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can I use a normal AC isolator with DC batteries?


SilverNodashi
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Hi, 

 

As the title suggests, can I use a normal AC Isolator for a DC battery bank? I know AC and DC is a bit different, but I don't know if these Isolators can't handle DC currents, or even an Arc when you open / close it?

 

If it's possible to use an AC isolator, what do I need to use on a 48V, 540A bank?

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Even using a DC Isolator make sure there is no load. I have seen footage of a DC Isolator that was switched on under load and all that happened is the the switch was welded in the on position. From my minimal experience with 32V DC lights AC components do not last.

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The problem is, that a DC spark, caused by a large battery bank could cause a LOT of damage!

Using fuses only won't stop the arc / spark.

Saying not to disconnect the batteries under load also doesn't help much. If the battery bank is full, it will still arc when you connect the inverter, even when off. Neither Axpert or Victron inverters have a DC isolator, yet the Microcare does.

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Silver, just stop right there dude!

 

Fuses was specifically created for DC circuits and that is why they still exist today. Otherwise everything would have been circuit breakers today.

The correct device to use with fuses for isolation, is called fuse holders.

In order to isolate, the fuse is physically pulled out of the circuit and holder.

The holder is insulated so the operator cannot get shocked.

 

Microcare units use a normal AC circuit breaker on their DC side :(

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When you "pull a fuse" or close the fuse holder you will in any case have a spark - no matter what.

 

Tip: Best way to minimize the spark / arc when you connect batteries to a inverter which is SWITCHED OFF, is to use a resistor in series with the connection. The initial high current condition is caused when the capacitors on the DC line inside the inverter is charged.  Once the capacitors are charged the current flow becomes 0 or extremely low.  A suitable resistor would be something like a standard 240V 100W globe - connect it in series with the battery connection to the inverter, give the capacitors a few seconds to charge and then remove the globe from the circuit.  Best place to connect the globe is in parallel with the isolator or fuse in the battery line, while the isolator is in open (disconnected) mode or the fuse is removed from the circuit.

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When you "pull a fuse" or close the fuse holder you will in any case have a spark - no matter what.

 

Tip: Best way to minimize the spark / arc when you connect batteries to a inverter which is SWITCHED OFF, is to use a resistor in series with the connection. The initial high current condition is caused when the capacitors on the DC line inside the inverter is charged.  Once the capacitors are charged the current flow becomes 0 or extremely low.  A suitable resistor would be something like a standard 240V 100W globe - connect it in series with the battery connection to the inverter, give the capacitors a few seconds to charge and then remove the globe from the circuit.  Best place to connect the globe is in parallel with the isolator or fuse in the battery line, while the isolator is in open (disconnected) mode or the fuse is removed from the circuit.

I don't know if I fully agree with you. 

 

A newly opened inverter, which is "SWITCHED OFF" (why do you guys should this out, like no one ever switches off these things??) still causes a spart / arc to happen. I have actually blown a fuse through such an arc, and on a car melted a terminal as well. 

There's a reason why your car has relays for all the heavy load stuff, after the fuse. A normal switch will burn out if you pass 40A or 60A though it, let alone 250A, 520A or even 800A. 

 

The Microcare inverters have them built-in, the Axpert manual says you need to put a "DC breaker" both on the battery side, and the PV side.

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I like your idea Superdiy of pre-charging the capacitors.

 

To make the resistor connection more permanent you could wire it in series through a pushbutton with a spring return.

 

This way you could leave the resistor / pushbutton permanently connected across the fused isolator or circuit breaker.

 

Is there a requirement for the isolation device between battery and inverter to be a visual break?

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interesting Thread guys.

Might be a silly question from our side. If the inverter is off, however the internal components will be energized apon battery connection.

Will the max initail current draw of the batteries be very much within saftey range? From our limmited understanding the off inverter will not be drawing more than 15A? To energize its internals?

SuperDIY tell us more about your globe/resisitor method.

Sounds fantastic.

P

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SuperDIY

 

I hope I understand you correctly. Remove a fuse and wire a globe in its place close connection. This charges capacitors. Remove globe carefully and re-insert fuse close connection again and power up the inverter.

 

Yes, you understood correctly.

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I like your idea Superdiy of pre-charging the capacitors.

