Schematic for capacitor pre-charge

[s/v Jedi]

I did test a Victron Phoenix VE.direct 24V 375W inverter for power draw. I used a very good EEVblog Davy Jones multimeter that measures micro Amps.

With the inverter switch off, there is no draw.

With the inverter switch on, but the remote switch circuit off, there must be some draw because a microcontroller is looking at that remote on/off signal. That said, I could not detect it so I think it is less than 1 uA, somewhere in the nA area so insignificant.

With the inverter powered on without a load connected, or in Eco mode without a load: this varies between models and is simply listed in the spec sheets.

[rslifkin]

I haven't disconnected or de-powered the DC side of my Multiplus since I installed it. When I first did the install, I cheated on the pre-charge. Turned on the AC input, turned the inverter on, confirmed it was outputting power to charge the non-existent batteries on the DC side (which would mean charged capacitors). Then I closed the DC breaker for the unit. No big power spike when I closed the breaker, so it worked.



I'm not sure if all versions of the Multiplus (or other inverter/charger units) can be powered up on AC first like that, however. And if you've got an inverter-only unit, of course that trick wouldn't work.

[s/v Jedi]

Originally Posted by Pete7

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.

Pete

I decided on a 25 Ohm 25W resistor for a 24V system and a 15 Ohm 25W resistor for 12V. This keeps the current below one Amp.

For this small inverter with 4mF capacitance we can calculate tau: RxC so 25 x 0.004 = 0.1 second. We can consider the capacitor to be fully charged after 5tau so that is only half a second.

For a big inverter with 40mF this is a factor 10 higher so it becomes 5 seconds. You will actually see the LED inside the switch become brighter during that time so you know when it’s done, but even if you would turn the big switch prematurely, the capacitor will already have enough pre-charge to prevent damage. You don’t need to fully charge it, just enough so that it has developed some internal resistance

Edit: this also came up on my “cheap BMS ABYC compliance” thread: how to calculate delays using capacitor and resistors. I checked on this and found the term tau isn’t in general use in the US. Search for “RC time constant” instead. I find that more of an explanation of what tau is but then again I’m from the old world where we use Latin :-)

[noelex 77]

Originally Posted by goboatingnow

My inverter when turned off either manually or remotely draws nothing once off

Cutting off the DC supply to the inverter will always result in zero power draw. The draw when turning the inverter off manually (if this is an option for your inverter) depends on design of the inverter and whether this is done at the inverter or via a remote panel (in which case the inverter needs to maintain communication with the remote panel).

I have just checked the power draw of our inverter when manually switched off at the inverter itself and it is drawing 1.25w. Low enough to ignore in most circumstances, but it is not zero. More importantly, this is not practical, as the inverter is located in a inconvenient position.

[team karst]

Originally Posted by s/v Jedi

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.

Good idea...


I don't like big sparks in a bilge; metal erosion, eye damage....

That 1N4007 is rated at 1A, which it will see early in the charge process (24V, 25Ohm). I'd go with a 6A model. Less heat, and you don't wont this diode to short out....

[Cwens]

For the cost of a resistor, a zener diode, and a relay, manufacturers could be building this "soft start" feature into the inverter itself. Why don't they?

[Pete7]

Originally Posted by ChrisOwens

For the cost of a resistor, a zener diode, and a relay, manufacturers could be building this "soft start" feature into the inverter itself. Why don't they?

They are in the market of selling inverters. If the inverter breaks after a few years and you buy another, well that's good business. If they sell you an inverter that lasts 3 decades, what repeat business do they have?

Pete

[s/v Jedi]

Originally Posted by team karst

Good idea...


I don't like big sparks in a bilge; metal erosion, eye damage....

That 1N4007 is rated at 1A, which it will see early in the charge process (24V, 25Ohm). I'd go with a 6A model. Less heat, and you don't wont this diode to short out....

