30 replies to this topic

[ECOBRAP]

Hi all,

 

Long story short, my battery is shot, and I couldn't get a new one before I left for the track. I jump started the car in the morning and it was ok for the rest of the day.

 

That being said, my car felt a little down on power, especially higher up in the rpm range, so I was wondering if a weak/dead battery can affect performance? Either by making the alternator work overtime to run accessories like the coolsuit, or affect the spark/fuel, or something else?

 

My butt dyno would say it was pretty significant, 3-5hp, which is why I am skeptical, but some have said it is possible? Any insight or experience on this would be appreciated!

 

Best,

Matt

 

 

[davew]

Ever heard an alt belt squeel when a Miata is first started, then go away once the battery is back to full charge after a few seconds. That shows how much drag an alt can create.

 

FYI, a Honda battery is the same footprint but taller and will work in a pinch. Also garden tractor batteries are the same size with different terminals that require adapters.

 

I sell a lot of batteries at the track. More than I would expect. My theory is that the batteries can not take the vubrations of rumble strips or the hi g-load that we put them through.

 

Dave

[ECOBRAP]

Thanks Dave, do you think a weak/dead battery can affect power even if the alternator is good?

[Tom Hampton]

Thanks Dave, do you think a weak/dead battery can affect power even if the alternator is good?

Yes.  If the battery is weak, it can continuously put a load on the alternator.  The 1.6 Miata alternator puts out a max of 60A, which is roughly 1 HP (60amps * 14.5 Volts = 870 watts ~ 1.1 HP).  Automotive alternators are typically less than 40% efficient at high-RPM (since they are designed to run at street-RPMs).  So, at 60 Amps the engine load would be on-the-order of ~3 HP. 

 

How close to that figure you are depends on how BAD your battery is, and how much current is actually being drawn from the alternator.  But, worst case, YES it could be that bad. 

[ECOBRAP]

Great thanks guys, will replace the battery this week and investigate the alternator to see if that has been affected as well.

Will update this thread if the car feels back to normal at the next event.

[manthony121]

I'm not sure I'm buying this.  I am not a mechanic, but I do understand basic physics.  The alternator takes some of the engine power to turn the rotor.  How much power it takes depends on the friction in the bearings.  I don't see how the electrical output of the alternator would effect how much power it takes to turn the rotor.  However, I don't know what happens to the "excess electricity" if the battery is already fully charged.  I don't see how it could be used to boost engine power output, unless it is part of an electric/gas engine hybrid system?

[Tom Sager]

I'm not sure I'm buying this.  I am not a mechanic, but I do understand basic physics.  The alternator takes some of the engine power to turn the rotor.  How much power it takes depends on the friction in the bearings.  I don't see how the electrical output of the alternator would effect how much power it takes to turn the rotor.  However, I don't know what happens to the "excess electricity" if the battery is already fully charged.  I don't see how it could be used to boost engine power output, unless it is part of an electric/gas engine hybrid system?

Bearing drag is minimal.  When the alternator is called upon to produce enough current to charge a low battery, there is significant drag due to the magnetic field that's utilized to produce electricity.  If the voltage regulator senses low voltage, more magnetic drag is needed to produce more current. 

[Tom Hampton]

I'm not sure I'm buying this.  I am not a mechanic, but I do understand basic physics.

I don't think you do. Energy can't come from "nothing". You can only convert it from one form to another. Electric current (the flow of electrons) IS energy---technically it requires energy to push those electrons around. The alternator converts mechanical energy into electrical energy.
 

The alternator takes some of the engine power to turn the rotor.  How much power it takes depends on the friction in the bearings.  I don't see how the electrical output of the alternator would effect how much power it takes to turn the rotor.  However, I don't know what happens to the "excess electricity" if the battery is already fully charged.  I don't see how it could be used to boost engine power output, unless it is part of an electric/gas engine hybrid system?

An alternator is a stator (electromagnet) that spins through several coils. As the stator magnetic field moves through the coils the electrons get "pushed" by the magnets. This creates an electrical potential which we call Volts. As long as the output terminals of the alternator aren't connected to anything the alternator doesn't take much force to spin (mostly bearing drag). However, once the alternator is connected, current begins to flow. This current flow causes a magnetic force inside the alternator that is opposing the rotation of the stator. This force is proportional to the current flow. Its like trying to push to North-poles of magnets together or pull a north and south poles apart.

