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Old 03-02-2025, 06:23   #1
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Inverter Inrush Currents and Lithium Power Systems

We were discussing fuses in another thread and I got to thinking about this.

As I understand it, inrush currents are limited by battery internal resistance. So with the far lower internal resistance of lithium, the inrush current when first connecting an inverter/charger might be even higher (depending on whether this is limited by the caps themselves; I have no idea). And unlike a regular lead battery installation, we have electronics like BMS's involved.


At first I was worried this might blow a fuse, but then I realized there were many other potential problems.


ChatGPT helped me find DIY solar discussions where this is mentioned as a serious problem, with some people experienced fried BMS's. Others stated that contactors would be destroyed by this inrush.

One solution discussed pre-charging the inverter caps through a resistor or 24v incandescent light bulb.

Someone else suggested that you can charge the caps by connecting the inverter to AC power first -- that the caps would charge from the AC power.

Anyone on here had any such experience?

Yet something else to worry about, which I had not thought of earlier
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Old 03-02-2025, 06:34   #2
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Re: Inverter Inrush Currents and Lithium Power Systems

I have a Renogy 3000W 12V/240V inverter and they recommended a 350 Amp T class fuse so that is what I will use with my new lithium install. Maybe contact your inverter supplier?

I am also concerned by inrush ams stall current on my 12V DC windlass.

From what I've read the 350 Amp T class and BMS with 200 Amp continuous / 350 A peak should work with either active.
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Old 03-02-2025, 06:36   #3
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Re: Inverter Inrush Currents and Lithium Power Systems

When I commissioned my current Multiplus with AGM batteries setup I did power up the inverter on AC first, then turn on the DC feed to the inverter. Not a single flicker from the lights or anything on the DC side when I flipped the switch (and no blown fuses), so as far as I could tell, it avoided the huge inrush. I was watching the inverter info through VE.Configure when I did this and it did look like the charger came up to the set voltage on the DC side (I didn't meter it to confirm) even without batteries connected.

Logically, if the inverter powers up the charger and shows DC voltage without having DC power applied first, then it should charge the caps from AC. The 1st gen Multiplus units seem to do that, but I don't know about the MPII.
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Old 03-02-2025, 06:40   #4
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Re: Inverter Inrush Currents and Lithium Power Systems

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Originally Posted by rslifkin View Post
When I commissioned my current Multiplus with AGM batteries setup I did power up the inverter on AC first, then turn on the DC feed to the inverter. Not a single flicker from the lights or anything on the DC side when I flipped the switch (and no blown fuses), so as far as I could tell, it avoided the huge inrush. I was watching the inverter info through VE.Configure when I did this and it did look like the charger came up to the set voltage on the DC side (I didn't meter it to confirm) even without batteries connected.

Logically, if the inverter powers up the charger and shows DC voltage without having DC power applied first, then it should charge the caps from AC. The 1st gen Multiplus units seem to do that, but I don't know about the MPII.

This is the easiest solution by far if it works.


Why doesn't the Victron manual discuss this? With lead batteries with no fuses -- ok -- but even in this case, the spark was still HUGE! Could have caused a fire if anything inflammable had been nearby. And scarred the terminals.
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Old 03-02-2025, 06:43   #5
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Re: Inverter Inrush Currents and Lithium Power Systems

I have two Blue Seas contactors, one for each battery. I had been planning to put the remote switches at the nav table so I could shut down either or both batteries for whatever reason.


Now I guess I won't do that.



If all the above info is correct, then I guess I will need to have an appropriate resistor (or incandescent lamp) in the battery box in case I need to pre-charge the caps when I'm off shore power. Otherwise, connect to AC first.
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Old 03-02-2025, 06:47   #6
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Re: Inverter Inrush Currents and Lithium Power Systems

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Originally Posted by Dockhead View Post
I have two Blue Seas contactors, one for each battery. I had been planning to put the remote switches at the nav table so I could shut down either or both batteries for whatever reason.


Now I guess I won't do that.



