battery bank disconnect

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battery bank disconnect

Postby Megunticook on Sun Nov 26, 2017 9:34 am

I'm installing a small (400 amp hour) 48V battery bank as part of my grid-tie with battery backup system using the Radian 8048.

Do I need a disconnect switch for the battery bank, or can the breakers installed in the Load Center serve that function?

The battery bank will be located roughly 6 feet from the load center. Do I need to install a fuse right next to the batteries or will the circuit breaker in the load center provide ample protection?

Thanks.
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Re: battery bank disconnect

Postby Kent Osterberg on Sun Nov 26, 2017 10:51 am

The dc breaker(s) in a flexware panel are suitable for both purposes of disconnect and overcurrent protection for most battery systems. But they do not provide fault protection inside the flexware panel - even with the breakers in the off position, a misplaced tool can short the battery with disastrous consequences. Because of that possibility, the panel should clearly labeled that all sources of power should be de-energized before working inside. So, a class T fuse near the battery is a good idea. NEC 240.21(H) permits (doesn't require) overcurrent protection as close as practical to the battery terminals even if the battery location is considered a hazardous location because of hydrogen gas.

Here's where a Class T fuse may be required for additional overcurrent protection: for a system with industrial batteries or a system with parallel sets of industrial batteries, or systems with paralleled battery cables, it may be necessary to have a Class T fuse(s) near batteries. It never hurts to include a Class T fuse in the battery cable, near the battery.
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Re: battery bank disconnect

Postby Ravenswood on Mon Nov 27, 2017 5:49 pm

When I built my system every reference recommended fusing of the batteries when there are multiple strings - but there was little information on how to do it and very few systems seemed to include this. As shown in the attached photo, I combined the bus bars and fusing on the outside of the battery box (behind a lexan cover). This allows me to easily disconnect the battery feed to the load center/inverter (once it is shut off using the breaker in the load center) and there is no need to work inside the battery box at all. The cost was minor compared to the peace of mind that I have when it is necessary for me to work inside the load center. I would highly recommend a disconnect of some type.
Battery Fusing.jpg
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Re: battery bank disconnect

Postby Kent Osterberg on Mon Nov 27, 2017 8:34 pm

Ravenswood, you came up with a nice looking solution for paralleling battery strings. Thanks for sharing the idea. It makes for an easy and safer place to disconnect the batteries when needed, but you still have to be really careful to not short the batteries with a misplaced tool.
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Re: battery bank disconnect

Postby Megunticook on Tue Nov 28, 2017 4:40 am

Thanks everyone. I ordered a 200A T fuse to put on the positive cable right near the battery terminal (although I only have 1 string). Am just picking up 8 Rolls-Surette S-550 batteries later today and my CEO is coming to inspect next week. If all goes well I'll be online late next week. It's been a long-haul (first time self install, started back at the end of September!).
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Re: battery bank disconnect

Postby Megunticook on Wed Nov 29, 2017 5:57 am

I have another question now: the 8048 manual calls for 2 sets of 2/0 AWG cables from the battery bank to the inverter.

Do I need another 200A T fuse, so each positive cable has it's own fuse?

And do I need to double up on the cables connecting the batteries to each other as well?

There is just a single string of 8 6V batteries.
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Re: battery bank disconnect

Postby Kent Osterberg on Thu Nov 30, 2017 1:08 pm

The 2/0 battery interconnect cables are essentially in free air and have an ampacity of 225 amps for 60°C rated wire and 265 amps for 75° rated wire, so it is perfectly safe to use single 2/0 cable for the interconnects. You just lose more energy than if you doubled them up. If the interconnects are each 18" long they will add up to a bit over 10 feet and at 200 amps the wire loss amounts to almost 0.4% in the interconnects alone. You could reduces the loses by doubling the the interconnects up or by using 4/0 interconnects. If the main battery cables are each 10' long of doubled up 2/0, the total of all the wire loss in the doubled up cable is also 0.4%.

If you are going to use 200-amp class T fuses for backup, put one in each cable.
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Re: battery bank disconnect

Postby McG on Sat Dec 09, 2017 2:32 pm

Hi guys, I read through this article and still I am not 100% on top of it.
Maybe because my scenario is different. I am fitting 10 x 100W flexible solar panels to the RV roof. It is 2 arrays of 5 panels in series. The 2 arrays are paralleled to the FM80 Controller. The controller is connected through a switch (see pic) to a fuse of sorts (See attached pics). I think it is a 60A which I'll need to up to 80A for the array size.

S2040001.jpg
Fuse


S2040002.jpg
Switch


S2040003.jpg
Fuse rating


Is the fuse the correct type?
Do I need any other fuses in this solar system?
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Re: battery bank disconnect

Postby Megunticook on Mon Dec 11, 2017 5:56 am

Not an expert on fuse types and it's not clear what sort of fuse goes in there.

Remember that you want to protect the wiring as well as the equipment, so double check the ampacity of your wires.
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