[James York]

I'm not sure I'm buying this.  I am not a mechanic, but I do understand basic physics.  The alternator takes some of the engine power to turn the rotor.  How much power it takes depends on the friction in the bearings.  I don't see how the electrical output of the alternator would effect how much power it takes to turn the rotor.  However, I don't know what happens to the "excess electricity" if the battery is already fully charged.  I don't see how it could be used to boost engine power output, unless it is part of an electric/gas engine hybrid system?

 

If you can figure out how to make electrical power w/o using some form of other energy to convert, you will be extremely wealthy.

[Ron Alan]

There is a reason we occasionally get checked for a "working" alternator!  Though I never thought much about the draw a "bad/weak" battery can create...I think about all the little Honda generators that scream to life when they are called on to run an air compressor or microwave in the paddock! I guess an alternator trying to charge a dead battery may never get a break?

[Bench Racer]

There is good reason why folks are known to implement a WOT switch to disengage the alternator.

My sister has a Buick Lacrosse which has a electric motor/generator which works backasswards adding 15 drive hp thru the belt system under acceleration conditions.

[manthony121]

So what I'm getting, from reading replies to my post, and other research I've done, is that the amount of energy being supplied by the alternator effects how hard it is to turn the alternator, and hence, how much power it takes from the engine?  It makes sense, but I had not heard of that before.  I had always thought the amount of power a generator produces only depends on how fast the rotor is turning, and whether the power is used for something (like charging a battery) or thrown away made no difference.

 

Thank you all (again!) for the education.

[Tom Sager]

There is a reason we occasionally get checked for a "working" alternator!  Though I never thought much about the draw a "bad/weak" battery can create...I think about all the little Honda generators that scream to life when they are called on to run an air compressor or microwave in the paddock! I guess an alternator trying to charge a dead battery may never get a break?

A strong 7 cell battery might keep that alternator free and loose. 

[Tom Hampton]

So what I'm getting, from reading replies to my post, and other research I've done, is that the amount of energy being supplied by the alternator effects how hard it is to turn the alternator, and hence, how much power it takes from the engine?  It makes sense, but I had not heard of that before.  I had always thought the amount of power a generator produces only depends on how fast the rotor is turning, and whether the power is used for something (like charging a battery) or thrown away made no difference.
 
Thank you all (again!) for the education.

In some power generation systems what you describe is more true. It is NOT true for an automotive alternator. Instead of generating a constant amount of power and "throwing away" the excess, its never generated in the first place. The stator is an electromagnet. The current through the stator controls the strength of that magnet. So, if you reduce the current through the electromagnet, you decrease the strength of the magnetic field, which in-turn reduces the amount of electro-motive-force induced into the alternator coils, and thus the required torque on the alternator pulley.

[Jim Drago]

FWIW...I have never seen anywhere near what many are posting on HP loss on a dyno from the alt.    

[Bench Racer]

FWIW...I have never seen anywhere near what many are posting on HP loss on a dyno from the alt.    

I see some minimal numbers such as 3 hp. 3hp/130hp = 2% Which leaves a question, what is dyno noise?

[Tom Hampton]

FWIW...I have never seen anywhere near what many are posting on HP loss on a dyno from the alt.

Not surprised.

 

The number above (~3 HP) would be a worst case, for a battery in an abnormal failure mode---with the alternator supplying the max rated current of 60 Amps. A battery that just ages-out normally would put no-load on the alternator in that aged-out state.  Only a damaged battery, with a short-circuited cell or two, could cause the necessary load condition---that's not just a "dead" battery. 

 

I don't know what the designed nominal operating load is for the Miata.  But, I'm sure its WAY less than 60 Amps.  So, you can scale the HP number accordingly as the real Amps diminish.   Its 0.058 HP per Amp + friction losses. 

[Jim Drago]

I see some minimal numbers such as 3 hp. 3hp/130hp = 2% Which leaves a question, what is dyno noise?

Dave

I cant really tell what you are saying here.. In SM 3 HP is anything but minimal.  Dyno noise by someone experienced in SM and tuning and good dyno practices IMO is less than 1 hp/1 ft lb. 

[ECOBRAP]

I had someone generous enough to share their data with me (jealous that they literally have a sensor on every part of the car).

 

Their battery voltage was static at about 14.5-14.6 volts, while mine was static at about 13.6 volts. Picked up a new battery, and will test a new alternator later in the week...

 

Hoping that it gives me this 3hp you are all arguing about

[Bench Racer]

Jim, my 3 hp mention was a number someone posted in this thread. I'd almost kill for 3 hp. No, 3 hp wouldn't get Clinton my vote. Thanks from a reliable Spec Miata shop, you answered my question 1hp/1ft lb.