Using DC to DC charger to precharge inverters?

[justinm001]

I had an issue over the weekend where my batteries BMS's were tripping and needed to precharge the 2x5000w Quattro inverters which have massive capacitators. I know the SOK batteries have a precharge setting but it a bit complicated and I need to google it everytime.... I really need to print a label with any instructions and just stick to the batteries. I messed with the batteries but was having an issue keeping them out of SCP even with breaker off and when they did I messed up turning on the precharge option.

So I turned the key on my RV to on and used my 4x400w 24/48 DC to DC chargers and the inverters powered up instantly with no issues. This happened a few times yesterday and each time it worked perfectly.

I know the capacitators use a ton of power for milliseconds to charge which cause the issues with the BMS thinking they're short circuiting. But is it safe to use a DC to DC charger? Should the battery breaker be off or on or does it not matter? The chassis batteries are 4 huge lead acid starter so inrush shouldn't be an issue with those, just unsure how it affects the DC to DC converters. Also Victron only makes max 400w for the 24/48 so even though I have 4 I worry it could be using 1 for that split rush and unsure how much power the capacitators actually pull, or if the DC to DC converters safely limit the power so it doesn't matter.

 

[HRTKD]

I had considered using a standard RV converter to do the pre-charge. I don't think that would have been much different from using the DC-DC charger like you did.

 

[Crowz]

A pencil is a bunch cheaper

If you want something simple and permanent then add a resistor on a push button switch and have it wired inline. Push the button to pre-charge when needed.

 

[DIYrich]

Disconnect the chassis batteries. They may be able to handle the discharge, but the fuses may not be too happy.

 

[justinm001]

A pencil is a bunch cheaper

If you want something simple and permanent then add a resistor on a push button switch and have it wired inline. Push the button to pre-charge when needed.

Dc to dc charger is free as it's used to charge from the alt. Also simple as I just turn the key to turn them on and then flip on my batteries.

Just unsure if it affects anything with the limits of the DC-DC converters or if they limit the power or what happens and since it's milliseconds idk if a CT would register it and it's on the other side of my shunt

 

[justinm001]

Disconnect the chassis batteries. They may be able to handle the discharge, but the fuses may not be too happy.

Fuses are thermal and thus don't protect against quick inrush current lasting under a few seconds.

Lots of motors have massive startup currents like AC units so they'd blow if this was an issue.

I do have fuses on the alt wire and coming out of each of the DC-DC converters

 

[hwy17]

My HLG-185H-54A can charge and power on my Schneider XW+.

When I first bought the XW I didn't have any 48v battery and could've really used that knowledge when I was just wanting to power it up for configuration.

 

[justinm001]

My HLG-185H-54A can charge and power on my Schneider XW+.

When I first bought the XW I didn't have any 48v battery and could've really used that knowledge when I was just wanting to power it up for configuration.

The DC-DC converters worked perfectly multiple times and its just like your power supply. What I don't know is if its safe for them. I think it is since the can limit the output power but just wanted to be certain as I don't want to fry $1000 in converters or cause any other issues. Idk the technology inside converters or how they work.

If they properly limit the output power thats perfectly fine as the inverter capacitators don't need instant power they could charge off 1 watt it'll just take longer which doesn't really matter. But massive spikes in current can cause all kinds of chaos and not sure if there's any protections.

 

[hwy17]

The DC-DC converters worked perfectly multiple times and its just like your power supply. What I don't know is if its safe for them. I think it is since the can limit the output power but just wanted to be certain as I don't want to fry $1000 in converters or cause any other issues. Idk the technology inside converters or how they work.

If they properly limit the output power thats perfectly fine as the inverter capacitators don't need instant power they could charge off 1 watt it'll just take longer which doesn't really matter. But massive spikes in current can cause all kinds of chaos and not sure if there's any protections.

Yeah I would have the same concerns. they're 60 bucks if an AC plug in precharger is a convenient backup for you. Otherwise a precharge circuit with a light bulb and a momentary switch can probably be made for less.

 

[AntronX]

10 ohm wirewound resistor will limit precharge current to 5A. You can get 10 of them for $7.

 

[justinm001]

10 ohm wirewound resistor will limit precharge current to 5A. You can get 10 of them for $7.

I get this BUT it doesn't solve the issue. Say I'm driving with my DC-DC chargers on (they're always on) and my inverters reset for whatever reason (rare but happens) It'll blow my battery BMS's and use the DC-DC charger anyways because the system is on.

I also don't want to add additional components and another process to my crazy wired system. If I get an alert that my inverters are off I can easily troubleshoot with the wife over the phone to flip the RV key to on and see if it comes online then flip the battery breakers on/off and see if the red alm is still on. I think the BMS resets itself if in SCP mode and the voltage resumes so likely turning the key would fix everything then she can turn it off and see if still fixed.

