Inverter capacitor charging spark!

[atatistcheff]

We all know that when you initially connect an inverter to power you get a spark as the capacitors charge up. For bigger inverters this spark is pretty significant. If the final connection is to your battery it means you get a tiny "weld" on the battery terminal each time you do this. I, for one, don't like to see these blemishes on my expensive LiFePO4 battery terminals. I'm wondering what folks do to avoid this - or am I just being too picky? Seems like you're going to get these marks on either the inverter or the battery terminals one way or the other unless someone has a solution that works for them.

 

[Rider]

Well, yeah, a bit OCD.... BUT...

I have a 100A switch between the battery and the inverter, so no touching wires to pre-charge. Just connect your wires and throw the switch.

 

[burtinator]

Just take a lightbulb en put it between the cable and the battery, the lamp will limit the current, after a few seconds when the capacitors have been charged just remove the lamp and connect the batteryterminal.

 

[Supervstech]

I am on the switch side of the fence. A big inverter shouldn’t be directly connected to a battery... it should have a breaker for protection from over current...
I have seen some wired with two switches connected to the inverter. One switch has a large capacity resister on a thin gauge wire to slowly fill the inverter capacitor without the big surge of voltage hitting the inverter.

 

[HighTechLab]

You really don't want to have this instant connection happening. If you are using LiFePO4, these cells have such low internal resistance they can deliver a MASSIVE amount of instantaneous current. It is not unheard of to actually melt the capacitor leads as they are pretty thin and on initial charge the capacitors can take in a HUGE amount of current, more than the actual leads can handle.

I talk about it in my video of upgrading to my LiFePO4 bank, start at 7:15

The best solution is to have a "Pre Charge" resistor as mentioned previously. In my case I still had a few shelves of Lead Acid that has high internal resistance so then when I moved over to the lithium the caps were still full.

You can use an external power supply as well to charge up prior to connecting on.

 

[PHoganDive]

You can put a momentary pushbutton switch next to the main power switch for the inverter. Use it to connect a lightbulb across the main switch contacts. Then before you turn the main switch on, hold the pushbutton on for a few seconds to charge up the caps through the lightbulb, then throw the main switch.
If you have a new labelmaker, you could even label it something like, 'press pushbutton for 5 seconds before throwing main switch'.

 

[Rider]

You really don't want to have this instant connection happening. If you are using LiFePO4, these cells have such low internal resistance they can deliver a MASSIVE amount of instantaneous current.

I'll agree with this. I have a small inverter (1000/2000w) on a single battery and the spark is minimal anyway, wasn't thinking in terms of big wattage. Larger inverters with large capacitors need a pre-charge.

 

[atatistcheff]

You can put a momentary pushbutton switch next to the main power switch for the inverter. Use it to connect a lightbulb across the main switch contacts.

What voltage/wattage bulb are we looking at here?

 

[PHoganDive]

The voltage needs to be the same or greater than the DC voltage of the system, i.e. if a 48 volt system, then the bulb needs to be at least 48 volts. In that case a standard 120 volt bulb would be fine. Wattage should probably be in the 25-100 watt range. That's enough to charge the caps reasonably quickly, without having too big a surge current when first turned on.

 

[Will Prowse]

Usually LiFePO4 BMS OCPD is triggered from charging most caps, but yeah if you are doing raw cell system, get a resistor on there and pre charge them. Hightechlab covered it nicely

 

[eddie1261]

In the case of having a bolt on fuse, your connection is to the fuse rather than the post. And as far as putting a switch between the battery post and the inverter, I remember one of the videos saying the connection should be directly to the battery. Did I get that wrong? I mean, the outward manifestation of that spark is just a visual. Doesn't that quick blast of charge into the capacitors happen no matter what? It seems like it would be the sudden movement of the energy that is cause for concern rather than whether we see a spark or not.

 

[gnubie]

I have a 24v system and use about 400 ohms of resistors to precharge the inverter's capacitors. The resistor string would be rated at 5W so it easily copes. I have a 3 position breaker and have set the 1st on position to be precharge so it's a case of turn to position 1 for 30 seconds, turn to position 2 for normal operations.

HighTechLab has it right about capacitors melting leads off but there is also often damage you can't see and its cumulative. The join between the leg and the sheet inside the capacitor will degrade with the very high inrush current and push the ESR of the capacitor up, and that's something you don't really want.

 

[Rider]

And as far as putting a switch between the battery post and the inverter, I remember one of the videos saying the connection should be directly to the battery.

In my mind, if there is no switch, you can never turn off the inverter. To turn it off, you have to disconnect battery cable somewhere. With a switch inline, I can turn off the inverter and isolate the battery with no effort.

 

[MrNatural22]

Well, yeah, a bit OCD.... BUT...

I have a 100A switch between the battery and the inverter, so no touching wires to pre-charge. Just connect your wires and throw the switch.

Yep that’s what I do also. Push the breaker disconnect button between the battery and inverter. Connect everything then reset the breaker. No sparky️️

 

[eddie1261]

In my mind, if there is no switch, you can never turn off the inverter. To turn it off, you have to disconnect battery cable somewhere. With a switch inline, I can turn off the inverter and isolate the battery with no effort.

With an MPP all in one, everything is on or off. I am now lost as to the actual topic here. I take it that someone was concerned about the spark when connecting the the cable between the battery positive and the inverter positive. At some point those capacitors have to get charged. As Will's video said, this is normal and I don't get the outcry. It's only going to happen once and it lasts what, 1/100th of a second? If it's normal, I don't understand the perceived risks.

 

[Will Prowse]

With 12v systems, and sealed lead acid/bms protected lifepo4, its not a big deal. If you are doing a 48v system with a large inverter (such as the 5kw monster I am using for my shed system), it is ideal to pre charge the caps. To be honest I bypass ocpd and let the spark happen if it is 12v. I havent found any data sheets to make me concerned. But higher voltage banks and large inverters? You can do some real damage. I will actually be covering this in great detail in a new video.

 

[grizzzman]

I guess that's the advantage of a FLA bank with solar. I remove my LFP battery to the a/c-heat in the house when not camping. the caps for both my inverters stay charged.

 

[DulcePinel]

Hi...most modern day electronic devices have a "front end" EMI/RFI filter to prevent incoming electrical spikes from damaging the internals and keep internal noise from propagating to the incoming line.
Be aware that many of these devices, inverters especially, will not work properly on an unfiltered 12 VDC circuit. The input capacitance will "filter" the full-wave rectified A/C voltage raising the input voltage to something in the range of 18 VDC.
Even if the inverter should work this may result in "backfeeding" the 18 Volts to other more sensitive (to overvoltage) devices on the same circuit.

pcba prototype

 

[backyardshane]

What voltage/wattage bulb are we looking at here?

i use a monentary switch and a 30ohm resistor

 

[John Frum]

As lifepo4 gains market share Inverters should be engineered to handle the inrush.
If there were 2 inverters and the only difference between them was one could tolerate this scenario I know which one I would buy.