diy solar

diy solar

Will this work for pre-charging inverter capacitors?

FilterGuy said:
Wow! If it is getting hot when the switch is in the "ALL" position then there is a fair amount of resistance across the '2' circuit in the switch. What voltage are you seeing across the resistor?
Click to expand...

Ok, no that's not what I meant. I meant: "when apply 24V to the 20Ohm resistor from a variable power supply, it gets hot pretty quickly", I haven't built my system yet (still trying to find some wheeled and/or stackable containers to put it all in) but then I wasn't thinking thru the reality of the parallel circuit dropping overall resistance. Nevermind. I should think more before posting.
 
Supervstech said:
Yes, at both 24 and 48v, the bulb glows dimly in pos 1,
Click to expand...

Two questions:
1) Is that with the inverter off
2) Is it glowing constant or just for a few moments?

If it is glowing constant, then something more than the capacitors are drawing current.

Supervstech said:
not in all or 2 though.
Click to expand...

That is what I would expect.
 
FilterGuy said:
You can turn the switch to '2' but it does not make any significant difference. With the switch in 'ALL' it is effectively creating a short across the resistor so the current across the resistor will be near zero. I say near zero because their is a very small resistance across the switch and that leaves a minuscule voltage across the resistor. ) As I write this, I realize that the resistor parallel to the switch reduces to total resistance between the bms and the inverter. Consequently leaving the switch on 'ALL' actually has a minuscule advantage over "2".
Click to expand...

Wow thank you so much for this revelation @FilterGuy, I was already thinking about crazy mono/bistable relays combinations to try to get an automatic pre-charging circuit that would deactivate itself on its own an leave the least relay coils powered while running ><, but definitely after reading you I'm also pretty convinced that it's not necessary to disconnect the pre-charge resistor.

I understand it can be counter-intuitive, but I stand behind the explanation of @FilterGuy: with a resistor put in parallel to the main wire the equivalent resistance of the circuit decreases (only very marginal though), and therefore because P = R*I^2 the dissipated Joule power is also decreasing, full stop. It's basically what people are doing when they sometimes put 2 wires in parallel to increase the equivalent gauge of the supply. Just basic electronics in the end, but counter-intuitive for sure when all we are trying to do is removing resistance everywhere :)
 
FilterGuy said:
Two questions:
1) Is that with the inverter off
2) Is it glowing constant or just for a few moments?

If it is glowing constant, then something more than the capacitors are drawing current.



That is what I would expect.
Click to expand...
Off.
Only a few seconds.
I am sure it would glow with the inverter on also, but I dont wanna try it.
 
FilterGuy said:
If anyone has the engineering background, please check me on this
Click to expand...
That looks good FilterGuy. I also agree with all your further assumptions. The fact that the time is so short is why the contacts don't just vaporize the first time. The other thing torturing the poor switch is switch bounce. All mechanical switches have it. The switch is actually opened and closed a few times in the process of closing. I've had four switches weld shut over the years while closing on capacitor banks. The nasty thing is you never know about it until you want to open them. LOL.
 
Supervstech said:
I am sure it would glow with the inverter on also, but I dont wanna try it.
Click to expand...
I was wondering about the longevity of a resistor being powered continually.

Then i got to thinking, how would you know if the resistor failed? Switching to position 2 would be bad if the resistor failed.
I think the light bulb idea is a good visual indicator that its working.
 
MisterSandals said:
I was wondering about the longevity of a resistor being powered continually.

Then i got to thinking, how would you know if the resistor failed? Switching to position 2 would be bad if the resistor failed.
I think the light bulb idea is a good visual indicator that its working.
Click to expand...

The only time I have seen a resistor fail is if has been abused in some way (Typically too much current). Note the power rating called out for the resistor is 25 watt. This is enough to handle 27 volts continuously, but keep in mind that they are only going to see any substantial current for a few moments when the switch is first switched to 1.

Driving a 120 volt light bulb at 24 volts won't phase it a bit either. In the old phone company mechanical switching systems, they used light bulbs as resistors in 48 volt circuits and they would last for decades without failing.

In the end, I don't think the reliability is going to be an issue with either the resistor or the lightbulb (the exception being that the lightbulb is more susceptible to physical abuse).