 

[HRTKD]

A momentary shutdown of the inverter may not affect the capacitors as they're still connected to the DC side.

I get the not wanting to complicate the system any more than it already is. In my case, it turned out that my LiFePO4 battery bank had no problems with the inrush when the Multiplus 12/3000 was initially connected without any help from other charge sources. That is no help to you since you already know that your battery bank can't handle it.

 

[justinm001]

Yeah I would have the same concerns. they're 60 bucks if an AC plug in precharger is a convenient backup for you. Otherwise a precharge circuit with a light bulb and a momentary switch can probably be made for less.

What is this precharger? Is it for 48V? I have a 48V AC charger or trickle charger on my list of things to buy and hardwire as a backup. Also my inverters minimum input charge is 13.2a which is a bit high if I want to run a 100ft extension cord to a 15a outlet (Victron limits the 13.2a to 80% after a period of time)

 

[hwy17]

What is this precharger? Is it for 48V? I have a 48V AC charger or trickle charger on my list of things to buy and hardwire as a backup. Also my inverters minimum input charge is 13.2a which is a bit high if I want to run a 100ft extension cord to a 15a outlet (Victron limits the 13.2a to 80% after a period of time)

The HLG-185H-54A is an LED driver I have that I have used. It's not a charger with charge profiles, but you can adjust the output voltage to your desired float and it will do a sort of CC/CV trickle charge.

 

[AntronX]

I think you need to fix your BMS tripping problem first. You may have unequal current sharing across those 48V batteries which causes one to go into overcurrent fault which leaves second battery to carry full load by itself which makes it trip as well.

 

[justinm001]

A momentary shutdown of the inverter may not affect the capacitors as they're still connected to the DC side.

I get the not wanting to complicate the system any more than it already is. In my case, it turned out that my LiFePO4 battery bank had no problems with the inrush when the Multiplus 12/3000 was initially connected without any help from other charge sources. That is no help to you since you already know that your battery bank can't handle it.

What I noticed is the capacitors drain over minutes depending on why the inverter is off. If you disconnect the DC from the inverter and hook up a voltmeter when off you can watch the voltage slowly drain from the inverter, which I'm assuming is the capacitators.

Problem is I have dual 5k quattros configured in split phase so when 1 blows they both do then they both start at the same time. The SOK precharge works even with both but its just not easy to do. I wish they would have just put a precharge button or option in the BMS.

BTW the MPPT seems to do the same as the DC-DC converter and will precharge but it depends on what happens as if there's no DC then it shuts off.

 

[HRTKD]

Yeah, if the inverter is actively providing AC power then the capacitors are going to be drained right away. But your DC source is still connected.

I'm thinking that you're looking at an edge case. In most situations where we talk about pre-charge, it's when the inverter is first commissioned or when it's brought back online after maintenance of the system. That is usually a controlled situation where you can manually intervene.

 

[justinm001]

I think you need to fix your BMS tripping problem first. You may have unequal current sharing across those 48V batteries which causes one to go into overcurrent fault which leaves second battery to carry full load by itself which makes it trip as well.

Posted what happened in here and still no clue what happened. The BMS tripping is very rare but the needing to precharge happens whenever there's an update or I'm doing maintenance on my system as I shut it all down.

But its very very rare I need close to 200A of 48V (10kw) power in an RV. Even more since I have all my important loads going through another 5kw redundant LFP system beforehand.

 

[justinm001]

Yeah, if the inverter is actively providing AC power then the capacitors are going to be drained right away. But your DC source is still connected.

I'm thinking that you're looking at an edge case. In most situations where we talk about pre-charge, it's when the inverter is first commissioned or when it's brought back online after maintenance of the system. That is usually a controlled situation where you can manually intervene.

Yes but even so using the DC-DC seems to be the easiest and best method, provided its safe and ok to use.

I know many have DC-DC chargers for their alts and many turn off their inverters when not in use so I'd think its a simple solution for others to use if they have it equipped already.

I can't figure out if these things have overload protection or how they handle overload scenarios. The "smart" ones that are chargers say they have overload protections. But I'm assuming they have to have them built in as they're converting voltage and limiting how much power they provide. I have them in parallel and they're supported in parallel.

This part of electricity is confusing to me as I have a massive 24v 270a alternator but these things provide 8.5a (400w) 48v each so something has to limit how much power they can provide or pull as I'm routinely pulling thousands of watts to the inverters and the DC-DC is only giving its limit.

IDK what would happen if I shut off my batteries and tried to pull more power from the inverter than the DC-DC charger can provide... I'm assuming the voltage would drop and the inverter would shutoff for low DC voltage.

 

[HRTKD]

I know many have DC-DC chargers for their alts and many turn off their inverters when not in use so I'd think its a simple solution for others to use if they have it equipped already.

Turning OFF the inverter and disconnecting the inverter are two different things. Turning the inverter off still has it connected to the DC side, so the capacitors are and remain fully charged.