Properly fusing a parellel battery bank


Old 08-23-12, 10:10 AM
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Properly fusing a parellel battery bank

Hey all,

I am new to batteries and electrical systems, really. But I'm building a battery bank and wondering how I should add fuses to make the system safer. It's going to be run outdoors around people so I want to make sure I don't start a fire

Here is a picture of my battery bank. The green lines indicate where I think I'm supposed to place fuses. Is this too much work? Can I just add one fuse inline to the inverter for the same effect? Any guidance would be awesome! Thanks!

( The draw on the system will only be about 300 - 350 watts at any one time I believe... so also... I have 80 amp fuses, and was wondering if inline fuse holders are no good, or... ?)

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Old 08-23-12, 10:29 AM
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First, determine the maximum amperage draw by whatever you are attaching to the battery pack. Then place a properly sized fuse inline with the positive cable between the pack and the load.

350 watts / 12 volts = 29.16 so use a 30 amp fuse.

From your drawing it appears that you would bridge between positive and negative terminals and that would immeadiatly blow any fuse you placed there.
Old 08-23-12, 11:09 AM
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I figured I had it wrong. Placing a fuse on the + for the inverter is much easier... and I was hoping that was what I could do.

Old 08-23-12, 11:51 AM
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Is 350 watts the inverter input or output?
Old 08-23-12, 12:40 PM
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You need to size the fuse based upon the peak rating of the inverter plus a fudge factor of about 10%. If your inverter has a peak rating of 500 watts then you should use a minimum of a 45 ampere fuse and 50 would not be too large. Slow-blow fuses are definitely preferred and make sure it is rated at 12 volts DC as direct current will "pull an arc" much farther than alternating current. This may not be a big deal at 12 volts but at higher DC voltages it is very important.
Old 08-23-12, 12:56 PM
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If my "analysis" of you photo is correct , the batteries are connected in "parallel" , i.e., all Positive terminals are linked together with Black "jumper" wires , and all Negative terminals are linked together with Red jumper wires.

Connecting a fuse directly across the Black and Red connection-points would be a "dead" short- circuit , or a resistance of Zero Ohms across the Battery terminals. As a result , the battery current would be ---- current ( measured in Amps ) = Battery Voltage [12 volts ] / .000001 ( near Zero resistance ). Just compare that value to the Amp-rating of a 30 amp fuse .
Old 08-23-12, 04:31 PM
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Thumbs up Welcome to our forums!

As said, you only need to install a single fuse at the batteries on the positive term.

You say that your current draw will only be between 300-350 watts but do you have a 350 watt inverter or a larger one with an anticipated smaller load?
A 350 watt inverter should not really have larger than a 40 amp fuse.
An 80 amp fuse would support about an 800 watt inverter.

That looks to be reasonably large wire on the batteries but can't see the supply conductor size
What gauge is this wire?
You would want to fuse this set up for the capacity of the inverter providing the fuse will properly protect the supply wires.

Curious to know what you will charge the bank with?
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