Necessity of a Pre-Charge Resistor

[eXodus]

some visitors mention manually connecting an incandescent lightbulb as a resistor to pre charge the inverter DC bus.

the bulb illuminating provides feedback on when current slows down. disconnect after light becomes dim sort of thing.

in this way "manual sensing" pre charge is also a method ??

This is my favorite way of pre charing. 12v 55w car headlight bulb. They got a nice high resistance.
The light bulb lights up quite bright and stays on for a few seconds. So a lot of current is flowing. Great visual confirmation

Not buying a resistor for once in a blue moon.
I always got spare bulbs in my glove compartment.

 

[jvin248]

some visitors mention manually connecting an incandescent lightbulb as a resistor to pre charge the inverter DC bus.

the bulb illuminating provides feedback on when current slows down. disconnect after light becomes dim sort of thing.

in this way "manual sensing" pre charge is also a method ??

I like this light bulb method. I already have a light-bulb-limiter built to repair guitar amps.

I've seen the "use a carpenter's flat pencil" trick and while it seems to work, it's too variable -- and anyone have a pencil explode when they are doing that?


Because pre-charging is such an issue and dangerous at higher voltages and larger amp systems, perhaps some smart folks can come up with the equivalent of "Uncle Doug's" light-bulb-limiter as an easy cheap insurance method to start up or alter solar systems? Make it sticky on the top of the forum.


.

 

[pollenface]

Because 24v and up inverters can draw MASSIVE amps when first connected.

so why is this a problem, do i risk blowing something up?

edit - actually, I have a bench power supply, I could connect it at 0v and crank the volts up over 10 seconds or so before connecting to batteries if it pleases the court

 

[LeoThomson]

Not disputing the need to pre-charge the inverter, but why isn't that circuit build into the inverter?

 

[Don B. Cilly]

It probably is.
My inverter (a CCC - Cheap Chinese Clone - 24V, 3KW) has a little (like very little) ON/OFF switch.
I turned it on and off countless times, when you turn it ON it takes about a second and a half to do so.
No sparks, no high-amp draws, no problems.

And if you think how easy it is to build such a circuit into the ON switch... even the Chinese can do it ;·)
-

 

[eXodus]

Not disputing the need to pre-charge the inverter, but why isn't that circuit build into the inverter?

Cost, pretty substantial circuit needed for big amps. This not a 2 cent part. You are looking at 2-3$ worth of parts plus engineering.

For using it once in the inverters life. I mean seriously, most people will connect the system one time, and the never again.

 

[eXodus]

It probably is.
My inverter (a CCC - Cheap Chinese Clone - 24V, 3KW) has a little (like very little) ON/OFF switch.
I turned it on and off countless times, when you turn it ON it takes about a second and a half to do so.
No sparks, no high-amp draws, no problems.

And if you think how easy it is to build such a circuit into the ON switch... even the Chinese can do it ;·)
-

That switch is not disconnecting the capacitors. It's just disabled the computer board. They stay charged at all times.

 

[Danke]

I think I’m gonna build a push button for my inverters, maybe put a lightbulb in series for visual confirmation.

My Push Button Pre-Charger Install for the SW 4024 Inverter

Up until Last week I had never heard of pre-charging a larger inverter before connecting the battery cables. I just lived with the spark. But I was watching a "Will" video about a DIY LFP build and saw that he used a resistor before connecting the Positive Terminal. This peaked my...

diysolarforum.com

 

[LeoThomson]

Cost, pretty substantial circuit needed for big amps. This not a 2 cent part. You are looking at 2-3$ worth of parts plus engineering.

For using it once in the inverters life. I mean seriously, most people will connect the system one time, and the never again.

Sad.

 

[TomC4306]

Only necessary when connecting battery to inverter.
How often do you disconnect the battery from the inverter?
$2 for a pair of 50 ohm 100 watt ceramic resistors that live in a small ziplock baggie in my toolbox.
Easy peasy.

 

[Supervstech]

It probably is.
My inverter (a CCC - Cheap Chinese Clone - 24V, 3KW) has a little (like very little) ON/OFF switch.
I turned it on and off countless times, when you turn it ON it takes about a second and a half to do so.
No sparks, no high-amp draws, no problems.

And if you think how easy it is to build such a circuit into the ON switch... even the Chinese can do it ;·)
-

Big difference between turning inverter off and connecting the inverter to the battery.
A precharge limiter is needed when connecting the battery to inverter.
If it remains connected at all times, it isn't needed after.
If in a mobile situation, where everything g is disconnected repeatedly, it needs to be hard wired...

 

[eXodus]

Sad.

Mass manufacturing is hard. Few extra parts can make or break your profits. Someone just made a decision based on: how many inverter are actually getting damaged when doing that. How many of those people will file a warranty request. If this is less cost then adding this feature it will not be done.

You can always buy a victory for 2-3x the price which has most of those safety features included.

