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prime the caps in my inverter by connecting it directly to my converter

John Frum

John Frum

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I have a powermax pm3-30-24lk and a samlex pst-1500-24.

Here is the idea...
With the converter in fixed output mode I connect the dc leads directly to the samlex inverter.
The cc/cv power supply will decrease the voltage to limit the the current to 30 amps thus avoiding the inrush stress on the inverter capacitors.
I wonder if I would just be stressing the converter instead.
Please advise.
 
Is 30Amps enough to actually power the inverter?
I think this would work as well as using a resistor to precharge the caps.

I don’t think it would hurt the converter... I don’t think anyway...
 
If you're sure the converter has a CC mode then it's ok.

But what's wrong with using a simple resistor?
 
30 ams * 27.2 = 816 watts.
I hope that will be enough to get the unit to power up and run an led lightbulb just for a basic smoke test.
 
What about the slow start cct’s used on aircon units ? Never looked at one but there must be some inductor in there that resists the initial current flow and thereafter it is only the resistance of the coil that will have a small volt drop.
 
pierre said:
What about the slow start cct’s used on aircon units ? Never looked at one but there must be some inductor in there that resists the initial current flow and thereafter it is only the resistance of the coil that will have a small volt drop.
Click to expand...
I'm not understanding what you are asking here.
I'm an HVAC tech, so I probably have what you are talking about.
 
smoothJoey said:
I have a powermax pm3-30-24lk and a samlex pst-1500-24.

Here is the idea...
With the converter in fixed output mode I connect the dc leads directly to the samlex inverter.
The cc/cv power supply will decrease the voltage to limit the the current to 30 amps thus avoiding the inrush stress on the inverter capacitors.
I wonder if I would just be stressing the converter instead.
Please advise.
Click to expand...
It would probably work fine..... However the converter is probably not designed to dump current into a 0 volt load (Dead short) which is the initial condition of the empty capacitors. Consequently, the current limiting circuit will be operating in a voltage-current range that it was not intended for. Will it work? Probably.
 
Supervstech said:
I'm not understanding what you are asking here.
I'm an HVAC tech, so I probably have what you are talking about.
Click to expand...
I am referring to cct’s that can delay/ramp ( di/dt ) the inrush current due to the capacitors in an inverter when first applying voltage. I thought that this discussion was all about that. Sorry if I missed the bus on this one :cry:
 
BiduleOhm said:
If you're sure the converter has a CC mode then it's ok.

But what's wrong with using a simple resistor?
Click to expand...

Nothing wrong at all using simple resistor. No high power inverter should self activate when battery applied. Might have to power processor and display so just be sure resistor is not too high in value. Prime objective is to avoid pitting breaker contacts.
 
Just trying to understand this procedure when turning on an inverter that may have not been on for a bit.
Most inverters in an RV installation have a remote inside the RV for turning the inverter on and off when not in use. So would one have to do the capacitor method to charge the inverter caps previous to each time the inverter is turned on via the remote inside the RV? Or is there a better way? Also how long would the inverters caps hold the charge when turned off? ?
This may have been covered in previous threads but I can’t seem to find them ?
 
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On all inverters I am aware of, the capacitors are between the DC input and the On/Of switch/circuit. Consequently, once the inverter is successfully connected to the battery, you do not have to pre-charge again till the DC is disconnected.

1593661849746.png

How fast the inverter capacitors discharge after the DC is connected will vary by inverter, but it is usually within a couple minutes.

One thing I have never figured out is how to deal with the case when the BMS shuts off discharge. When it decides to turn back on there is likely to be a surge. This is a good reason to make sure your inverter low-voltage stop is greater than the BMS low-voltage shut-off. (The Chargery has an add-on that will turn on a charge circuit for a few seconds first, but many/most BMSs do not have this.)
 
FilterGuy said:
One thing I have never figured out is how to deal with the case when the BMS shuts off discharge. When it decides to turn back on there is likely to be a surge. This is a good reason to make sure your inverter low-voltage stop is greater than the BMS low-voltage shut-off. (The Chargery has an add-on that will turn on a charge circuit for a few seconds first, but many/most BMSs do not have this.)
Click to expand...

BTW: It may not be as bad as I make it sound. The most likely reason for the BMS to shut off discharge is if the battery or cell is too low. Many (Most?) BMS's will not turn back on till the battery gets charged to an acceptable level. I have not played with it, but I suspect whatever is charging the battery enough to turn the BMS back on will also charge the inverter capacitors. Consequently, there should not be a surge when the BMS turns on.
 
