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Old 14-11-2022, 19:48   #1
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Schematic for capacitor pre-charge

I know that these posts are not very interesting but I need to design and test all this for our new installation so will share here anyway.

I made this as an addition to the inverter remote on/off switch. This new part allows you to pre-charge the capacitors inside the inverter or charger before connecting it to the batteries. It provides a feedback with the brightness of the LED inside the switch.

It is designed for 24V systems, but I tested at 12V as well and did the math for 48V. Pre-charge current is limited to 1A, which can easily be handled by the switch. This prevents sparks flying and burning the contacts inside the main switch.

The diode makes the LED inside the switch turn off when releasing the switch. I observed operation without the diode and while this does count to the lifespan of the LED and costs 7mA current, this also discharges the capacitors when turning the main switch to OFF. Always pro’s and con’s.

So there is a possible complication and that is what happens when the inverter is switched on while pre-charging. My tests show that it simply powers up. Next test was with a load on the inverter: a small night light. The inverter turned on again, pulling 200mA through the current limiting resistor and powering the load. But it has a red led flashing the code for low battery and I measure only 18V at the terminals. Next I tried a larger load and it shuts down, flashing the low battery error and the green led flashing the code that it turned off due to an alarm. The unit tries turning on regularly but this immediately makes the voltage drop again.

During all of these tests I tried sparking the positive terminal by touching it with a straight 24V lead but the pre-charge works perfectly and there are no sparks.

I have a nagging feeling that maybe a setup is possible that can also discharge the capacitor without an additional switch. I hope someone here can come up with it before I do
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Old 14-11-2022, 20:03   #2
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Re: Schematic for capacitor pre-charge

Why do you want to do this? What problem is it trying to solve?
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Old 14-11-2022, 20:12   #3
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Re: Schematic for capacitor pre-charge

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Originally Posted by ItDepends View Post
Why do you want to do this? What problem is it trying to solve?
The big capacitors inside inverters and chargers present a short circuit to the battery when connecting it. This leads to sparks flying and burned contacts. It’s also damaging to the capacitors.

To prevent this, you must pre-charge the capacitors through a resistor. This by itself often leads to problems and often without any feedback if this works or not.

With the price of components as low as it is today, you can build and install this permanently for under $10.
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Old 14-11-2022, 20:54   #4
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Re: Schematic for capacitor pre-charge

Why would I routinely turn off the DC power connection to my inverter? Its power draw in the "off" position is totally trivial in the general scheme of things. I haven't ever felt the need for a separate switch for the inverters. Fuses of course, but no switches...

The only time inverters get disconnected on my boat is if I shut off the 24V power to the whole boat for maintenance purposes. Maybe three of four times a year? That's been happening on my boat for 25 years, with inverters installed the whole time. The original main battery disconnect switches are still there and functioning just fine. I have had them apart, and they have no "burned contacts." It's really hard to get excited about, or even interested in this... Especially since one of the inverters is original with the original caps...

Those times when I have connected battery terminals "live" I certainly do not see ANYTHING like a short circuit arc.

It also seems to me that any well designed inverter would manage this theoretical problem internally...

What am I missing?
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Old 14-11-2022, 22:36   #5
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Re: Schematic for capacitor pre-charge

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Originally Posted by ItDepends View Post
Why would I routinely turn off the DC power connection to my inverter? Its power draw in the "off" position is totally trivial in the general scheme of things. I haven't ever felt the need for a separate switch for the inverters. Fuses of course, but no switches...

The only time inverters get disconnected on my boat is if I shut off the 24V power to the whole boat for maintenance purposes. Maybe three of four times a year? That's been happening on my boat for 25 years, with inverters installed the whole time. The original main battery disconnect switches are still there and functioning just fine. I have had them apart, and they have no "burned contacts." It's really hard to get excited about, or even interested in this... Especially since one of the inverters is original with the original caps...

Those times when I have connected battery terminals "live" I certainly do not see ANYTHING like a short circuit arc.

It also seems to me that any well designed inverter would manage this theoretical problem internally...

