Will this work for pre-charging inverter capacitors?

krby

Solar Addict
I'm building a mobile-ish setup that will have a battery disconnect (one of the big RV / Boat from Blue Sea or similar) switch between the battery and the inverter charger. I'll leave it off most of the time. I would like to avoid the high current inrush and spark at the switch when switching back to the "on"position. I'm understand and have used the resistor pre-charge method in the past for R/C helicopters, but I want somethign simple to operate.

I'm thinking I could build a parallel pre-charge circuit between the battery and the inverter. This would have smaller wire, an appropriate resistor, and a momentary button. The power-on procedure would be like this:
  1. Disconnect is in OFF posiition
  2. Press momentary button, count to 5, release the button
  3. Move disconnect to ON position.

Will this work? I'm breaking the pre-charge circuit before making the main circuit, but the capacitors shouldn't discharge in a few seconds, right?
 

gnubie

Photon Sorcerer
That will certainly do the pre-charge. Use a meter to watch the voltage rise the first time to gauge how long the button should be pressed. If the inverter is off, most of them are soft off these days, the charge on the capacitors won't be depleted significantly over a few seconds between you releasing the button and moving the switch to on.

You could also use a 3 position switch - off, 1st on position is precharge, 2nd on position is full current available to inverter.
 

krby

Solar Addict
You could also use a 3 position switch - off, 1st on position is precharge, 2nd on position is full current available to inverter.

Cool, thanks for the confirmation. That's actually the thing I'm thinking about, but when I wrote it felt hard to explain so I wrote the first idea with the separate button. Re-timing. Ya, will do.
 

krby

Solar Addict
So, in order to size the resistor vs how long the pre-charge will take, I need to know the capacitance of the inverter. I don't have my inverter yet, but I'd like to get a resistor ordered for the initial bench-test hookup. Watching @Will Prowse 's precharge video, he links to a 30Ohm 25W resistor, and he says to use it for a few seconds. He's demonstrating on 24V, so working backwards, that inverter has probably somewhere between 25,000 and 100,000 microfarads.

Can someone give me a ballpark of the capacitance on a 24V 3000W inverter? I expect it'll be different from each, I'm just hoping they'll be in the same ballpark, is it 10,000 microF or more than 1F? In case someone has one and is willing to discharge and measure it, I'm looking at the Victron Multiplus 24/3000VA.
 
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Johan

The law of conservation of misery.
@krby Do you use a BMS? If so, which one? This may open some additional options. Also, you may not need to break the precharge circuit at all before making the main circuit, because as soon as you connect the main circuit, the resistance of the main circuit is nearly zero, so nearly all current will pass through the main circuit and nearly none through the precharge circuit. This principle is used by e.g. Jack Rickard as shown in the user manual p23 of the " ESP32 BMS Controller Assembly". Theoretically, without Jack's hardware, you could achieve similar behavior as Jack' s system using two contactors (kind of high-current relay) in series, of which the second one has a precharge resistor R in parallel with itself as shown in Figure 1. Works like this: Contactor 1 closes first. This allows current to flow trough itself (duh) and R, but not yet through contactor 2. This allows the precharge. Next, after a time delay of your choosing, contactor 2 is closed (effectively fully bypassing R, but R is not disconnected). The time delay can be easily set using a cheap compact simple programmable timer (This link is key to this solution! So no design & soldering of capacitors, diodes etc., like here with a soft starter). Assuming that the contactors are controlled by the BMS, now you only need to (dis)connect the (low) power connection from the battery to the BMS to turn off/on the whole system without sparks, IF the BMS allows such. This avoids the (perhaps negligible) risk that someone in a hurry quickly moves the switch/button past the precharge contact, possibly damaging the inverter. Why neglegible: the precharge spike may last only in the order of milliseconds.

precharging circuit v1.png
Figure 1: The manual disconnect in rightmost purple box can be closed, powering the buck, in turn powering the BMS (or maybe it is already on through power on pin 4 when a charger is present), in turn performing checks, and if all OK, closing both contactors with a pause between them. Later on, the manual disconnect can cut the buck power, in turn opening the contactors and cutting power to the BMS so it does not drain the battery. (download PDF version to read small text).

