Need a Battery expert!

[tirwin]

hmmm... I'm in the same boat as the OP. I read this thread and I think some of my brain fell out.

I recently posted a question looking for advice as to whether I should upgrade my alternator. I just bought a new Valeo 75A. The 90A are about 4X the price for just 15A improvement. Is it worth 4X the price?

In my '83 I have an Alpine PDX5 amp (4x75W + 1x300W RMS), A/C, 55/60W headlights with relays, an MSD Streetfire CDI with Blaster 2 coil and the Optima yellow top.

So can someone summarize when should you really upgrade the alternator and when should you use the yellow vs red tops?

[Gene Smith]

When the average load of your electrical items exceeds the average capacity of your alternator you're using your battery's charge to supplement.

Police vehicles, ambulances and boats use special alternators, smaller alternator belt pulleys and and high tech voltage regulators that provide current at low RPM. Sitting at idle your 75A alternator is not putting out close to 75 amps. It's output is closer to 4-8 amps at idle. If your cruise RPM is 2500 your alternator "should" put out 40- 60 amps. To get the "rated output" of 75 amps your alternator would need to have maybe 3000or more rpm engine speed. Remember alternators have belt pulleys that make them run at higher RPM than engine crankshaft. It "might" be possible to reduce the pulley size to get better performance at low RPMs. What that would do to the cooling fan load is anyone's guess...Heat reduces the efficiency of alternators too. Luckily the 911's alternator is bathed in cool air from the fan.

The only real way to figure if your alternator is sufficient is to measure the voltage at the battery with your electrical items in use.

Measure your battery's voltage with engine off and everything off.
Then measure voltage with engine running.
Then add each electrical load while watching the voltage.

When you dip below the voltage of your battery's voltage with everything off while engine is running because of stereo and lights, wipers and AC cranked up you'll know you need more capacity from your alternator.

If you blip the throttle up from idle to say 2000 RPM and voltage returns to over 12.6 you have enough alternator at cruise but require more battery capacity to carry the electrical load at idle.

Beware- any deep discharge of regular lead/acid batteries drastically shortens their life.

That's the difference between RV/Boat Deep Cycle batteries and normal "car" batteries. OPTIMA batteries are supposed to tolerate deep discharges better than "car batteries".

Best practice is never to discharge any battery below 50% charge. That's about 12.1 volts on a battery at rest.

[tirwin]

Gene, great explanation!

[rick-l]

Originally Posted by mysocal911

Not really! Basic electronics defines currents for parallel loads, i.e. different size batteries require different charging currents which adds additional loads to an alternator besides all the others system elements, e.g. FP & lights.

No matter how big it is a lead acid cell’s voltage is about 1.95 volts discharged and 2.1 volts charged. That is simple chemistry. If you force more current through it when it is fully charged (elevate the voltage) it gives off the extra energy as heat and converting water to hydrogen and oxygen. In the SAE test (last post) when the 13.5 volts is specified they figure the battery is fully charged and not being overcharged and acts as a lossy diode to regulate the voltage similar to how it is used in a car. I = V / R tells you how much current is going through the test resistor.

Originally Posted by mysocal911

Remember, an alternator is a voltage source device, i.e. basically no current limiting, and NOT a current source.

An alternator is NOT a voltage source. According to LENZ'S LAW The Induced current is such as to OPPOSE the CHANGE in applied field. The faster you turn the alternator the more the flux in the windings change and the more current flow you induce. When you restrict the current flow (outside the alternator) a voltage is developed. Back to the V=I*R thing.

Originally Posted by mysocal911

Also, the state of charge of a battery affects it's sinking current when charging, i.e. very low battery can sink in many cases 70-80 amps when charging. That will damage some alternators.

That 70-80 amps is limited by the alternator similarly to how the rating was obtained in SAE J56 test standard. Also when the battery is discharged you want the alternator to run at its rated current to charge it as quickly as possible.

The bottom line is a bigger lead acid battery is not going to stress the alternator any more than a small capacity battery, it will just be required to operate longer at its rated current which it is designed to handle.

[mysocal911]

"The only real way to figure if your alternator is sufficient is to measure the voltage at the battery with your electrical items in use."

Actually, the best way is by using a clamp-on ammeter measuring the current
from just the alternator lead (not the battery's) with the worst case loads.

"An alternator is NOT a voltage source."

A voltage source is a device with a very low output impedance versus a current source
which has a very high impedance output with a defined output current. A voltage
source is a device which attempts to maintain an output voltage no matter what the
load is. Try increasing the alternator's output load current and you'll easily find that
the current exceeds its rating easily, i.e. its current is NOT limited by its rating.

