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run-the-race avatar image
run-the-race asked

Struggling to connect 70mm2 lug to Battery Switch ON/OFF 275A

If I try connect the lug with the flat side of the lug facing down, then it cannot sit flust with the 1st nut (the 2nd nut tight the lug again the 1st nut)

lug1.png

However if I try with the flat side of the lug facing up, then the lug sticks beyong the box and I cant mount it flush to a wall.

lug2.png

If I loosen the 1st nut out, so that the lug can sit flush against the nuts, then the thread bar jiggles and feels loose, and I fear the other side of the thread bar is not making contact with the switch internals. Is this okay or should it be avoided?

lug3.png

Thanks for any advice.

switch
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3 Answers
klim8skeptic avatar image
klim8skeptic answered ·

@run_the_race the removable panel/blank in the middle photo looks large enough to allow cable/lug connection.

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run-the-race avatar image run-the-race commented ·

Unfortunately the panels on all 4 sides are the same size.

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daza avatar image
daza answered ·

i wouldn’t lose that as that may well keep some of the internals at bay, you want these things to sit flush as this is a high resistance point all by itself having these not making good contact on the isolator isn’t going to help, just my 2 pence

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Craig Chamberlain avatar image
Craig Chamberlain answered ·

To be honest, these switches are pretty poor and if you ever switch them on or off under load then you will probably need to buy a new one because the arc caused by switching under load will make the contact resistance increase and they will heat up.

That all said, if you do decide to use this switch then it is ESSENTIAL that the bottom nut is snugged up against the plastic otherwise the contact surfaces inside the switch won’t sit flush and you’ll get a high resistance connection and potential fire. You can open up the switch carefully by removing the four screws and you’ll see what I mean. Just be very careful to reassemble it correctly.

I know what you mean about the lug not sitting flush but I have 70mm2 lugs on my switch and it just about screws flush to the wall. Maybe your lugs are different to mine though, I got mine from 12voltplanet.co.uk. What you might be able to do is put the flat part of the lugs in a vice with soft jaws and bend the lug slightly to make it exit the switch parallel to the wall. But just be very careful not to overdo it and make sure you don’t put a curve in the connection surface.

HTH

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run-the-race avatar image run-the-race commented ·

Thanks for that detailed info very helpful, I opened up the switch and it was broken in side with all my fiddling trying to get it to fit. I hammered the lugs to change the angle of the cable and now it fits. I hope the cables did not work lose with the hammering, but they are covered with heat shrink, and still feel tight.

Ps On the packaging it says cranking current 1200A. And it still not okay for switching under load?I am probably going to be using like 80A max of the 275A rating. But I will try to not switch it under load, it's just an isolator from the 48V from the batteries to the inverter, and another one to the MPPT.

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Craig Chamberlain avatar image Craig Chamberlain run-the-race commented ·

I wouldn't want to be putting 1200A through one of these switches for very long at all. It'll probably only be rated at that level for a very few seconds. Even 275A continuous current will cause some heating IME.

However, the ability of the switch to carry a given current isn't really the main issue. The issue is damage to the internal conductive surfaces due to switching off under load or switching on without either soft-start battery modules or a pre-charge mechanism. This is because, when you first apply battery voltage to a fully discharged inverter, the internal capacitors of the inverter are suddenly charged from 0V to ~48V and this can cause a surge current well in excess of the 275A switch rating and maybe even beyond the 1200A cranking current. But even worse, it tends to be applied over a relatively small contact area inside the switch because the switch is only just closing and an electric arc is created a bit like on a welder. This arc is very high temperature and will melt the contact surfaces and blacken the area around them with carbon. The end result is that your contact surface is reduced and is made dirty by the carbon. This creates a high resistance (comparatively) connection which drops voltage and therefore heats up. It doesn't take much to end up with a switch that runs too hot to even touch at 100A continuous load.

My battery modules are Pylontech US5000's and they have a soft-start feature. This means you can engage the isolator switch BEFORE you power up the batteries and then the batteries will come in slowly to pre-charge the inverter capacitors and avoid any surge. An alternative to this is to install your own pre-charge circuit if your battery modules don't include this feature.

Also, in my own setup I didn't bother with DC isolators for my MPPT to Battery connection. Instead I just have fuses appropriate to protect the cables and then of course I have the DC isolators between the solar panels and the MPPTs. With the PV isolators open, the current to the MPPTs is just standby current and I can't imagine any operational use-case where I would want to isolate my MPPTs from my battery modules. If I needed to replace an MPPT then I would just shut down the entire system first.

I also don't have isolators between my batteries and my bus bar (Lynx Distributor) but again I have 125A fuses for each battery module. Fewer interruptions/connections in the DC side means less voltage drop and higher efficiency.

Have a read at the following thread for more discussion on the 275A switch.

https://community.victronenergy.com/questions/201896/battery-switch-gets-too-hot-would-victron-switch-o.html

And this thread talks about the power on sequence of the inverter which might be helpful also:

https://community.victronenergy.com/questions/313922/what-is-the-correct-order-to-power-up-a-mp2.html

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