If all the above info is correct, then I guess I will need to have an appropriate resistor (or incandescent lamp) in the battery box in case I need to pre-charge the caps when I'm off shore power. Otherwise, connect to AC first.
I've seen mention of people wiring a permanent pre-charge circuit into their system and a switch that cuts just the DC power to the inverter. Then if you've cut the DC power for some reason, you turn on the pre-charge circuit, wait a few seconds, then turn on the main inverter DC power.

I think Victron still expects a lot of their installs to be done by professionals and often in fixed locations where they get powered up once and DC power may not be turned off for a decade. So they don't consider it a big concern, even though it is for a marine application.
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Old 03-02-2025, 07:00   #7
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Re: Inverter Inrush Currents and Lithium Power Systems

Quote:
Originally Posted by rslifkin View Post
I've seen mention of people wiring a permanent pre-charge circuit into their system and a switch that cuts just the DC power to the inverter. Then if you've cut the DC power for some reason, you turn on the pre-charge circuit, wait a few seconds, then turn on the main inverter DC power.

I think Victron still expects a lot of their installs to be done by professionals and often in fixed locations where they get powered up once and DC power may not be turned off for a decade. So they don't consider it a big concern, even though it is for a marine application.

Where did you see that?



I guess we can imagine such a circuit with a momentary switch which you hold and count down. Maybe a lock out. Seems like a bit of a faff. I would probably be ok with a resistor for emergencies when I'm off shore power AND can't get the generator running for whatever reason (assuming that AC pre-charge trick is correct).


Maybe I'll write to Victron tech support about this.
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Old 03-02-2025, 07:11   #8
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Re: Inverter Inrush Currents and Lithium Power Systems

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Originally Posted by Dockhead View Post
Where did you see that?



I guess we can imagine such a circuit with a momentary switch which you hold and count down. Maybe a lock out. Seems like a bit of a faff. I would probably be ok with a resistor for emergencies when I'm off shore power AND can't get the generator running for whatever reason (assuming that AC pre-charge trick is correct).


Maybe I'll write to Victron tech support about this.
I think it was on the Victron forums somewhere that I saw it. And yes, I think it involved a momentary button. Basically hold it down for a few seconds, then flip the main switch on.
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Old 03-02-2025, 07:32   #9
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Re: Inverter Inrush Currents and Lithium Power Systems

Quite an interesting question that I had not considered for boat systems. I work in the big battery world, and our converters and inverters are 500kW to 5MW. And every single one of them has two pre-charge circuits. The batteries have a pre-charge circuit, they will not close main contactors to the bus until the bus is at equal voltage (which means the bus has to be at no load when closing), and then the power conversion equipment has pre-charge contactors and will not close the main contactor until their internals match the bus voltage. Both pre-charge contactors are interlocked with the mains, once the main closes the pre-charge opens to prevent minor continuous flow through the resistor.

I work with that setup every single day, just take it for granted, and yet never thought about it from a boat perspective. But, as electrical systems get larger, inverters get larger, and batteries become lithium I could easily see it as an issue. A little surprised that the bigger vendors don't include it automatically as part of the system.

[Edit] but see that Nick had a conversation about it a couple of years ago:

https://www.cruisersforum.com/forums...ge-270514.html
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Old 03-02-2025, 08:10   #10
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Re: Inverter Inrush Currents and Lithium Power Systems

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Originally Posted by PippaB View Post
Quite an interesting question that I had not considered for boat systems. I work in the big battery world, and our converters and inverters are 500kW to 5MW. And every single one of them has two pre-charge circuits. The batteries have a pre-charge circuit, they will not close main contactors to the bus until the bus is at equal voltage (which means the bus has to be at no load when closing), and then the power conversion equipment has pre-charge contactors and will not close the main contactor until their internals match the bus voltage. Both pre-charge contactors are interlocked with the mains, once the main closes the pre-charge opens to prevent minor continuous flow through the resistor.

I work with that setup every single day, just take it for granted, and yet never thought about it from a boat perspective. But, as electrical systems get larger, inverters get larger, and batteries become lithium I could easily see it as an issue. A little surprised that the bigger vendors don't include it automatically as part of the system.