What am I missing?
Probably… modern inverters or you have circuit resistance somewhere. Even the 375W inverter I am testing with shows a 4mF input capacitance on my multimeter and gives a big spark. The large solar controllers and the big Multiplus units weld banana plugs during testing. I believe a Multiplus 3000 has 40mF (that’s 40,000 microFarads).

Any good electrolytic capacitor shows as a dead short when it’s empty. If you don’t get the big sparks then something is wrong… maybe you have LA batteries instead of LFP or there is some circuit resistance elsewhere?

As to internal circuitry… I’m not sure why they don’t have it. But they do recommend pre-charging capacitors. This has been discussed many times and the consensus is that this has become a problem with new battery technology which has extremely low internal resistance. I already saw a significant difference when I started using the TPPL batteries from Odyssey. They advertise a short circuit performance of 5kA and I believe it.

You have no DC switch for the inverter? Then you can’t even pre-charge the capacitors because you would feed the whole DC system instead of just the inverter. All example diagrams I have seen include the switch as well as a main battery disconnect switch. Good reasons to switch it off: storage mode, lightning storms, maintenance, system reset, batteries switched off, prevent contacts welding when switching back on etc.

I must admit that I don’t always pre-charge my units because I’m too pressed by time or can’t find the resistor etc. Last timeI did that, the spark inside the Blue Sea Systems switch was so bright that the whole housing lit up like a big red indicator light.
Now I can just push the green button
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Old 14-11-2022, 22:53   #6
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Re: Schematic for capacitor pre-charge

Absolutely standard equipment/procedure in the industrial battery/inverter world. Just surprised that marine inverters (some anyway) don’t have this built in as standard equipment.

https://dynapower.com/dc-pre-charge-units/

https://lithionicsbattery.com/wp-con...sh-Current.pdf
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Old 14-11-2022, 23:31   #7
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Re: Schematic for capacitor pre-charge

We use one of these sorts of thing if the whole system has been shut down for a while. Actual value is lower than image. Stops the sparks when connecting.

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Old 14-11-2022, 23:58   #8
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Re: Schematic for capacitor pre-charge

It’s a new problem with small lithium batteries. That connecting the inverter surges the current draw and the internal bms shuts down. And you lose all battery power.

If you don’t have lithuim. Or have a large lithium bank thst can handle the surge without shutting down. . It’s not an issue. People have been sparking themselves with inverters for 30 years without issue.
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Old 15-11-2022, 00:36   #9
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Schematic for capacitor pre-charge

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Originally Posted by smac999 View Post
It’s a new problem with small lithium batteries. That connecting the inverter surges the current draw and the internal bms shuts down. And you lose all battery power.

If you don’t have lithuim. Or have a large lithium bank thst can handle the surge without shutting down. . It’s not an issue. People have been sparking themselves with inverters for 30 years without issue.


A better approach is nicks power resistor ( or a mosfet in rush current limiter then pre charging caps I have a good mosfet inrush limiter circuit in use elsewhere so it could be adapted.

In my bms I am looking at the bms allowing inrush currents as my bms has a programmable time delay on over current.
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Old 15-11-2022, 01:38   #10
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Re: Schematic for capacitor pre-charge

Thanks for the circuit, Nick .
Quote:
Originally Posted by ItDepends View Post
Why would I routinely turn off the DC power connection to my inverter? Its power draw in the "off" position is totally trivial in the general scheme of things. I haven't ever felt the need for a separate switch for the inverters. Fuses of course, but no switches...
Inverters do draw some standby current when on, but not powering anything.

The amount of energy wasted depends upon the inverter model and the way it is configured.

My Mastervolt 24v 2500w inverter has one of the lowest standby current draws at around 0.27 A. This is about 160 Whrs (the equivalent of around 13 AHrs for a 12v system) per day.

To successfully power a boat using solar while still having all the modern conveniences, especially when cruising areas of poor solar insolation, it is helpful to minimise energy draws that serve no practical purpose. I would suggest measuring the standby current draw for your inverter. It may be more than you think.
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Old 15-11-2022, 04:40   #11
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Re: Schematic for capacitor pre-charge

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Thanks for the circuit, Nick .

