How many amps current surge during Inverter capacitor charge?

[Barry Timm]

Am in discussion with someone about the need to pre-charge the capacitors in a Multiplus 3000VA connected to Lithium batteries.

He says there is no brief surge of thousands of amps. He says he measured the surge with an oscilloscope and sees only 104 amps maximum. Perhaps he's on a battery BMS that can clamp the surge down or something?

Thoughts?

[Dacian Todea (electrodacus)]

I doubt the surge current will be that small. I do not have a Multiplus 3000VA so I can not confirm but if the capacitors are directly across the inverter terminals the surge will be only limited by the battery internal impedance and cables resistance so many thousands of A.

There may be some inverters that have some build in precharge circuit but I doubt that is the case with the Multiplus.

[sailingharry]

It's a widely held belief.  I've never seen any test data to support the need for precharge.  More importantly, I don't think the manual for my Multiplus suggests such action.  I know I've never done it.

[Dacian Todea (electrodacus)]

sailingharry,   Are you saying the surge current when connecting the inverter is very small for Victron Multiplus or that is not important that sparks will damage the connector or switch when connecting the battery to inverter ?

[Dacian Todea (electrodacus)]

Here is a video showing a Victron multiplus connected to a battery https://www.youtube.com/watch?v=DOrJ5AH9yjM

[Ken King]

Here is a design for a fairly inexpensive solution.  While not using that inverter I experienced an arc the first time I attached a 2000w inverter to 280ah lifepO.  Have used it for several years now.

[alex sfakianos]

Excuse a bit of a thread jack but it's at least on the same topic...

I set my boat up for 12V and have very little need for an inverter. But I do have a small Victron Phoenix 500 inverter for a few charging devices. The Victron manual doesn't say anything about pre-charge. I realize that if you are using a switch to turn it on there can be a good spark that perhaps over time will damage the switch. But what if the connections are permanent and the inverter is turned on and off with the remote switch? Would it still be necessary to install a pre-charge circuit or use a light bulb?

[Habana7]

The surge is not that big and this is only the case during installation and that is only once, each inverter has a soft on/off that you use afterwards, I assume that you do not disconnect the cables every evening.

Op woensdag 3 april 2024 om 02:42:29 UTC+2 schreef alex sfakianos:

[michael clark]

I also wonder about this as once the inverters are connected to the battery I never disconnect them but I do turn them off by their switch...

if I turn them back on I have not done a precharge as many people talk about...

For some inverters the capacitors wind down when shutting down.... maybe all of them... not sure

I have a large inverter that is off and it has LED indicator lights inside still glowing so I must be drawing a minuscule amount of power even when the power switch is off...

others(inverters) I do not know how they react or if a precharge is required when turning the inverter on or off???

supposedly the precharging will put less stress on the inverter components.... but I have only done a pre-charge for the initial connection to the battery and never when turning the inverters on and off.

most of the inverters are left on 24/7 but to conserve power at times,  I have shut some off and turned them back on another day...

I have the same concern.... do these need a precharge if you turn the switch on and off...

???????

[sailingharry]

The inverter switch (on/off on the unit) only controls the AC generating circuitry.  The capacitors are always directly connected to the battery (in order to turn that part on and off, the inverter would require a large, high current, solenoid type switch -- you don't hear that thump on and off.  In an inverter/charger, you do hear that, but that is the AC side transfer switch).  The pre-charge is only an issue when connecting the unit to the battery.  The unknown is how long that takes to bleed off when disconnected -- if you leave your boat with the DC switch on 100% of the time (bad practice, but I do....), there is no issue.  If you are more responsible, and turn your battery switch off every time you leave the boat, then after some period of time (minutes?  hours?  days?  weeks?) your capacitors are discharged and it could be an issue.

Note, a decent capacitor keeps its charge a "long time" and once charged is an open circuit.  So, in theory, it can go a "long time" before needing a pre-charge, and if left connected when not in use should draw "very small" currents.  I used quotes to emphasize that these are relative terms, and I have no idea about the actual values.

[Dacian Todea (electrodacus)]

Alex,  Using the remote ON/OFF switch will not disconnect the capacitors so they will always be fully charged and no precharge circuit will be needed no matter what inverter you have.

That said such a small inverter will not have huge input capacitors so it is not really and issue to connect and disconnect from battery. Just the DC cables that will be thinner will have enough resistance to not allow for extreme fast charge so no super large inrush currents at connection.

pilgrimvalley,  Unless the ON/OFF switch is a large circuit breaker that disconnects the DC input the input capacitors will not discharge are remain fully charge even when the ON/OFF switch is in the OFF position.

Normally you only need a precharge at initial installation in order to protect the connectors. The large sparks when connecting the cables can damage the connector and then you will have increase connection resistance.

If you have a switch that disconnects the DC wires from inverter then that switch will benefit from a precharge circuit as it will extend the life of the switch.

[Bob Hughes]

The youtube video (in the March 21 post) shows why I strongly prefer using an automotive style incandescent light bulb for pre-charge versus a power resistor. While bench testing my SBMS - Victron Multiplus II 3000 - EVE LF304 4S setup, I discovered two unexpected behaviors of the MP II, that I would not have noticed if I had used a power resistor, as suggested in the "Inverter disconnect.pdf"

1) You cannot pre-charge the capacitor if the MP rocker switch is in the I or II position AND the MP mode is set to OFF in the Victron Connect app. In this configuration, the MP is trying to draw power from the battery terminals and this prevents the capacitors from charging. This behavior is the same, regardless of whether the AC-IN is powered or not. To charge the capacitors, I had to first put the rocker switch in the O position (completely off). 

2) Connecting the V-Sense input (to 13V) pre-charges the capacitor at a nice slow rate. I discovered this accidentally when I pushed my pre-charge momentary switch. The incandescent bulb didn't light up. Then I measured the Victron MP battery terminal voltage. It was 13 V. I expected it to be zero because I had not yet connected the battery. Turns out, I connected the V-Sense input first, and that pre-charged the capacitors. To verify, I disconnected the V-Sense and discharged the capacitors with another bulb. I then connected the volt meter to the MP battery terminals and connected the V-Sense input again. It took about 10 seconds to charge the capacitors up to 13V through the V-Sense input. For comparison, my type 1003 incandescent bulb charges the capacitors in about 3 seconds.

I was a bit surprised that the V-Sense input is not electrically isolated from the Victron battery terminals. But at least they put a decent resistance in the circuit. If this was a documented feature of V-Sense, I might have just used that instead of building a pre-charge circuit. Oh well. It's already done.

Note: The MP rocker switch has to be in the O position (completely off) for this to work. If the rocker switch is in the I or II position, the capacitors do not charge.

One more ramification of this V-Sense behavior, when putting a seasonal MP II in long-term storage (RV, boat, summer cabin), you probably need to disconnect the battery terminals AND the V-Sense. You would not want to slowly drain the battery through the current path from V-Sense input to loads connected to the MP battery terminals

I think these two examples show some of the advantages of using an incandescent bulb in a pre-charge circuit, instead of a power resistor. I would not likely have noticed either of these behaviors if I had been using a power resistor. And as the youtube video points out, a light bulb test will warn you if you have a dead short BEFORE you connect the battery leads.

[Dacian Todea (electrodacus)]

Bob,

Thanks for the detailed explanation of what happens.

That behavior will be valid for all inverters not just Victron.

That is also the reason I prefer that EXT IO3 directly controls the ON/OFF switch so when battery is fully charged there is no load on the battery unlike a software solution on the inverter side where some micro controller still needs to be ON to monitor signals as some times that self consumption can be significant.