24v cold start

[Dmoser]

I have seen threads stating that 24v starting systems are better in cold weather, my question is why? Yes, voltage is the force that pushes current but i was always under the impression that the more current you have going through an electric motor the stronger it will be. 12v with two batteries doubles the amperage going through the motor so shouldn't it be just as, if not more effective for starting an engine?

this is more of a question out of general curiosity more than anything.

 

[AceHigh]

My automotive days began with 6 volt starter systems.

I will take 24 volt any single day.

 

[Matt65]

Increased Voltage reduces the Resistance. This also reduces the heat generated, or energy lost in the circuit. This is one reason AC transmission lines are at such a high kV.

Resistance= Voltage/Current

 

[mistaken1]

I have seen threads stating that 24v starting systems are better in cold weather, my question is why? Yes, voltage is the force that pushes current but i was always under the impression that the more current you have going through an electric motor the stronger it will be. 12v with two batteries doubles the amperage going through the motor so shouldn't it be just as, if not more effective for starting an engine?

this is more of a question out of general curiosity more than anything.

Two 12V batteries in parallel will NOT put double the amperage through a starter motor.

They are capable of delivering double the amperage but the actual amperage draw is a function of the design of the motor.


P=I x E
Power = Amperage x Voltage.

Double the amperage for a given voltage and double the power.
Double the voltage for a given current flow and double the power.

How much power a motor uses is a function of the motor design so a normally functioning 12V starter motor that is designed to use say 200A on single battery will still use 200A when connected to a dozen 12V parallel batteries.

So if a motor is designed to provide x amount of torque and x RPMs at 12V and we want that motor to produce more torque and more RPMs then we need to increase the power that the motor uses. We can double the current or double the voltage. As Matt65 points out doubling the voltage is a more efficient way of increasing the power of our motor.

IOW if we have two motors that are identical in mechanical characteristics (torque and RPM) but one is 12V and the other is 24V the 24V motor will use less current and consequently smaller wires and control devices.

 

[Dmoser]

Thanks. I guess I just needed some simple math

 

[mistaken1]

Thanks. I guess I just needed some simple math

The point is two batteries can deliver twice the power of a single battery. Whether that power is at twice the voltage (series) or twice the current flow (parallel).

So you are right in the sense that two batteries in parallel can deliver the current to turn a starter motor for a longer period of time than a single battery.

Or if you factor in cold weather that can reduce the amount of power a single battery can deliver then two batteries in parallel at individually reduced output can still deliver every bit of current that a given starter may draw.

 

[jdemaris]

I have seen threads stating that 24v starting systems are better in cold weather, my question is why? .

There is NO difference. A 12 volt starter wired properly has exactly the same turning force as a 24 volt starter.

Two 12 volt batteries hooked in series to make 24 volts have the same power as two 12 volt batteries hooked in parallel to make 12 volts.

24 volt starter on a 6.2 is rated around 2500 or 3000 watts and is designed to work at 18-20 volts.

12 volt starter on a 6.2 is also rated around 2500 or 3000 watts and is designed to work at 9-10 volts.

Both make the same torque at the same cranking RPMs.

Back in the late 50s, 12 volt starter technology at the consumer level had not caught up yet and 24 volts was more comon in big diesels. That ended in the mid 1960s. The US military uses 24 volts in many things to adhere to an old standard and not because it's better. The only potential gain with 24 volts is allows cost cutting by using smaller wires. It also greatly complicates things when series-parallel switches are used to make it a hybrid 12'24 volt system.

 

[Gunner0311]

Smaller wires on 24v system?`

I am more confused than ever!

 

[jdemaris]

I am more confused than ever!

The higher the voltage the smaller wire needed. That's why power companies use ultra-high voltage for long-range transmission lines.

With a 6.2 diesel Blazer or truck? If the correct size cables are used, both systems work the same. But the higher voltage system can get by with much smaller wire to do the same work.

If the battery cable length - from batteries to starter is 4 feet long . .

With 12 volts - a #1 gauge cable is needed to keep voltage drop below 2.8%
With 24 volts - a #4 gauge cable is needed to keep volage drop below 2.8%

If the cable length is 6 feet long . .

With 12 volts - a #2/0 gauge cable is needed to keep voltage drop below 2.6%
With 24 volts - a #2 gauge cable is needed to keep volage drop below 2.6%

 

[Matt65]

This is pretty helpful as well: http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc