diy solar

diy solar

Inverter capacitor charging spark!

Thank you both.
My only concern is the 15 amp draw (mostly fan) on the diesel-fired boiler purge cycle, when it "sees" 12V from the pre-charge circuit. I bought a heavy aluminum - finned 10ohm resistor for that pre-charge circuit.
The other draws are very nominal.

Doug
 
DW SD said:
Thank you both.
My only concern is the 15 amp draw (mostly fan) on the diesel-fired boiler purge cycle, when it "sees" 12V from the pre-charge circuit. I bought a heavy aluminum - finned 10ohm resistor for that pre-charge circuit.
The other draws are very nominal.

Doug
Click to expand...
You definitely do not want that fan turned on when you try to pre-charge.
 
FilterGuy said:
That is not my experience..... I have built systems with the pre-charge on the DC loads and the Inverter.... it worked fine. (Keep in mind that I always make my pre-charge resistor a very low value. Clearly, this would not work well if you have huge DC loads but smaller loads are ok.

Having said that.... my typical build will have the precharge on the inverter only.
Click to expand...

I haven't built any, so your experience counts for more.

But I think we're saying the same thing. Resistor has to be low enough value to power whatever DC loads are present and still charge capacitor most of the way to battery voltage. Small load is almost the same as no load, just capacitor.

15A heater fan/ignitor is too much (unless resistor is 0.05 ohm for 0.75V drop (12W dissipation). That'll limit capacitor charging current to 250A, and will dissipate 3kW for a moment while charging capacitor. 30' of 12 awg wire would serve as such a resistor. 15' of Romex, join white and black wire at far end.
 
cinergi said:
Food for thought ...

I've blown 300 amp Class T fuses due to inrush.
I've destroyed a Chargery DCC SSR due to inrush.
I've damaged a battery disconnect switch due to inrush.
I now use a contactor. Inrush will quickly destroy its relay contacts.

Why expose the capacitors to this? Even if it's unclear whether this shortens the life, why risk it?
Why expose your battery to this? We don't know how bad this is for LFP cells but we do know that LFP cells can be locally depleted with too much current

Your BMS should generally never disconnect (and thus reconnect). If you're worried about daily reconnection of the BMS, I suggest you change the parameters of the connected equipment to shut down prior to the BMS disconnect. The BMS is a last line of defense. Not an on/off switch.

Until I have a pre-charge circuit, I just keep a resistor connected across the contactor so that my capacitors are always charged.

Caveat: my system is a lot more power hungry than most. 48v system with 2 5kVa inverters = a lot of inrush.
Click to expand...
I have a 60v 60a Daly BMS that is bad after 5 weeks. My inverters are on a timer with about only 500 Watts on them at night. Daly is sending me a replacement and now I'd like to protect the BMS with some type of surge absorber. Is there anything I can use?
 
Danner said:
I have a 60v 60a Daly BMS that is bad after 5 weeks. My inverters are on a timer with about only 500 Watts on them at night. Daly is sending me a replacement and now I'd like to protect the BMS with some type of surge absorber. Is there anything I can use?
Click to expand...
How is the timer configured? Are you cutting the dc input?
why not control the inverter on switch? Leave the batteries connected always, and only shut off the inverter with the timer... WAY better control, and the capacitors never discharge...
 
Supervstech said:
How is the timer configured? Are you cutting the dc input?
why not control the inverter on switch? Leave the batteries connected always, and only shut off the inverter with the timer... WAY better control, and the capacitors never discharge...
Click to expand...
I'm switching off/on the grid tied inverter everyday. But my Daly bms is already bad.
 
Yes, A very old post. Since I made that post I realized the Perco Switch is not rated for 48V systems. Please download this resource. It has a switch and wiring for a 48V setup.

diysolarforum.com

Inverter Disconnect Switch with Precharge

To get the paper, click on the orange button at the top of this page. This circuit is designed as a disconnect switch that allows the user to pre-charge the inverter capacitors before turning the switch completely on. 26 June 2020 update: At...
diysolarforum.com diysolarforum.com

As far as the 10-ohm resister goes, it would probably work fine. The max current would be around 6A (60V/10ohm=6A).
This is higher than the 2A I designed for but is still probably ok.

However, the potential bad scenario is if the inverter is turned on and the switch is left in the precharge position. In most cases, the inverter will turn on but immediately go to a low voltage fault mode so the current through the resistor will remain low. However, it is a 10W resistor so at full voltage of the battery, it can only handle 10W/59V = 0.17A for a sustained period.

If I were in your position, I would set it up with the 10ohm/10W resister and try leaving it in the precharge state for a little time while the inverter is on. If the resister does not get excessively hot, you are fine. If it gets too hot..... get a larger resister.
 
FilterGuy said:
Yes, A very old post. Since I made that post I realized the Perco Switch is not rated for 48V systems. Please download this resource. It has a switch and wiring for a 48V setup.

diysolarforum.com

Inverter Disconnect Switch with Precharge

To get the paper, click on the orange button at the top of this page. This circuit is designed as a disconnect switch that allows the user to pre-charge the inverter capacitors before turning the switch completely on. 26 June 2020 update: At...
diysolarforum.com diysolarforum.com

As far as the 10-ohm resister goes, it would probably work fine. The max current would be around 6A (60V/10ohm=6A).
This is higher than the 2A I designed for but is still probably ok.

