Discharged battery's effect on amperage draw

[Randy6414]

I need one of the electrical gurus to help set me straight on this: does a deeply discharged battery have any effect on how many amps something such as a bilge pump will draw?

For example, if a 12v bilge pump draws 6 amps at 12v (72 watts), would a week battery cause it to try to "pull" more amps from the battery (keeping total wattage the same), and possibly exceeding the fuse capacity, blowing it?

In short, does a low battery increase the risk of blowing a fuse, or does it not have any effect?

Thanks,

Randy

 

[Guest]

Fuse

Hi Randy, not likely. Once a 12 volt battery reaches 12 volts it has been deeply discharged to about 40% of it's rated capacity. To let it draw down further (ten, nine, eight volts, etc.) will cause the bilge pump to operate slower and slower until there is insufficient charge to start and run the pump motor. The fuse is there to protect the motor from a short or power surge that a weak battery is unlikely to produce. My advice to you is to always keep your battery charged to 12 volts minimum so that it will not fail prematurely resulting in a boat loss.

Terry Cox

 

[Peggie Hall HeadMistress]

Another important reason to keep battery fully charged

Low voltage to any electric motor damages the motor, eventually causing the device to fail.

 

[MoonSailer]

Amps,volts and resistence. They are all related. Volts pushes the electrcity and amps is how much is being pushed, resistence must be overcame. But all things being equal as to resistence: an increase in voltage will cause an increase in amperage. An increase in resistence will cause a decrease in amperage. I like the water hose analogy. Amps is how much water, volts is the water pressure, resistence is the size/length of the water hose. http://www.sengpielaudio.com/calculator-ohm.htm

 

[MoonSailer]

Charger??? Wiring??? I wonder if a powerful charger and poor wiring might cause a surge of amps and blow a fuse. An alternator puts out 14.4 volts and with a very weak battery accepting a lot of amps due to low resistence. Maybe the ground and positive wires on the pump reversed....just a thought. http://www.evdl.org/pages/hartcharge.html

 

[Bob Poff]

It draws less current at the lower voltage.

Ernst Ohm says : Volts/Amps = Resistance .
And Volts/Resistance = Amps.

So you motor that dtaws 6A @ 12V would have an R of 2 ohms (12/6=2).

Then 2 ohms across say 10V would draw 5A (10/2=5).

An easy way to remember Ohms Law is this figure:
E
---|---
I | R

E=I*R
I=E/R
R=E/I

E is Voltage
I is Current
R is Resistance

And a silly little ditty someone once told me to remember their placement in the figure is :

An Indian is hunting a Rabbit on the ground while the Eagle soars above.

 

[Allan12210]

I always heard that (V)ermont = (R)hode (I)sland

 

[kloudie1]

OK, but power is Isquared R .. so if ya ask a motor to generate the same power at a lower voltage, the current will have to increase because it rises faster than the E=IR would lead ya to believe.. Not many motors will behave that way, but there are some that will.. I need an EE to help me with this one.. Seems like I remember that it was AC motors that would do that and only one type of AC motor?? (like a refrig compressor or an AC compressor??)

 

[Randy6414]

It draws less current at the lower voltage.

That's what I was looking for. I kept wrestling with this in my head, thinking that the bilge pump, motor, or whatever device would try to pull more amps at reduced boltage trying to keep the wattage (power) the same. I now realize that I was incorrect. Thanks to all of you who replied.

Randy

 

[Ross]

This is a case of words and terms being misused and misunderstood. The motor in question will develope its rated power at 12 volts. At ten volts it will be rated for a much lower power output. There are ac motors that are designed to be connected to either 120 volts or 240 volts. This is accomplished by the manner with which the internal wires are connected to the supply wires. The power that a motor develops depends on the load applied. Too large a load and the motor will overheat. A 2 hp motor connected to a half hp load will draw the same current as a half hp motor connected to a half hp load.

 

[Bill Roosa]

Motors are inductors

So you can't use V=IR to calculate the running resistance.
In a stalled motor V=IR
In a running motor with no load the back EMF (ElectroMotive Force) will cause the motor to have enough inductive resistance to stop all but the little amount of current needed to overcome the internal bearing resistance and drag from the brushes.
If you put a light bulb in series with a motor (AC or DC) and turn on the switch the light will momentarily brighten as the motor comes up to speed and begins to produce back EMF (back voltage). Once the motor is up to speed the light will only glow dimly if at all. As a load is placed on the motor the light will glow brighter. Since lights glow from current passing we can deduce that a stalled motor will draw lots more current than one running at speed.
So if you run a motor at a lower voltage it runs at a lower speed and produces less back EMF which would make it draw more (than normal) current.
Back EMF comes from the motor coils cutting magnetic lines of force as they turn in a magnetic field. That is why P=I^2*R where R is the resistance you measure with the ohm meter when the motor is off. If you draw more power out of the motor the motor slows, makes less back EMF, and draws more current.
What you have is a motor that "thinks" it is under a higher load when you operate it at a lower system voltage. It also makes the cooling fan run slower and can cause the motor to overheat.
Course on marine grade equipment electrical motors are required to not catch fire if you stall the motor and apply the rated voltage and they should do that for at least an hour.

 

[Ross]

Re: Motors are inductors

Bill, Just an aside. That is correct with ac motors and shunt wound dc motors but not so with series wound dc motors. Series wound dc motors have no speed limits if the availble power is unlimited.