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Old 10-12-2018, 14:53   #1
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battery fuse is opening

I moved the batteries to a new location and put in a 200 amp fuse for the battery (both house and starting) tried to start engine the starting battery was weak and tried to turn over but did not have the juice to start the engine it blew the fuse, when switched to both the batteries they were almost dead they feebly tried turned over slowly and both fuses blew . when I charged the battery. it started just fine. no fuse blew,
whats going on ??

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Old 10-12-2018, 16:18   #2
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Re: battery fuse is opening

The starter motor will draw more current at low battery voltage than it does at normal voltage. I'd consider using a higher rated fuse on the start battery. It will still open if there is a short circuit (very high current) but survive the situation you have described.

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Old 11-12-2018, 05:21   #3
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Re: battery fuse is opening

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Originally Posted by Jim Cate View Post
The starter motor will draw more current at low battery voltage than it does at normal voltage.

A DC starter motor will draw less current with less voltage but the duration of start will be longer meaning the fuse is in its trip delay curve longer. A healthy engine will normally start (loaded to unloaded starter duration) in anywhere from .75 seconds to about 1.25 seconds. With low voltage/low battery the loaded duration can often exceed 6+ seconds, if it cranks at all. If the starter is "stalled", because the battery does not have the oomph to turn over the motor, this looks very similar to a dead short to the fuse. If you hold it stalled long enough, or do quick successive repeat tries, and the fuse is too small, pop....

Either way a 200A fuse for starting a diesel is very much on the too small side if you want to avoid nuisance trips. For most small aux engines 250A - 300A would be a minimum fuse size you'd want to start a motor.
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Old 11-12-2018, 05:53   #4
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Re: battery fuse is opening

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Originally Posted by Maine Sail View Post
A DC starter motor will draw less current with less voltage but the duration of start will be longer meaning the fuse is in its trip delay curve longer. A healthy engine will normally start (loaded to unloaded starter duration) in anywhere from .75 seconds to about 1.25 seconds. With low voltage/low battery the loaded duration can often exceed 6+ seconds, if it cranks at all. If the starter is "stalled", because the battery does not have the oomph to turn over the motor, this looks very similar to a dead short to the fuse. If you hold it stalled long enough, or do quick successive repeat tries, and the fuse is too small, pop....

Either way a 200A fuse for starting a diesel is very much on the too small side if you want to avoid nuisance trips. For most small aux engines 250A - 300A would be a minimum fuse size you'd want to start a motor.



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Running 12v motor on 6v?

I have a 12 v electric motor I want to run on 6v to slow it down will this work or just bugger the motor up?

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Best Answer: there are only two things that you need to consider:
1) loss of power- if the motor is rated at 1/64hp it will try to maintain that at any voltage. it will do well, but 50% of the voltage is a lot to lose.
2) increased current- to attempt to maintain the rated hp the motor will increase its current draw. this can cause complete failure if the motor overheats


I see so many statements that seem to conflict regarding reduced volt/increased amps.
Of course being a starter motor one would hope it wouldn't be run for more that a few seconds before being allowed to cool.


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Old 11-12-2018, 06:02   #5
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Re: battery fuse is opening

I have 300 amp fuses on my batteries (the start and the house) at the terminals. Just saying.
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Old 11-12-2018, 09:20   #6
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Re: battery fuse is opening

If you have the equipment to test for this, the test is simple. Take a starter motor and put it in a bench vise. Connect it to a fully charged battery, now trigger the solenoid. Measure the amperage being drawn with a clamp meter (best if you can measure both in-rush (peak crank-over current capability) and average current). Now discharge the battery and do the same. You will quickly see that the starter motor cannot develop the same current. Lower voltage at the starter means less avaible cranking current.
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Old 11-12-2018, 13:05   #7
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Re: battery fuse is opening