Figure 1 system's disadvantage: BMS gets power through port 2 or 4. So even after a manual disconnect cutting power to port 2, the BMS can theoretically still drain the battery while powered by mains, if present, through port 4. This can be avoided by a better (?) design, shown in Figure 2.

precharging circuit v2.png
Figure 2: New version. The manual disconnect in bottommost purple box can be closed, powering the BMS, in turn performing checks, and if all OK, closing both contactors with a pause between them. Later on, the manual disconnect can cut the BMS power, in turn opening all contactors and relay (download PDF version to read small text).

🤜Update:
Forget about the previous two designs, someone else has already invented the wheel: Tiny BMS has an integrated timer and a programmable precharge duration and the option to use the on-board FET to replace one (expensive!?) external contactor(y). For less than half the price of an Orion.

Alternatively, see also:
[1] http://liionbms.com/php/precharge.php
In addition to the two contactors used by Jack, this approach has relay "K1" [1] to disconnect the precharge resistor "when the system if off" [1] (I do not understand this - doesn't K3 already fulfill this function!?). Maybe in the rare case that K3 is arc welded closed, K1 should prevent a closed circuit as a back-up.
 

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Geriakt

Solar Enthusiast
So, in order to size the resistor vs how long the pre-charge will take, I need to know the capacitance of the inverter. I don't have my inverter yet, but I'd like to get a resistor ordered for the initial bench-test hookup. Watching @Will Prowse 's precharge video, he links to a 30Ohm 25W resistor, and he says to use it for a few seconds. He's demonstrating on 24V, so working backwards, that inverter has probably somewhere between 25,000 and 100,000 microfarads.

Can someone give me a ballpark of the capacitance on a 24V 3000W inverter? I expect it'll be different from each, I'm just hoping they'll be in the same ballpark, is it 10,000 microF or more than 1F? In case someone has one and is willing to discharge and measure it, I'm looking at the Victron Multiplus 24/3000VA.
I made a pre-charge for my Victron Multiplus 24/3000. I used a formula to determine my resistor but do not remember it. I am using a 7watt 120ohm resistor connected between my contactor posts. I open my manual battery switch and the resistor precharges the Multiplus. I wait 10 seconds and turn on the contactor.
 

krby

Solar Addict
Thanks all for the replies, I ended up with a 20ohm 50W resistor for just a few dollars. Once everything shows up, I'll measure the inductance and post back here so others can find it.
 

erik.calco

Solar Badger
Your diagram looks way cooler at night.
1574350753055.png
(no idea why it does that when you click on it)

Nice, diagram, though! 👍
 

Johan

The law of conservation of misery.
Your diagram looks way cooler at night.
View attachment 2344
(no idea why it does that when you click on it)

Nice, diagram, though! 👍
Thanks, solved: The image had a transparent background, changed to white. Also this forum compresses images, rendering the small print unreadable, so I attached PDF versions of the diagrams with infinite sharpness :) for those who like to view the devil's details.
 

electric

Solar Addict
Can someone give me a ballpark of the capacitance on a 24V 3000W inverter? I expect it'll be different from each, I'm just hoping they'll be in the same ballpark, is it 10,000 microF or more than 1F? In case someone has one and is willing to discharge and measure it, I'm looking at the Victron Multiplus 24/3000VA.
Approx. 80,000 uF or 80 mF or 0.08 F
EDIT: Your 20 Ohm 50W resistor will work just fine. At this value you really need no more than 1s to 1.5s of pre-charge time for safe operation at 24V.
 
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krby

Solar Addict
Approx. 80,000 uF or 80 mF or 0.08 F
EDIT: Your 20 Ohm 50W resistor will work just fine. At this value you really need no more than 1s to 1.5s of pre-charge time for safe operation at 24V.