Having said that, I have seen switches fail. Consequently if the "1" circuit on your switch fails, the lightbulb would not glow.
 
Keith C said:
Tiny BMS
and
the Chargery family.

Do look and understand the feature before buying. I think the Chargery one is a little add-on board. It might actually work standalone but don't quote me on that. :)
Click to expand...
Thanks!! I'll look at 'em
 
Supervstech said:
I am sure it would glow with the inverter on also, but I dont wanna try it.
Click to expand...

I don't blame you!!! This is a good reminder of what Keith C said earlier in the thread.
Keith C said:
Keep in mind that trying the slow startup on many inverters will be bad. As the caps charge and an already switched ON inverter's control circuitry reaches its startup enable voltage the inverter will attempt to start up. Thus instantly draining the capacitors down to below control voltage. This will shut off the inverter as the resistor tries to recharge the caps again. Then around you go. This is called a relaxation oscillator. This would be very bad as the startup process of an inverter is one of the hardest things they have to do. Chaining more than one of these together will toast many of them.
Click to expand...

If the inverter is on while you are trying to pre-charge you are putting the inverter into a strange state that it was not designed for. (Particularly if the inverter also has a load on it).
 
LOL! :)

Nowt wrong with getting the answer then having a good 'ole debate about it ;-)
 
tictag said:
I've read this entire thread but I am confused, has the simplest solution been considered?

View attachment 4356
Click to expand...

That is essentially the rotary switch design we have discussed.

The main thing is getting a switch that can handle the current.... and that is where the Perco battery selector switch comes in.
 
astronom said:
I understand it can be counter-intuitive, but I stand behind the explanation of @FilterGuy: with a resistor put in parallel to the main wire the equivalent resistance of the circuit decreases (only very marginal though), and therefore because P = R*I^2 the dissipated Joule power is also decreasing, full stop. It's basically what people are doing when they sometimes put 2 wires in parallel to increase the equivalent gauge of the supply. Just basic electronics in the end, but counter-intuitive for sure when all we are trying to do is removing resistance everywhere :)
Click to expand...

Astronom, I'm afraid we are getting away from the question of whether to leave the resistor inline after the caps have charged. I believe FilterGuy is correct but for a different reason altogether. Please bear with me here:

While it is true that two parallel wires are commonly used to increase the equivalent gauge that is not relevant here as we are discussing a circuit with one leg as a low resistance wire and the other leg as a resistor. True but not relevant.

My original concern was that leaving a resistor in a circuit after it is no longer needed is a waste of energy as some portion of whatever current flows through it will be converted to heat. This is also true but not relevant.

The relevant point FilterGuy made is that very little current will flow through the circuit leg with the resistor in it. This is likely to be so insignificant of an amount that you can basically ignore that leg of the parallel circuit. This I believe to be true and relevant.

I have ordered a resistor which should get here in a couple days. At that time I will charge the caps and measure any current after that to confirm the hypothesis but I expect any measurable current in the resistor side of the circuit to be insignificant. I will report back either way.
 
There are hundreds on Amazon for
FilterGuy said:
That is essentially the rotary switch design we have discussed.

The main thing is getting a switch that can handle the current.... and that is where the Perco battery selector switch comes in.
Click to expand...
There are hundreds on Amazon for £10-£20, just search 'battery isolator'. The ones I've just had a very quick look at go up to 1,000A.

Another avenue might be arc welding suppliers.
 
tictag said:
There are hundreds on Amazon for

There are hundreds on Amazon for £10-£20, just search 'battery isolator'. The ones I've just had a very quick look at go up to 1,000A.

Another avenue might be arc welding suppliers.
Click to expand...


Most of the battery isolator switches are single pole on-off. If you want to manage the charge circuit with the same switch, you need the single pole-multi throw off-1-all-2 battery selector switch. The selector switches I was familiar with would allow you to go from off directly to 2 (Bypassing the pre-charge).
This is an example of what I was familiar with: https://www.amazon.com/Battery-Isolator-Switch-Removeable-Batteries/dp/B00JWZCY5S

That is why I was glad to find the Perko that did not allow that.

You don't really need the '2' position, but the only downside is the extra position might be a little confusing to the user.

I poked around in welding supplies but did not find anything.... but that may only mean I did not look in the right spot.
 
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