A lowered voltage source reduces the starter motor's current draw potential, Current = Voltage divided by Resistance. Whereas when a motor starts turning it derives back electromotive force [i.e., it acts as a generator] which bEMF counters the source voltage. Current derives torque inside a motor. If there is inadequate current to derive torque greater than the load then the motor either stalls [does not turn] or runs slower until the motor torque equals the load. As Maine Sail indicated with your weakened voltage source, you likely were cranking for a comparatively lengthy time and the continuous, yet inadequate, current draw heated your 200 ampere fuse causing it to break open the circuit. If your fuse did not blow open and you kept current flowing through your starter [a solenoid and a motor] you could burn out the solenoid or the motor as they are not intended for continuous use cycles, albeit they should have thermal sensors to trip a relay.

Maine Sail provides astute advice in operating a starter, "do quick successive repeat tries" as short periods of current draw allow for heat to dissipate from the fuse circuit between successive tries, and also lets everything else cool. Also, voltage will sag yet further when a battery that is in a low state of charge is caused to have current drawn from it, thereby further inhibiting the ability of the motor to draw current to derive torque. Batteries "refresh" themselves to an extent as to yielding the voltage of their state of charge when one stops drawing current from them, so you can get a short burst of higher voltage upon initial loading when utilizing short tries at starting.

So yes you could install a yet larger fuse to your starter circuit [300+] but too high of capacity fuse can result in endangering the remainder of the conductors forming the circuit [evaluate the current rating of each of the components of your circuit, make your fuse the weakest component] and / or more appropriately just provide for an appropriate voltage source by charging the battery and thence result in a quickly turned over and started engine.

Diesel engines with their comparatively higher compression ratios require greater torque to turn over than do gasoline engines and larger engines similarly require greater torque. Also, diesel engines draw current for heating the glow plugs, although not a great deal of current.

A charged battery / higher voltage source derives greater current potential to the motor and hence greater torque capabilities to avail turning over and starting your engine in the intended very short span of time.

In simple reference, for electric motors, torque is a function of the amount of current [amperes - I], the length of current carrying conductor under the influence of a magnetic field [L] and the strength of the magnetic field influencing the current carried in the length of the conductors that is under the influence of the magnetic field [B].

Once a motor's architecture is defined, that is to say B and L are established, its torque output becomes primarily a matter of managing the current [I].

For equipping large and high compression engines, one typically provides for a larger motor with greater length of conductors [L] and also larger current potential via providing for larger dimension of gauge of conductors. Longer length of conductors increases resistance [and accordingly increases bEMF potential] both of which reduce current potential; larger dimensions of gauge of conductors decreases resistance and increases current carrying potential and capacity. One often also increases voltage for larger or diesel engines, from say 12 volts to 24 volts, or higher, stater systems, such as are found on our bulldozer, front end loader, and dump trucks which batteries are placed in series circuit and which we used to maintain the roadway to our family's retreat in the mountains and to hobby gold mine. As to the subject of fuses: Drilling and blasting with dynamite and ANFO being the funnest part of working hard rock, although it always seems that the last fuse to be lighted takes the longest to get ignited, all the while the other fuses are busy burning towards their blasting caps which is why we use considerable length of fusing; but operating the large Tonka Toys can be recreational also. We operate a 35kW hydroelectric power system to avail power to the family retreat / mining camp and to charge the high amperage batteries after the equipment has set idle for the long winter period, the four seasons of Montana noted as being June, July, August and Winter. Guess what season it is now?
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Old 11-12-2018, 17:22   #8
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Re: battery fuse is opening

A motor is an inductive load,as opposed to a simple resistance load(light bulb,etc)
Ohms law does not apply as simply as it does in simple resistance loads./ Len


Fuses should be sized to meet wire ampacity. You protect the wire with a fuse at power source that matches the wire ampacity.

If the load needs current(fire) protection,you place the appropriate sized smaller fuse at the load input.
Starter motors do not normally need or want fuse protection for the motor itself.There are so many variables that can increase starter current draw/heating of components during cranking.