[Edit] but see that Nick had a conversation about it a couple of years ago:

https://www.cruisersforum.com/forums...ge-270514.html
Bingo!

As usual, Nick is ahead of the curve.

This is brilliant. I had not thought it worthwhile thinking about a permanent installation, but since Nick has already designed it . . .

Great!


That thread does render this one superfluous.
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Old 03-02-2025, 22:16   #11
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Re: Inverter Inrush Currents and Lithium Power Systems

Hi.
I am a sparky and installer for Victron.

If you want a Lithium based system you should invest in a proper BMS.
It is silly not to do it.
A bit like trying to charge your new Tesla at home with an old school 12 V battery charger. Smile...
Lithium and wet might look the same but are two pretty different animals.

Any BMS worth the name and the cost have the pre-charge limitation.

Eg Lynx BMS:
The built-in pre-charge circuit pre-charges capacitive loads such as inverters or inverter/chargers before the contactor closes to prevent high inrush current.

Personally I have never had an experience where this was a problem.
(With the exception of "cold starts" where they can trip to easy)
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Old 04-02-2025, 02:46   #12
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Re: Inverter Inrush Currents and Lithium Power Systems

The Tao BMS has this pre-charge function, it is automatic, but unfortunately Tao has ceased operation.
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Old 04-02-2025, 02:58   #13
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Re: Inverter Inrush Currents and Lithium Power Systems

Quote:
Originally Posted by Sowilo View Post
Hi.
I am a sparky and installer for Victron.

If you want a Lithium based system you should invest in a proper BMS.
It is silly not to do it.
A bit like trying to charge your new Tesla at home with an old school 12 V battery charger. Smile...
Lithium and wet might look the same but are two pretty different animals.

Any BMS worth the name and the cost have the pre-charge limitation.

Eg Lynx BMS:
The built-in pre-charge circuit pre-charges capacitive loads such as inverters or inverter/chargers before the contactor closes to prevent high inrush current.

Personally I have never had an experience where this was a problem.n
(With the exception of "cold starts" where they can trip to easy)
Where did you get the idea that anyone is not using a BMS?

My system is built with JK inverter BMS's, two of them, the 200A type.


I'm not aware of any BMS's currently available which have this function. The Lynx one is only for Victron batteries.
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Old 04-02-2025, 05:28   #14
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Re: Inverter Inrush Currents and Lithium Power Systems

An excellent explanation of the cause, effect, and solution to inverter inrush currents”
“Inverter Pre-Charge Circuit Inrush Current” ~ by Ametherm, Inc.
https://www.ametherm.com/blog/inrush-current/inverter-pre-charge-inrush-current/

They have numerous other technical articles, on the subject, listed at the bottom right, of the above link.


Below is a typical pre-charge circuitry for battery operation and timing diagram, showing how the circuit operates. (Courtesy of Lithium -ION BMS)





In its most basic form, the pre-charge circuit operates as follows:

OFF: When the system is OFF all relays/contactors are off.

Precharge: When the system is first turned on, K1 and K3 are turned on to pre-charge the load until the inrush current has subsided. R1 shows the location of the thermistor in the pre-charge circuit.

ON: After pre-charge, contactor K2 is turned on (relay K1, must be off to save coil power).
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Old 04-02-2025, 05:31   #15
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Re: Inverter Inrush Currents and Lithium Power Systems

Hi Dockhead.
Ahh, Maybe a bit cloudy (sorry for that) but the keyword being "Proper".

What I mean is that once you have choosen a candidate for battery - check that it has a proper BMS. It can be a combination of internal and external units. I think a proper BMS should protect the batteries for over current as well as over/under charging.

For example I have recently seen a Renogy based system that trusted a BMS made of a combination of two, an external system for over- and undercharge - and an internal for over current. Not easy to reset but it works.

If you have Victron batteries and maybe other Victron units a solid choice is a Smart BMS that does it all.

I mean it is not so easy and a misstake can be expensive.
But if I know the battery model I can tell how I think a proper BMS should be set up.

See my point?

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