There’s no need to worry: the diode 1A rating is continuous and my measurements confirm there is no heat buildup at all. The 1A is hard limited by the resistor and only occurs for a fraction of a second.

I did try to find ways to heat it up. Best scenario I managed was a 200mA continuous draw. This heated up the LED inside the switch but hardly anything in the diode. I set the LED, which has a 1.5k resistor built in for 12V operation to a 7mA current for high brightness which, to the eye, is equal to it’s nominal 9mA rating.

I think this circuit is bulletproof as it is. The switch also has a normally-closed contact and I hope someone can figure out a capacitor discharge without adding another switch…

[s/v Jedi]

Originally Posted by ChrisOwens

For the cost of a resistor, a zener diode, and a relay, manufacturers could be building this "soft start" feature into the inverter itself. Why don't they?

There is more to it than that. When automating this, they need a big contactor or a bank of MOSFET’s as well, because they can’t physically turn that main switch on or off. The capacitors can not be directly connected to the DC terminals.

Cost and size limitations will be big reasons to leave it out. Also, they require qualified installers and they know that the capacitors require pre-charging.

[goboatingnow]

Originally Posted by noelex 77

Cutting off the DC supply to the inverter will always result in zero power draw. The draw when turning the inverter off manually (if this is an option for your inverter) depends on design of the inverter and whether this is done at the inverter or via a remote panel (in which case the inverter needs to maintain communication with the remote panel).



I have just checked the power draw of our inverter when manually switched off at the inverter itself and it is drawing 1.25w. Low enough to ignore in most circumstances, but it is not zero. More importantly, this is not practical, as the inverter is located in a inconvenient position.

All electronics have some quiescent draw. Once it’s very very small it’s essentially ignorable.

In my case the on off powers down the switcher control. What’s left is micro amps largely irelevant

I’d never disconnect the dc feed habitually two risky and likely to lead to trouble. D.c is permanently connected at all times.

In electronics zero is an imaginary number anyway.

[s/v Jedi]

The components used can easily be identified on Amazon by using the names I added to the diagram for a search and using the pictures as visual identification.

I grabbed the small resistors and diodes from my stockpiles but very affordable assortments are available as well as the specific components used individually (but price per component goes up of course unless you buy hundreds)

This needs to be installed close to the main switch that is bypassed by it. If you don’t have one yet then it’s a good time to get one installed and check those big DC cables as well. I like the small Blue Sea Systems switch as pictured in my diagram for this, also available on Amazon.

If you need to make new DC cables or only a short one to install a switch, and you don’t have the big crimper for the cable lugs, here is a tip: when you buy the lugs and cables at their store, West Marine allows you to use their assembly bench that has this crimper. I never actually used it but I was at a store recently and saw they still have these. You can also order pre-made cables.
I urge you not to solder your own, nor use those hammer-type crimpers. This cabling is what can start electrical fires, make sure it is in perfect condition

[Dirk01]

Maybe that article gives you some hints for your design?

https://www.sensata.com/sites/defaul...whitepaper.pdf

Please be aware, that due to the added induction of the wiring the voltage on the contacts could reach +70% (or a bit more) of the battery voltage (and some ySeconds later).

Cheers
Dirk

[Dirk01]

Or you just use this: https://www.rec-bms.com/wp-content/u..._0_Timeset.pdf

And maybe the main contactor should be a toggle version? A permanently ON coil of the main switch draws a lot of juice also. Look at this maybe: https://tbs-electronics.nl/wp-conten...BP-EN-REVB.pdf

A save toggling relay is offered by taoperformance if you only have an ON/OFF switch. You have to get a customer status for login first.

Cheers
Dirk

[Emmalina]

Personally I have a 4 farad cap on my 12v system for this sort of thing a lot easier !

[ahun]

We figured you only really have to do the precharge the very first time you power up the inverter. Unless the system is turned off months and months on end the capacitors stay charged up enough without causing any problems when restarting. So it should be anyway.