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Can a cable have capacity?


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I want to use a different technology service battery but my bow thruster battery is connected to the service battery. I thought it would just be a simple job of moving the cable from the pos of the service battery to the starter battery but nothing is ever simple.

Boat is quite simple, no solar and no inverters.

All testing done with both isolators off and no mains connection.

So I have 3 x25mm2 cables connected to the pos of the service battery, I take each one off in turn and measure to see if its live. I find one that measures 12.6 volts, I put some tape over the end and go to front of boat and disconnect the pos 25mm2 cable going to the BT battery, I also put some tape on it.

I go back to the engine bay and measure to see if the cable I disconnected earlier is dead. To my supprise it measures 12.0 volts but slowly drops over the next 2 minutes to zero. 

So I reconnect the pos cable to the BT battery, confirm I get 12.6volts at the engine bay end and then disconnect the BT battery again. The same happens, I get 12.0 volts initialy dropping to zero over the next two minutes.

I am pretty sure that there is nothing else connected to this cable but not 100% sure. Even so I can't think what could do this.

 

Any ideas?

 

 

  

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? Large capacitror in the circuit betwixt the main batteries and the bow thruster unit?

OR

A smaller battery mounted mear the thruster to allow for voltage drop between the main bank and the thruster?

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41 minutes ago, sueanddaren said:

I want to use a different technology service battery but my bow thruster battery is connected to the service battery. I thought it would just be a simple job of moving the cable from the pos of the service battery to the starter battery but nothing is ever simple.

Boat is quite simple, no solar and no inverters.

All testing done with both isolators off and no mains connection.

So I have 3 x25mm2 cables connected to the pos of the service battery, I take each one off in turn and measure to see if its live. I find one that measures 12.6 volts, I put some tape over the end and go to front of boat and disconnect the pos 25mm2 cable going to the BT battery, I also put some tape on it.

I go back to the engine bay and measure to see if the cable I disconnected earlier is dead. To my supprise it measures 12.0 volts but slowly drops over the next 2 minutes to zero. 

So I reconnect the pos cable to the BT battery, confirm I get 12.6volts at the engine bay end and then disconnect the BT battery again. The same happens, I get 12.0 volts initialy dropping to zero over the next two minutes.

I am pretty sure that there is nothing else connected to this cable but not 100% sure. Even so I can't think what could do this.

 

Any ideas?

 

 

  

Inverter capacitors discharging.

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19 minutes ago, OldGoat said:

? Large capacitror in the circuit betwixt the main batteries and the bow thruster unit?

OR

A smaller battery mounted mear the thruster to allow for voltage drop between the main bank and the thruster?

What would have a large capacitor in it?

If there is another battery I haven't found it in the last 3 years and I have no idea where it could be. 

20 minutes ago, Tracy D'arth said:

Inverter capacitors discharging.

Haven't got an inverter.

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27 minutes ago, OldGoat said:

? Large capacitror in the circuit betwixt the main batteries and the bow thruster unit?

OR

A smaller battery mounted mear the thruster to allow for voltage drop between the main bank and the thruster?

Just realised you mean the BT battery. Yes I have a BT battery but that is what Iam disconnecting at the front and that is what is sending the 12.6volts to the back.

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I guess the bow thruster is connected to this cable and this probably has a remote electronic control unit.  This will have capacitors which will discharge during which the voltage will drop.  I assume your meter is a digital meter which will have an impedance of 10M ohms, which will discharge a capacitor fairly slowly.

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13 hours ago, sueanddaren said:

I want to use a different technology service battery but my bow thruster battery is connected to the service battery. I thought it would just be a simple job of moving the cable from the pos of the service battery to the starter battery but nothing is ever simple.

Boat is quite simple, no solar and no inverters.

All testing done with both isolators off and no mains connection.

So I have 3 x25mm2 cables connected to the pos of the service battery, I take each one off in turn and measure to see if its live. I find one that measures 12.6 volts, I put some tape over the end and go to front of boat and disconnect the pos 25mm2 cable going to the BT battery, I also put some tape on it.

I go back to the engine bay and measure to see if the cable I disconnected earlier is dead. To my supprise it measures 12.0 volts but slowly drops over the next 2 minutes to zero. 