 

To make the resistor connection more permanent you could wire it in series through a pushbutton with a spring return.

 

This way you could leave the resistor / pushbutton permanently connected across the fused isolator or circuit breaker.

 

Is there a requirement for the isolation device between battery and inverter to be a visual break?

 

 

Good idea, if you are going to disconnect the batteries frequently, but if not, then having a globe ready for when you need it is easier.

 

I don't think a visual break is a requirement, if I understand you correctly. If you have an isolator installed, depending on the specific isolator you might not be able to see inside it.

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interesting Thread guys.

Might be a silly question from our side. If the inverter is off, however the internal components will be energized apon battery connection.

Will the max initail current draw of the batteries be very much within saftey range? From our limmited understanding the off inverter will not be drawing more than 15A? To energize its internals?

SuperDIY tell us more about your globe/resisitor method.

Sounds fantastic.

P

 

Hi Pony, I'm not sure what you mean by the 15A statement?   :)

 

Here is the explanation: Inverters have capacitors on the DC rails, technically in parallel with the external battery bank.  If an inverter is switched off just the electronics which usually performs the (inversion) work stops, but the connection to the battery bank is not disconnected. To put it in layman's terms - the engine is stopped, but the fuel line is intact.

 

These capacitors in a new inverter, or an inverter which was disconnected from the battery bank for some time, are usually fully discharged meaning that the voltage on the capacitors sits at 0V. When you connect the battery bank to the inverter these capacitors will be charged up to the same potential as the battery bank connected to them. The current flow tapers off as the potential of the capacitors gets closer to the potential of the battery bank and eventually the current will drop to basically 0A. The initial charging current is extremely high if the resistance is very low (e.g. thick battery cables) and it only takes a few milliseconds to charge the capacitors - this current is not limited in any way and might be hundreds or even thousands of amps, but will flow for a few milliseconds only and usually fuses and circuit breakers won't react to such an overcurrent condition, because fuses and circuit breakers cannot react that fast.

 

If you increase the resistance between the battery bank and the inverter, a lower initial current will flow and it would take longer to charge the capacitors. Depending on the value of the resistor and the size of the capacitors the charging current should taper off to 0 within a certain amount of time. Using a standard 100W globe, connected in series with the battery bank and the inverter, the capacitors should be charged within a few seconds, the globe can then be removed from the circuit and the battery bank can then be connected directly to the inverter.

 

If you however leave the inverter disconnected from the battery bank after the capacitors were charged, the capacitors will slowly discharge because of internal leakage currents inside the capacitors and because the inverter might use some power to keep certain circuits powered e.g. the circuit monitoring when the "soft touch" power button is pressed.

 

Therefor, as mentioned before the easiest way to use the globe method is to connect the battery cables (in your permanent installation), but leave the fuse out of the circuit or leave the isolator in the open / disconnected position, depending on if you have a fuse or isolator in your installation. A good idea is to connect / solder a piece of wire (20cm) to each terminal of the globe - you can use thin wire 0,5mm2 or 1mm2 since the current will be low. Now push the bare end of one of the wires against the connection on one side of the fuse holder OR isolator and push the bare end of the other wire against the connection on the other side of the fuse holder OR isolator.  You might notice a small sparc / arc when the second wire touches the connection (terminal) - then you know that the connection was made and the capacitors should be charging. Keep the wires in position for about 10 seconds and then take the globe and wires out of the circuit. The capacitors should now be fully charged and you can insert the fuse OR close the isolator. 

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Guys,

You overcomplicating a very simple thing.

The arc you going to get when closing a fuse holder is not going to be huge. You propably not even going to notice it.

Here is an installation I did for work.

Small 10Amp fuse holder for the PV. (3180Wp at 456V)

Bigger 100Amp fuse holder for the Battries. (225Ah at 48V)

 

post-23-0-85454500-1435309633_thumb.jpg

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Something I came across on an Aussie forum.

 

"Weber claims that there is a pre-charge circuit in the PIP, just that it doesn't operate if the battery has only recently been disconnected". The PIP is the name the Axpert is marketed under in Australia and "Weber" has an MSc in Comp Sci and Electronics so has opened up the Axpert and I think knows what he is talking about. Perhaps not a huge arc with the Axpert, will see in the next couple of days.

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