ByronJ
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- Jul 7, 2012
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Hi John
I have been wondering why current flow between main and auxilary banks is not more of an issue and I think it is because, to a large extent, the current flow between the batteries is self limiting.
Essentially the greater the voltage difference between two batteries the more current will flow.
So lets look at what happens when we connect our auxiliary battery to the starter battery (using a switch not a VSR) with 5m of 25mm2 cable (the battery is in the back of the car).
First some numbers:
25mm2 cable is rated to carry a constant current of 170A, but at this current it would generate a voltage drop of 0.58v over 5m.
At 200A this rises to a voltage drop of 0.68V.
At 300A this rises to a voltage drop of 1.03V.
At 20A it generates a drop of just 0.07v
So if we take the alternator out of the picture and assume the starter bank is fully charged at 12.8v and the auxiliary is heavily discharged by 75% and so is at 12v. At the instant we connect the batteries current will flow from the starter to the auxiliary battery as there is a difference between them of 0.8v.
However as the current flow increase so does the voltage drop across the connecting cable. As the current reaches 170A (our cables safe capacity) the voltage drop introduced is now 0.58v thus the difference between the batteries is only 0.22v. Actually the starter battery voltage will also be pulled down by the discharge. I just checked my battery and the voltage at the terminals drops by 0.2v when the ignition is switched. So at 170A there would likely be no voltage difference beween the batteries and thus no current flow.
It would also be impossible for the current to reach 300A as at that point the starter battery will appear to the auxiliary to have a voltage of 11.7v.
In reality I suspect the closed system will balance and a steady current flow be reached almost immediately. What this current would be we don’t know. It could certainly be a fairly large current flow but we can see from the rapid voltage drop introduced as the current reaches 170A that it is very unlikely to exceed 170A and is likely to be much less than that.
Now I just came to this conclusion on my own so I could be completely wrong, but it seems to make sense to me...
I have been wondering why current flow between main and auxilary banks is not more of an issue and I think it is because, to a large extent, the current flow between the batteries is self limiting.
Essentially the greater the voltage difference between two batteries the more current will flow.
So lets look at what happens when we connect our auxiliary battery to the starter battery (using a switch not a VSR) with 5m of 25mm2 cable (the battery is in the back of the car).
First some numbers:
25mm2 cable is rated to carry a constant current of 170A, but at this current it would generate a voltage drop of 0.58v over 5m.
At 200A this rises to a voltage drop of 0.68V.
At 300A this rises to a voltage drop of 1.03V.
At 20A it generates a drop of just 0.07v
So if we take the alternator out of the picture and assume the starter bank is fully charged at 12.8v and the auxiliary is heavily discharged by 75% and so is at 12v. At the instant we connect the batteries current will flow from the starter to the auxiliary battery as there is a difference between them of 0.8v.
However as the current flow increase so does the voltage drop across the connecting cable. As the current reaches 170A (our cables safe capacity) the voltage drop introduced is now 0.58v thus the difference between the batteries is only 0.22v. Actually the starter battery voltage will also be pulled down by the discharge. I just checked my battery and the voltage at the terminals drops by 0.2v when the ignition is switched. So at 170A there would likely be no voltage difference beween the batteries and thus no current flow.
It would also be impossible for the current to reach 300A as at that point the starter battery will appear to the auxiliary to have a voltage of 11.7v.
In reality I suspect the closed system will balance and a steady current flow be reached almost immediately. What this current would be we don’t know. It could certainly be a fairly large current flow but we can see from the rapid voltage drop introduced as the current reaches 170A that it is very unlikely to exceed 170A and is likely to be much less than that.
Now I just came to this conclusion on my own so I could be completely wrong, but it seems to make sense to me...
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