side to keep it properly charged. I've also been thinking of adding a couple solar panels outside the trailer and running the output of that to a solar charge controller that can handle AGM batteries. That would work just as well.
 
That's useful information and aligns with what I suspected. With a big enough BMS or enough numbers of BMS it's not a problem.
I put in a pre-charge on my system. I've got 4 120a (continuous) BMS's, but my my understanding is that the capacitor charge inflow is way, way beyond that. I can't remember the numbers when I researched it, but I think it was thousands of amps (maybe 10's of thousands?), but just for a split second. I don't think that is a huge issue for a mechanical contactor/relay, but most of the internal BMS's have FET-based switching which can be damaged before has a chance to sense the voltage and shut down. At least that's the theory. I've never damaged one, but I did have one of my BMS's shut down (sensing a short) when connecting the invertor when I was initially building/testing my system. The pre-charge is like $10 of parts and a 20 minute job. I can't see the harm. I disconnect my main DC load center before I do the pre-charge. Keeping my cerbo in the loop, I can see when the pre-charge is done because I see the lights blinking on it and then I flip the main battery switch. My pre-charge circuit is just a jumper across the battery switch.

1637355639508.png
 
You can blow up the inverter caps with LiFePO4 batteries.

I haven’t seen or heard of that, although it’s wise to limit the temperature rises in any capacitor to maximise its lifespan.

The main reasons for using precharge are to limit discharge current from battery, and to prevent arc flash welding of contactor.

I’ve seen contactor welding result in pack failure due to the BMS being unable to disconnect the pack from a load.
 
OP here. I found a solution that works for me. The diagram above is an old setup. Below is my current setup. I went with a 1,1,1+2 switch so I can isolate the batteries to reset a protected BMS without unhooking the batteries. I also added a cutoff switch for the inverter. Kept a simple precharge circuit.

DC wiring 290RL rev 1 ALL.jpg
 
It seems like anything with capacitors in it like this should be designed to limit the inrush when first powering up. Guess it costs too much and they feel it's better to just stick it on the end user to deal with.

As it is, my plan is to just hook the inverters up to a small power supply or battery charger of the correct voltage, with low amps and let it charge it's caps up from that. Presumably, this should work? The inverters capacitors won't draw more from the power supply than it's willing to supply.

I was thinking of having something like this on a switch as my "precharge resistor". I just think the little screen would be neat, show you the amps the inverter is taking while it takes it and then show you the idle draw while the inverter is doing nothing before you switch to your batteries and then turn loads back on. It's so cheap it's almost worth trying it just to see what it does.

61gqmf7+doS._AC_SL1500_.jpg
 
It seems like anything with capacitors in it like this should be designed to limit the inrush when first powering up. Guess it costs too much and they feel it's better to just stick it on the end user to deal with.

As it is, my plan is to just hook the inverters up to a small power supply or battery charger of the correct voltage, with low amps and let it charge it's caps up from that. Presumably, this should work? The inverters capacitors won't draw more from the power supply than it's willing to supply.

I was thinking of having something like this on a switch as my "precharge resistor". I just think the little screen would be neat, show you the amps the inverter is taking while it takes it and then show you the idle draw while the inverter is doing nothing before you switch to your batteries and then turn loads back on. It's so cheap it's almost worth trying it just to see what it does.

61gqmf7+doS._AC_SL1500_.jpg
Might work. The inverter will try to power up unless you have it turned off, so it might tax the power supply. It doesn't take a lot to precharge the caps.
 
Might work. The inverter will try to power up unless you have it turned off, so it might tax the power supply. It doesn't take a lot to precharge the caps.

For sure, the inverter might try to run off it.. or even to charge it like a battery, which would be bad. Have to make sure all the solar input and AC loads are turned off. It should be able to power the idle load of most inverters if it can really do 400w of output.
 
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