DC voltage drop over long wires to bilge pump

[Meroo]

A voltmeter is definetly not what you would refer to as a "load". with the voltmeter we know that there is a potential difference of 12.6v at the end of the cable. lll know that the voltage sitting at the terminals of the battery is 13v. No current required the voltage is always there . Same principal as using a voltstickk with 240v. You can wave a voltstickk over the active of a 240v cable and get a positive indication even if the cable has been cut and the circuit is not complete. A voltstick indicates on the emf of a live cablee as voltage is always present when you have a source of electricityy coming from the other end. All you do when you use a voltmeter is measure the voltage between your positive and negative and the voltage at the end of your cable will be less then at the point of supply (battery) . Again this is due to the resistance of the cable and that lower voltage will always be sitting there at the end of the cable irrespective of connected to a load or not

 

[blisters]

Anyhoo, Occasional panners post #8 is the best solution in this thread for MikeB05 to determine if he has a problem or not.
Jon

 

[AngerManagement]

The voltmeter is the load when measuring the voltage over a battery.
Jon

Yes but only a very light load so much so that it is not an actual load of any significance...

I have plenty of stuffed batteries that will measure close to 12V but even running a low voltage lamp will drop due to their high internal resistance.

In reality the only way other than to do the maths and assume min losses in the power supply; is to measure at the motor when it is running. And then is the pump Current Sensitive or Voltage Dependant (both when under load )

Thus you have a Supply V then a drop across one supply line (R1) the Load (R2) and the return line (R3) is the total Resistance of the cct.

V = I x (R1 + R2 + R3) Or V = (I x R1) + (I x R2) + (I x R3 )

And unless you have a Quality Meter your not going to be able to accurately measure the resistance.

But excluding the DunningKruger mob even a small resistance in the leads will have an impact on Voltage available to the Pump. Then add in Pump and Supply efficiency OR lack of and you have a pigs breakfast.

For DC and any load over any distance - Go a big as you can and I mean big

 

[blisters]

The voltmeter is the load when measuring the voltage over a battery.
Jon

Yes but only a very light load so much so that it is not an actual load of any significance...

Yep that's so it doesn't cause a voltage drop over what you are measuring.
Jon

 

[Occasional_panner]

Looks like a good wee wee match going on.

Mike just do the bucket fill test timing it with both lengths of cable, if there is a big difference you need bigger cable, simple, you don't need to calculate anything you can see and time the difference with your eyes.

 

[lewy]

what connections have you used for the fuse.could be a voltage drop over the connections.

 

[Smoky bandit]

Hey mike...if ya using it around your area...you might as well get a petrol pump. ..you can get a good one for under $200 on ebay...the fine will be the same no mater what pump.....fuel ....or ...bilge. ...i to have turned to the dark side ...and brought a pump...if you cant beat em...Join em.

 

[MikeB05]

To everyone who has posted responses...thank you to all. I did not realise it was as complex an issue as it is...never the less, I have learned much from all your posts and responses.
again,
many thanks
Mike B

 

[condor22]

The simple fact in all of this is - voltage drop can be measured with or without a load on the circuit. The question is;

If there is no load, who cares what the voltage drop is?

The reality of voltage drop in any circuit is, when you want to use the power ie the bilge pump.

There are 3 primary factors to consider with voltage drop

1. Length of cable between the source and the load,
2. Cross sectional area of copper (cable size) the heavier the cable the more current it will carry for the same voltage drop or the less voltage drop for the same current.
3. Condition and age of the battery, an older battery in poorer condition will drop off more under load.

Before I rewired, I measured 3 points in my car/caravan hook up. 1. At the starter battery, 2. at the Anderson plug on the rear of the car and, 3. at the input to my van battery. I had 13.8v at the main, 12.6 at the Anderson and 11.2 at the van battery. The easy fix was to put a Redarc BCDC1220 at the van battery, it will run on as low as 9v and charge at 14.5 for the van AGM, so the 11.2 saved a complete rewire. And, yes I checked it with the 3 way fridge on and the engine of the car on, I also checked all the cables for over heating, no prob. I do have thermal circuit protection at all points.

So, simple solution if the battery is good and you need the same distance, use a heavier cable. Another option is to include a solar input to the battery, this will increase the volts at the battery and in turn to the pump, even with existing wiring.

I have a twin core 10M extension cable with Anderson at both ends (Supercheap Auto $61.49) I use it on my compressor to reach my van wheels when hitched up, it pulls near 40A and handles the load great. I think it's 8mm cable (area)