Check your battery terminals!

Bru said:
Dougal nailed it nicely, not all loads are resistive or simply on or off.

Increased resistance on a purely resistive load will, of course, result in reduced current in the circuit and that too is bad - your nav lights will be dimmer for example
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Thanks,
but what you said about increased current and battery drain only applies to loads which will draw a fixed power, in watts, such as equipment with switch mode power converters as suggested by William_H in #16.

What I would now like see explained is William's statement that, "A starter motor will turn more slowly at reduced voltage (volts being lost in bad connection) which will result in it trying to draw more current which could conceivably end up with a total more current out of the battery" IF the motor is drawing more current why is it turning more slowly. Surely if the current is greater it should turn more quickly ?
 
VicS said:
Thanks,
but what you said about increased current and battery drain only applies to loads which will draw a fixed power, in watts, such as equipment with switch mode power converters as suggested by William_H in #16.

What I would now like see explained is William's statement that, "A starter motor will turn more slowly at reduced voltage (volts being lost in bad connection) which will result in it trying to draw more current which could conceivably end up with a total more current out of the battery" IF the motor is drawing more current why is it turning more slowly. Surely if the current is greater it should turn more quickly ?
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William is correct. It all comes down to "back EMF". This back EMF is "electro motive force" which is produced when a motor armature turns within the magnet field created by the field windings (or a permanent magnet). As well as acting as a motor, the rotation causes the armature to generate a voltage, the back EMF, this opposes the current flow and results in it decreasing. The slower the motor rotates the less back EMF it generates, allowing the current to increase. If the motor is stalled, there is no back EMF, and the current is only restricted by the relatively low resistance of the now stationary armature windings. This will result, in some cases, of the motor overheating and "burning out". Starter motors will suffer from this if their applied voltage is too low. Specifications for a motor will state the "stall current" and it will be appreciably higher than the running current.
Richard
 
DickieT said:
William is correct. It all comes down to "back EMF". This back EMF is "electro motive force" which is produced when a motor armature turns within the magnet field created by the field windings (or a permanent magnet). As well as acting as a motor, the rotation causes the armature to generate a voltage, the back EMF, this opposes the current flow and results in it decreasing. The slower the motor rotates the less back EMF it generates, allowing the current to increase. If the motor is stalled, there is no back EMF, and the current is only restricted by the relatively low resistance of the now stationary armature windings. This will result, in some cases, of the motor overheating and "burning out". Starter motors will suffer from this if their applied voltage is too low. Specifications for a motor will state the "stall current" and it will be appreciably higher than the running current.
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Whilst what you say is correct, it's difficult to reconcile this with a situation in which a bad connection causes reduced voltage, rather than other causes. I find it hard to believe a scenario in which the connection is so bad, causing the voltage to be far too low, yet at the same time the connection is presumed to be good enough to supply ever larger currents.
 
pvb said:
Whilst what you say is correct, it's difficult to reconcile this with a situation in which a bad connection causes reduced voltage, rather than other causes. I find it hard to believe a scenario in which the connection is so bad, causing the voltage to be far too low, yet at the same time the connection is presumed to be good enough to supply ever larger currents.
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No, I agree with you. My explanation of back EMF was simply to explain to VicS how a motor will start drawing more current but produce less power.
The case we gave here of an unwanted resistance at say the battery terminals will simply drop the available voltage to a motor, resulting in it drawing more current and this then developing yet more voltage drop across the resistance. It is a law of dimenishing returns, this is why connections to high load items like starter motors have to be as perfect as possible. Also the initial surge from a non rotating motor is high (our old friend back EMF again) making the problem worse.
The same, to an extent, also applies to switch mode power supplies. True they will draw more current to try and maintain their output voltage at the correct level, but again there is a limit. The more they draw the more the available input voltage to them drops. As an example - if they are supplied via a slightly under sided cable (and hence lower voltage) they will still produce their rated output, but at the cost of drawing more amps. However you have reduced the amount of "headroom" they have before they fall off their perch and no longer can supply the correct output voltage.
Richard
 
pvb said:
Whilst what you say is correct, it's difficult to reconcile this with a situation in which a bad connection causes reduced voltage, rather than other causes. I find it hard to believe a scenario in which the connection is so bad, causing the voltage to be far too low, yet at the same time the connection is presumed to be good enough to supply ever larger currents.
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The best thing you can do is play with the numbers on the calculation website I linked to, the one that solves the quadratic equation for a constant power load.
When you say "so bad" I think you are thinking of a lost cause case, one where the resistance is too high for any useful battery use.
What is being referred to here is an intermediate stage where the connection is of the order of a few hundredths of an ohm, and demonstrating how bad even this small resistance can be.
 
In the day when I worked for BR, all the mainline diesel electrics could demonstrate this in spades. Using DC generators and DC traction motors, you could watch traction current build up as full power was applied, up to 2500A or more for Class 4s. But as speed increases, the current reduces (back EMF), and at a speed of around 30mph, the first of three to five stages of traction motor field diversion cut in, so the mag field in the traction motors reduces, allowing higher traction current again. Most locos had 3 stages (effectively 4"gears"), but the 45's had 5...

All went with modern high voltage alternators and AC motors... oh for the days of a cab ride in a noisy draughty cold class 50...
 
Gladys said:
In the day when I worked for BR, all the mainline diesel electrics could demonstrate this in spades. Using DC generators and DC traction motors, you could watch traction current build up as full power was applied, up to 2500A or more for Class 4s. But as speed increases, the current reduces (back EMF), and at a speed of around 30mph, the first of three to five stages of traction motor field diversion cut in, so the mag field in the traction motors reduces, allowing higher traction current again. Most locos had 3 stages (effectively 4"gears"), but the 45's had 5...

All went with modern high voltage alternators and AC motors... oh for the days of a cab ride in a noisy draughty cold class 50...
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Now that really is thread drift! :rolleyes:
 
DickieT said:
William is correct. It all comes down to "back EMF". This back EMF is "electro motive force" which is produced when a motor armature turns within the magnet field created by the field windings (or a permanent magnet). As well as acting as a motor, the rotation causes the armature to generate a voltage, the back EMF, this opposes the current flow and results in it decreasing. The slower the motor rotates the less back EMF it generates, allowing the current to increase. If the motor is stalled, there is no back EMF, and the current is only restricted by the relatively low resistance of the now stationary armature windings. This will result, in some cases, of the motor overheating and "burning out". Starter motors will suffer from this if their applied voltage is too low. Specifications for a motor will state the "stall current" and it will be appreciably higher than the running current.
Richard
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Hello Vic
Thanks Richard. Yes my statement that at lower voltage a starter motor will draw more current depends on a lot of other factors like the friction resistance of the engine at the speed the starter motor can achieve. So it is a case of it may or may not but possibly could. (Draw more current at lower voltage) olewill.
 
William_H said:
Hello Vic
Thanks Richard. Yes my statement that at lower voltage a starter motor will draw more current depends on a lot of other factors like the friction resistance of the engine at the speed the starter motor can achieve. So it is a case of it may or may not but possibly could. (Draw more current at lower voltage) olewill.
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And another thing.... If the lower voltage means slower cranking it will (well probably will) mean longer cranking... so more Ah to start your donk.
 
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