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Question for the techies Batteries


w1ntersun

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I will try to explain this as simply as possible

 

You have 4 x 100 amp batteries connected in parallel

You connect an invertor that draws 100 amps to the end battery.

Forgetting losses resistance etc.

What is the current drawn in each of the linking cable between batteries and what fuse size should be put in

 

Richard

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Not an expert answer I'm afraid, but I would use at least 150 amp rated starter or welding cable to link all four batteries, preferably with crimped terminals, and I would draw the inverter power from the positive of no 1 battery and the negative from no 4 battery so that all four work equally.

 

Site the inverter as close as is a sensible location to the batteries as power loss in a 13 amp ac cable is a lot less than in 12 vold dc cables.

 

Switch the inverter off when not in use as many of them take 12 to 24 watts just on standby and will flatten your batteries if left on.

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w1ntersun - 2013-05-04 10:59 AM

 

Hi, Thanks for the answer but its a hypertheticle question. A school science teacher said it dosn't matter where you connect the invertor and that the current is equal on all cables.

 

Richard

 

I'm don't have enough knowledge to say he/she is wrong but would you trust a hypothetical teacher to wire up your van!

 

 

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The current in a circuit will always be the same. So if connected as "Tracker" suggests there will be a circuit and the current will be equal at any point within it.

 

If both connections were made at say, battery 1 then the other batteries are not part of the circuit so different currents would be found depending where the measurement is taken. It's because cables have resistance, no matter how thick they are, so will affect the current/voltage.

 

Even if connected properly each battery will not be exactly the same as it's neighbour so will have a slightly different output under load.

 

Tricky stuff this electrickery!

 

H

 

 

 

 

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w1ntersun - 2013-05-04 10:59 AM

 

Hi, Thanks for the answer but its a hypertheticle question. A school science teacher said it dosn't matter where you connect the invertor and that the current is equal on all cables.

 

Richard

I think he's wrong. The current will be equal in all of the battery linking cables only if each battery's +ve terminal was taken to a single +ve connection point, each -ve terminal was taken to a single -ve connection point, and the inverter was connected to those single connection points.

 

Ignoring differences between batteries, battery internal resistance, resistance in cable and connections, etc, each battery would be supplying 1/4 of the current drawn by the inverter. So for a 100A load each battery would supply 25A. For a parallel connected battery bank, connected in the way you describe, with the load connected to battery 4, the current in the connecting cables would be: 1 > 2 = 25A, 2 > 3 = 50A, 3 > 4 = 75A, 4 > Load = 100A

 

The trouble is this doesn't happen in real life! See: http://www.smartgauge.co.uk/batt_con.html

 

Andy

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