Thanks! Also, I can't believe I didn't think of using 'uF'. I spent way too much time looking for a way to put the proper symbol for micro
 

krby

Solar Addict
Thanks! Also, I can't believe I didn't think of using 'uF'. I spent way too much time looking for a way to put the proper symbol for micro

Victron Multiplus 24V 3000VA arrived today (thanks to @Justin Laureltec at Bay Marine!) , so I got out a DMM to test the capacitance and after taking all the precautions, making it was discharged, etc. My multimeter only ranges up to 9999uF :-(
Oh well, I've got the 20Ohm resistor in hand and as @electric has said, that will work fine.
 

Keith C

Ahhh, the sharks have him.
You don't ever use the capacitance in the calculation anyway. You assume a dead - short and limit the current to what the switch can safely handle. So if you have 24V and a switch good for, say, 16Amps you simply use Ohm's Law. 24V/16A = 1.5ohms. That's the resistance you want. Then that resistor has to be large enough to dissipate the heat and handle the current. That represents about 350Watts but only briefly. A 30W or higher power rated resistor should work. Any higher value will be fine too.

You picked 20 ohms. That's only 24/20 = 1.2Amps. Peak power would be = 1.2 x 1.2 x 20 = 28W. But it tapers down from that quickly. A fine choice by the way.

In your application I'd add a simple panel meter like:
Fleabay LCD panel meter
hooked across the inverter's input. (you'd select 200V and have the added regulator choice)

It will show you at a glance if the caps are near the battery voltage. It will show you when the caps are discharged. It will show you when to release the "Pre-charge" button and close the main switch on the battery bank. It will also show you what your battery voltage is during start ups which helps keep you in the mental loop on the batteries.
 

krby

Solar Addict
You don't ever use the capacitance in the calculation anyway. You assume a dead - short and limit the current to what the switch can safely handle.
Your point about assuming the dead short is really important, but I think there is reason to look at the capacitance, it tells me how long I'll need to pre-charge.

The assumption I made but didn't say in my first post is that I was only selecting from a range of capacitors that the circuit could handle. I was planning no more than 30A so I started looking at 1 Ohm and above resistors. Pretty quickly, I narrowed my choices down to 5, 10, 15, 25 Ohm. At a dead short, all of these will work for the circuit I have planned (no more than ~5A) but I didn't know the capacitance so I wouldn't now how long it would take with each.
 

Keith C

Ahhh, the sharks have him.
This takes 2 minutes to 50V at 50mA to charge up.. 😄

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SCClockDr

Solar Addict
You don't ever use the capacitance in the calculation anyway. You assume a dead - short and limit the current to what the switch can safely handle. So if you have 24V and a switch good for, say, 16Amps you simply use Ohm's Law. 24V/16A = 1.5ohms. That's the resistance you want. Then that resistor has to be large enough to dissipate the heat and handle the current. That represents about 350Watts but only briefly. A 30W or higher power rated resistor should work. Any higher value will be fine too.

You picked 20 ohms. That's only 24/20 = 1.2Amps. Peak power would be = 1.2 x 1.2 x 20 = 28W. But it tapers down from that quickly. A fine choice by the way.

In your application I'd add a simple panel meter like:
Fleabay LCD panel meter
hooked across the inverter's input. (you'd select 200V and have the added regulator choice)

It will show you at a glance if the caps are near the battery voltage. It will show you when the caps are discharged. It will show you when to release the "Pre-charge" button and close the main switch on the battery bank. It will also show you what your battery voltage is during start ups which helps keep you in the mental loop on the batteries.
I'd like to offer an indicator alternative:
A voltage divided 3 or 5mm led that glows as the caps charge and quits when the charge current drops below 10-20% BMS capacity. (Draws only during the cap charging process.)
OR
That illuminates when the inverter side is @ 90-95% nominal battery voltage, meaning the caps are nearly charged. (Constant draw with inverter on.)
 
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