There are/were very few fuses in the starter feed of the countless zillions of road vehicles produced.



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Old 11-12-2018, 18:54   #9
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Re: battery fuse is opening

^^ and to add to Len's excellent post, you have to add it the internal resistance of a partly discharged battery and the fact that starter motors are series wound - meaning that their effective resistance decreases rapidly as they are slowed down e.g. when stalled, they have a very low effective resistance and will draw as much current as the battery can supply - until something melts .
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Old 11-12-2018, 20:12   #10
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Re: battery fuse is opening

The main thing here is that the inrush current is 5X or more the normal current. The inrush tapers off as the motor gains speed. The inrush period is greatly extended when the supply voltage is low because the motor can't get to speed quickly or at all. A stalled motor draws very high current, and also at the instant power is applied. Fuses and circuit breakers have time delay curves to accommodate normal motors but starters have exceptionally high inrush and are really more than normal fuses and circuit breakers handle well, even at ratings well above the specification for the wire. This is why starting circuits are exempt from fuse requirements and we rely on battery switches here in case something goes wrong. A 300A should work for your 50-ish HP engine but it may become weak with repeated starts and eventually give you a nuisance failure, perhaps at an inconvenient time.
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Old 12-12-2018, 15:16   #11
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Re: battery fuse is opening

"A motor is an inductive load,as opposed to a simple resistance load(light bulb,etc)
Ohms law does not apply as simply as it does in simple resistance loads./ Len"

Agreed if the motor has inductance then that added impedance comes to play and can have a significant impact on power factor and resultant current draw. For a motor with inductance, power factor tends to decrease significantly more when the motor is operated at slow speeds, e.g., less than 50% of rated speed and at stall such as commonly occurs when a starter is struggling excessively to crank an engine.

If the motor does not have significant inductance then it will operate much like a resistive load.

Presently most engine starter motors have significant inductance.
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Old 12-12-2018, 16:57   #12
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Re: battery fuse is opening

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Originally Posted by Montanan View Post
"A motor is an inductive load,as opposed to a simple resistance load(light bulb,etc)
Ohms law does not apply as simply as it does in simple resistance loads./ Len"

Agreed if the motor has inductance then that added impedance comes to play and can have a significant impact on power factor and resultant current draw. For a motor with inductance, power factor tends to decrease significantly more when the motor is operated at slow speeds, e.g., less than 50% of rated speed and at stall such as commonly occurs when a starter is struggling excessively to crank an engine.

If the motor does not have significant inductance then it will operate much like a resistive load.

Presently most engine starter motors have significant inductance.
Power factor doesn't really apply to an uncontrolled DC series wound motor i.e. a starter motor. These are simple DC motors where the voltage and current remain phase i.e. a PF of 1. The only thing to consider is the BEMF as you rightly considered in your previous post.

The posts above explaining why a low battery voltage (indicating a partially discharged battery - thus high internal resistance) leads to longer starting times and thus a prolonged current draw explain better why battery fuses might trip when battery voltage drops.

Ohms law explains it all when all factors are included. i.e. motor coil resistance, motor rpm (i.e. BEMF due to coil inductance and proportional to RPM), applied voltage, battery chemistry (expressed as battery internal resistance), lead resistance etc.
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Old 12-12-2018, 18:28   #13
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Re: battery fuse is opening

Impedance and Power factor only come into the equation in AC circuits. However, inductance is the reason that the (DC) motor does not continuously draw the current one would expect from its resistance value. The main electrical characteristic of an inductor is it opposes changes in current. When a motor is turning the brushes and commutator switch pairs of windings into the circuit and each time one is energized there is a lag in current rise, causing a reduction in the average current the faster it turns. When the motor is stalled there is only one minutely brief lag in the current rise and then Ohm's Law applies 100% - 100's of amps for even a very small engine starter.
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