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Battery cutoff switch on negative or positive cable?

Thodoris

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Jul 16, 2023
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Greece
As per the title, on which side should i install the battery disconnect switch? Positive or negative?
I am using a LiFePO4 51.2V rack battery and a battery disconnect switch like the one in the picture attached.
Thanks!
Screenshot from October 22, 2023, 12_34 PM.png
 
Blue Sea has similar switches some of which can support 80V (72 Volt system). WARNING, there are cheap knockoffs out there, be aware of that and don't furtle with switches, breakers or fuses, that is NOT where to pinch pennies.


Below is one my generic diagrams to help you out.
Parallel System-setup PNG.png
 
Thanks for the help regarding voltage rating.
I was also not happy from the replies i was getting from Victron. When i asked them about their 48V switch they were not clear and they never told me that I CANNOT USE their switch for my system. I was thinking that 48V was the nominal voltage rating, but i think it's dangerous to go on with that.

So i found this one which is rated up to 60V. What do you think?
 
When looking at these switches the Big Detail to watch for is not the Label that says 12V/24V or 48V... That is kind of a misnomer because for example a 24V LFP system must have a switch that can handle 30V, just like a 48V system has to handle 60V. 12/24/48 are Legacy from Lead Acid days but still used as the common reference.

Looked at the Specs PDF and that appears to be acceptable. I say appears, as this is a brand I have never heard of and do not knows its origins.
 
Thanks for the help regarding voltage rating.
I was also not happy from the replies i was getting from Victron. When i asked them about their 48V switch they were not clear and they never told me that I CANNOT USE their switch for my system. I was thinking that 48V was the nominal voltage rating, but i think it's dangerous to go on with that.

So i found this one which is rated up to 60V. What do you think?
It says it's rated high enough.
If you trust the brand, it should be fine.
 
Below is one my generic diagrams to help you out.
Parallel System-setup PNG.png
When I was putting my system together and trying to figure out which way was up, I found this diagram and made it my desktop background. It helped me a lot. Thanks for that.
IMG_20230811_084503117.jpg
 
It's rated for 48V systems, it's an OEM switch.
But the same rating applies for the VICTRON switch which is much more expensive.

Looks the same as this one: https://www.amazon.com/Master-Battery-Switch-Screws-12-48V/dp/B07S8XBZRJ
Great info by others already. I asked about the voltage because it's tough to find something rated for 60v, I went down the same path recently.
 
Great info by others already. I asked about the voltage because it's tough to find something rated for 60v, I went down the same path recently.
It's even harder to find one that's UL listed. I searched all over Amazon recently looking for a DC breaker switch, and couldn't find one that was 100A+, 60V+, and UL Listed. I seriously considered one that was around $140, which seemed way too high. It looked high quality, but I couldn't find anywhere that it was UL Listed.

I like the one that you linked in that thread.
 
Blue Sea has similar switches some of which can support 80V (72 Volt system). WARNING, there are cheap knockoffs out there, be aware of that and don't furtle with switches, breakers or fuses, that is NOT where to pinch pennies.

I can say from experience, that the Chinese/Amazon/Ali Expr. knockoffs costing around $10,00-$20,00 are a waste of your money.
If you run more than 40-50 amps through those cheap switches, then they easily heat up to 90 degrees C/194F. (measured with Flir)
And if you measure the voltage drop between the switch terminals, you certainly don't want to use it anymore.
 
Most of these switches are marketed for marine or automotive applications with lead acid batteries. 48 volt rating is adequate. I believe over 50 volts is considered high voltage so rating at 60 volts would probably require additional testing.

Some of the Chinese offerings claim 60V, but I wouldn't believe it.

I'm going with a BEP 701-PM by Marinco. It's only rated 48V, but I can be sure that it is truly capable of 275 A. I'm only adding this switch in case I need to deploy a pre-charge resistor. Also to isolate the class T fuse for replacement. There's a code for ESS requiring fuse isolation, so the layman can safely change the fuse. NEC 706.21(E)
 
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It really doesn't matter what amperage it's rated for. If it can't safety disconnect the voltage going through it.
But at a higher amperage, the voltage rating is even more important. Because more amperage can be flowing through the arc. That is created by the under rated disconnect.
 
I don't plan to open the switch with current flowing. Even Sol-Ark warns not to open the built in battery breaker with current flowing.
 
Someone a while back posted photo's of a cheapo knockoff switch on a 48V system that not only arced but totally melted. Only takes ONE oopsie and your life can be changed drastically & FAST. As I said before, stick to high quality safely components, breakers, fuses & switches, the otherside is not worth a few pennies saved.
 
It really doesn't matter what amperage it's rated for. If it can't safety disconnect the voltage going through it.
But at a higher amperage, the voltage rating is even more important. Because more amperage can be flowing through the arc. That is created by the under rated disconnect.
For me a Blue Sea e9003, 275 amp continuous rating at 48 volts is adequate. I have one on each 160 amp hour battery. Each battery has a 150 amp class t fuse for ocp. These switches will only ever get turned on or off when there is virtually no current flowing. Purely as an isolator.
Typically during my semiannual balancing festival. When everyone is in absorption or float.
 
Someone a while back posted photo's of a cheapo knockoff switch on a 48V system that not only arced but totally melted. Only takes ONE oopsie and your life can be changed drastically & FAST. As I said before, stick to high quality safely components, breakers, fuses & switches, the otherside is not worth a few pennies saved.
Do you remember which switch was used? I am concerned whether mine is one of those or not.
 
Do you remember which switch was used? I am concerned whether mine is one of those or not.
Unfortunately I do not, it was posted in here about 6-8 months ago or so. There have been a couple of posts over the years showing the results of cheap knock offs.

The GOTCHA that catches people with their pants down is the common assumption that when someone says 48 Volt, that this is the voltage limit they need to support but in reality is HAS to be 60 Volts. Even with Lead Acid when running Equalization you are above 48V, and LFP for example can see 58V without effort, let alone other chemisries....
 

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