Inverters do draw some standby current when on, but not powering anything.

The amount of energy wasted depends upon the inverter model and the way it is configured.

My Mastervolt 24v 2500w inverter has one of the lowest standby current draws at around 0.27 A. This is about 160 Whrs (the equivalent of around 13 AHrs for a 12v system) per day.

To successfully power a boat using solar while still having all the modern conveniences, especially when cruising areas of poor solar insolation, it is helpful to minimise energy draws that serve no practical purpose. I would suggest measuring the standby current draw for your inverter. It may be more than you think.
Nolex,

You seem to be confusing the difference between an inverter that is "ON", "ON-STANDBY", "OFF", and and one that is disconnected from DC power.

If your inverter draws 13 A-hrs a day in ON-SANDBY that would be quite normal. If it draws 13 A-hrs a day while turned OFF and connected, you need a new inverter.

I routinely turn my inverters "OFF" when not being used to save the standby power draw.

I do not routinely disconnect them from DC power. While OFF the power draw is too low to register on my system's instruments. I could put a milliamp meter in the circuit, but, why bother?
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Old 15-11-2022, 04:48   #12
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Re: Schematic for capacitor pre-charge

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As to internal circuitry… I’m not sure why they don’t have it. But they do recommend pre-charging capacitors.
I assume when you say "they recommend" you are referring to the makers of inverters and LiFePO batteries. I thought I was pretty familiar with Victron stuff, and I haven't ever come across this in their information. Can you steer me to it?
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Old 15-11-2022, 05:06   #13
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Re: Schematic for capacitor pre-charge

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Nolex,

You seem to be confusing the difference between an inverter that is "ON", "ON-STANDBY", "OFF", and and one that is disconnected from DC power.
Personally, I prefer to disconnect the DC input to any device not in use, especially inverters, but I agree if you are manually forcing your inverter to shut down using a hardwired switch then the power draw will be small.
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Old 15-11-2022, 06:28   #14
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Re: Schematic for capacitor pre-charge

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Personally, I prefer to disconnect the DC input to any device not in use, especially inverters, but I agree if you are manually forcing your inverter to shut down using a hardwired switch then the power draw will be small.


My inverter when turned off either manually or remotely draws nothing once off
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Old 15-11-2022, 06:36   #15
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Re: Schematic for capacitor pre-charge

To find info on this, I recommend to use your browser and type “victron capacitor pre-charge” in the search bar.

I have read recommendations from many manufacturers on this but never logged where it is… surely for Victron’s case it will be in the search results.

But there’s more to this: I am an electronics designer and with all the education put into me and a life of practical experience with exactly this, I simply can state the need for pre-charge in systems with lithium batteries as fact. It doesn’t matter which manufacturer or which product: if it has large capacitors connected directly to the DC terminals, and your batteries are LFP (very low internal resistance) then not pre-charging capacitors is damaging to all components in that circuit.

I am sure that Victron has good reasons to not put this circuitry (which can be fully automated) into their products, but that doesn’t change the fact that it isn’t there.

Also, the stories about sparking capacitors for 30 years etc. may have the same reasons as the lack of this circuitry inside the equipment: older battery technology has a much higher internal resistance, which greatly limits this current and brings it from damaging to annoying levels.

Of course I have been sparking capacitors my whole life as well. I can tell you the real fun starts at 600VDC

I also read about LFP drop-in batteries triggering their over-current circuits: yes, they are trying to prevent damage, but unfortunately their circuits are too slow for this because the damage occurs immediately and before detection can trigger. Many MOSFET based BMS’s have blown MOSFET’s that people don’t even realize because “it still works”. Yes it works because there are many MOSFET’s running in parallel, but the max. capacity has come down in linear relation to the ratio of surviving MOSFET’s.
It is not because of annoyance triggering a BMS that a pre-charge is required, it is to prevent damage. If you battery/BMS can deal with this without damage then you only move that damage to a weaker point. There are bunches of inverters with their whole battery terminals evaporated and people getting molten metal splatter in their eyes etc. This is not something to take lightly!
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