However, the potential bad scenario is if the inverter is turned on and the switch is left in the precharge position. In most cases, the inverter will turn on but immediately go to a low voltage fault mode so the current through the resistor will remain low. However, it is a 10W resistor so at full voltage of the battery, it can only handle 10W/59V = 0.17A for a sustained period.

If I were in your position, I would set it up with the 10ohm/10W resister and try leaving it in the precharge state for a little time while the inverter is on. If the resister does not get excessively hot, you are fine. If it gets too hot..... get a larger resister.
Click to expand...

What I'm doing right now is really just a temporary solution to quickly connect the inverter to the batteries before the proper size resistor arrives over the weekend. So I'm trying to "wing it" with what I have at hand right now. Later on I may or may not build the disconnect switch with the resistor built in, I do like that idea so I might just get on it when I have time.

One thing I'm not sure I understand though, when you say "
I would set it up with the 10ohm/10W resister and try leaving it in the precharge state for a little time while the inverter is on
Click to expand...
The inverter can't be on unless I connect it using this resistor, right? Or did you mean just do it and then let it run for a little bit longer to see if it's ok to use it repeatedly?
 
martan said:
The inverter can't be on unless I connect it using this resistor, right? Or did you mean just do it and then let it run for a little bit longer to see if it's ok to use it repeatedly?
Click to expand...
Yes.... kinda

The potentially dangerous scenario with the switch design is if the user puts the switch into the precharge position while the inverter is on and then leaves it there for an extended time. The inverter will almost certainly turn on and then immediately fault to a low-voltage error. In this condition, the inverter will be on but not inverting but will still be pulling some current to power the control logic. What we don't know is how much current the inverter will draw in this fault mode. If it is too much current it will overheat the resistor. This is unlikely.... but every inverter is different. My suggestion is to set up this bad scenario and see if it gets too hot.

If you are just using the resistor to manually precharge.... don't worry about any of the above. You will not be standing there holding the resistor to the contacts for an extended time with the inverter turned on..... and even if you do and it happens to get hot you will know it and disconnect.
 
FilterGuy said:
Yes.... kinda

The potentially dangerous scenario with the switch design is if the user puts the switch into the precharge position while the inverter is on and then leaves it there for an extended time. The inverter will almost certainly turn on and then immediately fault to a low-voltage error. In this condition, the inverter will be on but not inverting but will still be pulling some current to power the control logic. What we don't know is how much current the inverter will draw in this fault mode. If it is too much current it will overheat the resistor. This is unlikely.... but every inverter is different. My suggestion is to set up this bad scenario and see if it gets too hot.

If you are just using the resistor to manually precharge.... don't worry about any of the above. You will not be standing there holding the resistor to the contacts for an extended time with the inverter turned on..... and even if you do and it happens to get hot you will know it and disconnect.
Click to expand...
Ok great, thank you. I'll give it a try, maybe tonight. Would you say holding it to pre-charge for about 20-30 seconds with this 10 ohm piece should be enough? I can't hurt anything if I hold it longer for the very first time, if anything it just gets warmer or hot, right? That goes back to your "test" to hold it there longer while it's on. I really just worry about the first spark, for now.

Btw., I checked the switch if I wanted to make one per your instruction, they run for about $60 a piece, not cheap for sure, for what it is.
 
I do pre-charge with inverter power switch in the OFF position, you can put a DC Volt meter at the DC input of the inverter and see how long it will take for the Voltage at the inverter rises close to the battery Voltage.
 
martan said:
Ok great, thank you. I'll give it a try, maybe tonight. Would you say holding it to pre-charge for about 20-30 seconds with this 10 ohm piece should be enough?
Click to expand...
With a 10ohm resister, the current is ~3x higher than in my design so you only need it for a second or two. Probably even less. You might even see a tiny spark when you do it. (It won't be enough to worry about because the current will max out at less than 6A)

Just be sure the inverter is off and quickly put the lug on the terminal once you take off the resister.

BTW" I like to hook up the negative to both the inverter and battery. Then hook the positive to the battery inverter and do the resister trick at the battery. This way, you don't have a big energized wire hanging around and looking for a place to short out.
 
Last edited:
Bud Martin said:
I do pre-charge with inverter power switch in the OFF position, you can put a DC Volt meter at the DC input of the inverter and see how long it will take for the Voltage at the inverter rises close to the battery Voltage.
Click to expand...
Yes, that's what I was thinking, to check on the inverter side as well.
 
FilterGuy said:
With a 10ohm resister, the current is ~3x higher than in my design so you only need it for a second or two. Probably even less. You might even see a tiny spark when you do it. (It won't be enough to worry about because the current will max out at less than 6A)

Just be sure the inverter is off and quickly put the lug on the terminal once you take off the resister.
Click to expand...
Worked like a charm, inverter is connected.
 
pollenface said:
I imagine that if you only charge to 30v the spark/inrush will be significantly reduced
Click to expand...
Probably, but I really wanted to try this 10W 10 ohm resistor I had here laying around, if it can handle it , and it did just fine.
 
You must log in or register to reply here.

Similar threads

M
Super capacitor storage
Replies
7
Views
656
Warpspeed
W
F
How to avoid spark on when connecting my inverter to my batteries
2
Replies
38
Views
3K
timselectric
T
T
Inverter capacitor precharge procedure
Replies
8
Views
1K
MiSh
M
J
Battery Charging
Replies
6
Views
330
johnch121
J
S
Inverter to battery
Replies
13
Views
425
hardtop
hardtop
Back
Top