Thanks FG. The inverter in an RV unlike one used for home use (usually on 24/7) would be turned off via a remote switch for periods when not being used for ac power to avoid unnecessary battery drain. Turning the inverter back on later would cause the caps to be charged again. So one would need to use a resistor to absorb the initial burst charging the inverters caps back up again or risk sparks or damage and wear on the BMS or switch. My question was what is a simple method
of using a resistor to charge each time the inverter is needed?
Not an electrician so an easy to understand method and simple drawing would be great ?
 
Two things:

1) Turning the inverter off but leaving it hooked to the battery will *not* discharge the capacitors... even on the smaller car inverter. The capacitors will only discharge if the inverter is disconnected from the battery. Notice that in my representative drawing above, the inverter switch is *after* the capacitors. I have yet to find an inverter of any kind that is not wired this way.

2) Check out this resource for an inverter disconnect switch with a built in pre-charge circuit:
https://diysolarforum.com/resources/inverter-disconnect-switch-with-precharge.60/
 
Thanks, that answers my question. Using a resistor to charge caps would only be needed when batteries are disconnected/reconnected. Upon thinking about it it makes sense as the sparking issue ⚡️⚡️I’ve experienced happens when the inverter is off.
I’m using Two Lion energy 1300s in series and Will mentioned in a previous video the batteries go into safe mode from the burst out to the inverter and had to reset the BMS.
Add another page learned from the DIY forum. ?
 
Capacitor in-rush will be an issue for systems where the inverter is disconnected from the battery.
Here is just one scenario for a separate port bms.
The battery is drawn down and low voltage disconnect disables the discharge port.
The charger is still connected and will eventually rises enough to re-enable the discharge port.
In the meantime the caps have discharged.
I don't see how to easily have a self healing system that doesn't suffer from the cap in-rush issue.
 
I find in practical use that it takes more than a day for the caps to discharge enough to cause a spark.

YMMV
 
Does someone perhaps know the approx. value of the capacitance of say , a 1kW inverter , as measured at the input terminals of the inverter.
I am playing around with RC time constants and circuits as a way to pre-charge the inverter caps.
 
smoothJoey said:
Capacitor in-rush will be an issue for systems where the inverter is disconnected from the battery.
Here is just one scenario for a separate port bms.
The battery is drawn down and low voltage disconnect disables the discharge port.
The charger is still connected and will eventually rises enough to re-enable the discharge port.
In the meantime the caps have discharged.
I don't see how to easily have a self healing system that doesn't suffer from the cap in-rush issue.
Click to expand...

Shoot I missed the point again.
In the above scenario the charger will likely prime the caps before the battery is reconnected.
As @FilterGuy had already explained.
 
FilterGuy said:
On all inverters I am aware of, the capacitors are between the DC input and the On/Of switch/circuit. Consequently, once the inverter is successfully connected to the battery, you do not have to pre-charge again till the DC is disconnected.

View attachment 16630

How fast the inverter capacitors discharge after the DC is connected will vary by inverter, but it is usually within a couple minutes.

One thing I have never figured out is how to deal with the case when the BMS shuts off discharge. When it decides to turn back on there is likely to be a surge. This is a good reason to make sure your inverter low-voltage stop is greater than the BMS low-voltage shut-off. (The Chargery has an add-on that will turn on a charge circuit for a few seconds first, but many/most BMSs do not have this.)
Click to expand...
For inverters larger then about a 300 watts, any ON/OFF switch within inverter will be just shutting down the electronics controlling the inverter's MOSFET's drivers. They do not use a large and expensive switch (ie. your DC breaker/switch), so capacitors are always on the battery lines coming into the inverter.

For a 3kW to 8kW inverter they typically have 7,000 to 10,000 uF's consisting of several paralleled capacitors. Throwing 48v on these caps is not that tough on caps. They are more stressed by the constant ripple current of normal inverter operation, but the large charging surge current is hard on the DC breaker if it is used to turn on inverter. It causes pitting on its contact surfaces, like little spot welding pock marks that can increase breaker switch resistance and eventually cause it to start arcing on large inverter current draw. High surge current could cause the breaker's contacts to stick together by the spot welding action and not allow the breaker to pop open for its primary safety function. And there is aways the possiblity the high surge current immediately pops the breaker back open compounding the pitting process.

Discharging time on these caps is variable depending on models of inverter. Some inverters put a couple mA current bleed resistor across these caps to accelerate their discharge time. A day or more to as little as 5 minutes might not be unexpected.

BMS with MOSFET as series cutout switch doesn't have the problems of mechanical contacts, but another good reason a solenoid contactor cutout is a bad idea. There might be a small possiblity that when the BMS switch re-engages there is enough surge current created to pop the DC circuit breaker. I would expect the series resistance of BMS MOSFET's, battery line resistance, DC breaker resistance, and any current shunt resistance is normally enough to prevent popping circuit breaker. If you have this problem your DC breaker amperage size is probably not high enough or breaker is defective.
 
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