So I reconnect the pos cable to the BT battery, confirm I get 12.6volts at the engine bay end and then disconnect the BT battery again. The same happens, I get 12.0 volts initialy dropping to zero over the next two minutes.

I am pretty sure that there is nothing else connected to this cable but not 100% sure. Even so I can't think what could do this.

 

Any ideas?

 

 

  


A cable, like anything else, does have some capacitance. But a tiny amount, not something that would keep even a digital multimeter going for more than microseconds. So it sounds like there is some other device that has significant capacitance, that is connected to the cable.

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13 hours ago, Chewbacka said:

I guess the bow thruster is connected to this cable and this probably has a remote electronic control unit.  This will have capacitors which will discharge during which the voltage will drop.  I assume your meter is a digital meter which will have an impedance of 10M ohms, which will discharge a capacitor fairly slowly.

You might have something here, the BT manufacturers wiring diagram shows that the control unit is powered from the starter battery, but there are definatley no connections to the starter battery and the isolators were turned off.  I also can't belive they would have run another cable all the way to the back of the boat when the BT battery is sitting next to BT. I will have another look at the weekend to see if they have made a connection somewhere down the line.  

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11 minutes ago, sueanddaren said:

You might have something here, the BT manufacturers wiring diagram shows that the control unit is powered from the starter battery, but there are definatley no connections to the starter battery and the isolators were turned off.  I also can't belive they would have run another cable all the way to the back of the boat when the BT battery is sitting next to BT. I will have another look at the weekend to see if they have made a connection somewhere down the line.  

 

The extra cable to the back is for charging. The battery at the front is to give the BT enough power at that point near the BT. 

 

 

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1 hour ago, sueanddaren said:

You might have something here, the BT manufacturers wiring diagram shows that the control unit is powered from the starter battery, but there are definatley no connections to the starter battery and the isolators were turned off.  I also can't belive they would have run another cable all the way to the back of the boat when the BT battery is sitting next to BT. I will have another look at the weekend to see if they have made a connection somewhere down the line.  

There maybe a multi core cable running from the bow thruster to the controller panel, this would have +12v & negative  to power the electronics/indicator lamp/beeper etc, and a couple of signal wires (clockwise/anti-clockwise motor rotation).

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22 hours ago, Chewbacka said:

There maybe a multi core cable running from the bow thruster to the controller panel, this would have +12v & negative  to power the electronics/indicator lamp/beeper etc, and a couple of signal wires (clockwise/anti-clockwise motor rotation).

Managed to get back to boat today to have another look. As I suspected BT control unit is feed from BT battery not starter battery as manufacturer advises. However that means that whatever is giving the 25mm2 cable capacity is definetley not the BT control unit.

 

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47 minutes ago, sueanddaren said:

Managed to get back to boat today to have another look. As I suspected BT control unit is feed from BT battery not starter battery as manufacturer advises. However that means that whatever is giving the 25mm2 cable capacity is definetley not the BT control unit.

 

It is possible that your supply cable connected to the service battery goes to the bow thruster and then on to the bow thruster battery.  The bow thruster possibly has a 4 or 6 core cable running between the bow thruster and the control panel.  So when either end is connected to a battery the other end will read battery voltage, disconnect both ends and the cable will show the voltage from the control electronics capacitors. 

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5 hours ago, Chewbacka said:

It is possible that your supply cable connected to the service battery goes to the bow thruster and then on to the bow thruster battery.  The bow thruster possibly has a 4 or 6 core cable running between the bow thruster and the control panel.  So when either end is connected to a battery the other end will read battery voltage, disconnect both ends and the cable will show the voltage from the control electronics capacitors. 

Red 25mm2 cable comes from back of boat into a 100A fuse then onto pos terminal on BT battery, a red 60mm2 cable then comes of this terminal to a 350A fuse then on to an isolation switch then onto the BT.

Black 25mm2 cable comes from back of boat straight to neg terminal on BT battery, a black 60mm" cable then goes from this terminal to the BT.

The pos feed to the control unit comes off the BT battery terminal and I have to remove it to take the 25mm2 red cable off, so no the two arn't connected when I measure the voltage in the engine bay.

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