 

[Don B. Cilly]

Well, I connected it to the battery three times, when I first installed it, when I changed batteries and when I moved house.
I did not "see" any surges, but then maybe I simply didn't notice. There were certainly no arcs though.
-

 

[RCinFLA]

There are two primary issues of not limiting inverter capacitor initial charging current.

Subjecting breaker contacts to large surge current will pit the contacts in breaker increasing breaker series resistance the more times it is done.

With lithium-ion batteries, the large surge current can trip the BMS short circuit current protection.

Usually, there is not damage to filter capacitors due to their series ESR resistance.

How large can surge current be? That depends on total path resistance. For lead-acid batteries their internal resistance greatly limits the maximum surge current, not so for Li-Ion batteries. 1000 to 3000 amps is typical.

A HF 5kW 48v All-in-One inverter typically has four 4,700uF capacitors with 16 milliohms ESR each in parallel on battery input. With 14 milliohms total battery path resistance and with 55v battery, the peak current is about 2800 amps. It takes about 2 millisecs to exponentially decay to steady state.

There are inverters, like Xantrex XW series, and most LF inverters that have greater battery line input capacitors' values. Usually, the peak current is limited by battery path resistance and just the decay time is longer.

The 'at nauseum' attention given to this topic is way over discussed. Just a 0.5 to 2 ohm power resistor placed across breaker cable terminal lugs a fraction of a second before throwing breaker on is sufficient. Last time I removed battery power from my inverters was 17 years ago.

Well, I connected it to the battery three times, when I first installed it, when I changed batteries and when I moved house.
I did not "see" any surges, but then maybe I simply didn't notice. There were certainly no arcs though.
-

If capacitors are already charged, it can take hours to a couple of days to bleed the charge off.

 

[Supervstech]

Well, I connected it to the battery three times, when I first installed it, when I changed batteries and when I moved house.
I did not "see" any surges, but then maybe I simply didn't notice. There were certainly no arcs though.
-

Like said above, 12V doesn’t have much danger, and lead acid makes it less of an issue. Lithium though is scary.

 

[LeoThomson]

Mass manufacturing is hard. Few extra parts can make or break your profits. Someone just made a decision based on: how many inverter are actually getting damaged when doing that. How many of those people will file a warranty request. If this is less cost then adding this feature it will not be done.

I know how capitalism works, one of the things people miss is that it requires an informed consumer and most consumers are not (there is no way for the average Joe to be able to keep up). One of my pet peeves are how much heat LED light bulbs produce (I see heat as waste)and their relative short light span. As far as I am aware, the Dubai bulb is not sold outside of Dubai. Another other one is rechargeable battery chargers, charging two in series. I ended up getting an Ikea charger that does multiple single cells instead.

You can always buy a victory for 2-3x the price which has most of those safety features included.

I did, I rather save up a little longer to get quality products. Seemed to me the best solution for my needs at the time, it is a reputable brand that lasts, runs all my power tools, water pump, window air conditioners, fridge and freezer, induction cooker as well as lights without any problems. There might well be better products available for less these days, but things are changing fast and I can't keep up. So I just keep reading here once in a while, asking questions.

 

[eXodus]

I did, I rather save up a little longer to get quality products. Seemed to me the best solution for my needs at the time, it is a reputable brand that lasts, runs all my power tools, water pump, window air conditioners, fridge and freezer, induction cooker as well as lights without any problems

I usually buy good quality products in critical applications and high use items. But for things I don't use much - I go on the cheap/used and just treat them well.
Even the cheap products of today are not much worse then expensive stuff rom a few years ago.

My guess is that not too many people burn up their inverters when connecting them. I mean it's really not a big deal of pre-charging and it seems like some server rack batteries started to integrate that feature.

Further, I start seeing better BMS with current limiting features. So the issue might be solved from the supply side instead of the demand.

 

[Supervstech]

I usually buy good quality products in critical applications and high use items. But for things I don't use much - I go on the cheap/used and just treat them well.
Even the cheap products of today are not much worse then expensive stuff rom a few years ago.

My guess is that not too many people burn up their inverters when connecting them. I mean it's really not a big deal of pre-charging and it seems like some server rack batteries started to integrate that feature.

Further, I start seeing better BMS with current limiting features. So the issue might be solved from the supply side instead of the demand.

Even with current limiting, the instantaneous demand of the capacitor bank can destroy the fets in a bms...

 

[LeoThomson]

Even with current limiting, the instantaneous demand of the capacitor bank can destroy the fees in a bms...

I do not know what "the fees in a bms" are.

I thought the idea of a pre charge resister is to protect things from getting destroyed. If I want to prevent the fantom load from the inverter, I will have to disconnect it from the battery when I am not using it, or switch the inverter off. I can see that happen multiple times a day in a camper for example. Surely a manufacturer solution would be preferable and cheaper, regardless if it is in the BMS, the inverter or both.

 

[Ellcon123]

Precharge resistor is to prevent the spark when connecting up the inverter or whatever else may have large capacitors. Leaves a good pit on the surface plus it gives you a good scare.