"That 70-80 amps is limited by the alternator similarly to how the rating was obtained in SAE J56 test standard. Also when the battery is discharged you want the alternator to run at its rated current to charge it as quickly as possible."

Don't think so.

"In the SAE test (last post) when the 13.5 volts is specified they figure the battery is fully charged"

That's NOT always the case for an automotive battery, i.e. the battery is fully charged
when the alternator is in the charge mode.

Buy a clamp-on amp meter, discharge your battery over-night by leaving the lights
'on', and the next day start the engine and revv it to 3K RPMs while you monitor the
charging current. Hold the RPMs for 5-10 minutes.

Oh, order from Pelican Parts another alternator before the test so you'll have a replacement
after yours becomes marginal or fails.

[rick-l]

So every alternator on every car that has been jump started has burst into flames and failed.... WOW

[Gene Smith]

"Actually, the best way is by using a clamp-on ammeter measuring the current
from just the alternator lead (not the battery's) with the worst case loads". So Cal?

If that were the case modern vehicles would still have ammeters on the dashboard or computer display like vehicles from the 40's and 50's. They do not. They have voltmeters.
A simple voltmeter will show all needed to figure this out.

"Buy a clamp-on amp meter, discharge your battery over-night by leaving the lights
'on', and the next day start the engine and revv it to 3K RPMs while you monitor the
charging current. Hold the RPMs for 5-10 minutes."

I'm not sure how to engage an electric starter with a discharged battery.
I AM sure discharging a lead acid starting battery by leaving lights on all night will reduce the battery's lifespan.
A car's starting battery is not designed for a complete discharge and the cells plates can warp from this abuse resulting in a destroyed battery.

"revv it to 3K RPMs while you monitor the
charging current. Hold the RPMs for 5-10 minutes"

Holding an engine at 3000 RPM without a load isn't something I'd want anyone to do to my 911.
Especially a cold engine just jump-started by someone who just purposely discharged the battery...

For ten minutes?

Jackpot!

[mysocal911]

"If that were the case modern vehicles would still have ammeters on the dashboard or computer display like vehicles from the 40's and 50's. They do not. They have voltmeters."

Totally non-relevant! And a voltmeter is used to determine if the charging system
is functioning and NOT whether the alternator has enough current capability
to SAFELY power a non-stock electrical system. That requires an amp meter.

"I'm not sure how to engage an electric starter with a discharged battery.
I AM sure discharging a lead acid starting battery by leaving lights on all night will reduce the battery's lifespan.
A car's starting battery is not designed for a complete discharge and the cells plates can warp from this abuse resulting in a destroyed battery.
revv it to 3K RPMs while you monitor the
charging current. Hold the RPMs for 5-10 minutes"

Holding an engine at 3000 RPM without a load isn't something I'd want anyone to do to my 911.
Especially a cold engine just jump-started by someone who just purposely discharged the battery..."

The point, which was obviously missed, is that under those conditions (that can
occur on occasion) the alternator can exceed it's rated output and destroy itself.
And having a large battery, e.g. 80-100Ahr/1000CCA, that can provide high output
currents can also draw high charging currents exceeding the rated alternator's output.

[mysocal911]

Originally Posted by rick-l

So every alternator on every car that has been jump started has burst into flames and failed.... WOW

Under the right conditions, the alternator can be damaged when one revvs the engine
attempting to quickly charge a previously dead battery. That's why it's best to let
the engine idle or remove the battery and use an external charger.

Many 911SCs with Paris-Rhone (Valeo) alternators with internal regulators have a high
failure rate because they're marginally designed. The 911 3.2 alternators (90amp)
have failed because of excessive loading (bad batteries or too much current load).
The Bosch alternators used on the 964/993 are much better designed and thus more
reliable.

[rick-l]

Originally Posted by mysocal911

"An alternator is NOT a voltage source."

A voltage source is a device with a very low output impedance versus a current source
which has a very high impedance output with a defined output current. A voltage
source is a device which attempts to maintain an output voltage no matter what the
load is. Try increasing the alternator's output load current and you'll easily find that
the current exceeds its rating easily, i.e. its current is NOT limited by its rating.

Definitions:

• A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
  
• A voltage source is a two terminal device which can maintain a fixed voltage.

Which one sounds more like an alternator? I would say the first one. The alternator will put out maximum current based on RPM, number of windings on the field, current through the field etc. whether the battery voltage is 12.1 or 13.

Originally Posted by mysocal911

That's NOT always the case for an automotive battery, i.e. the battery is fully charged when the alternator is in the charge mode..

The charging feed back loop is closed by the regulator based on the voltage at the battery terminals. The regulator delivers full current (maximum magnetic field) to the field windings until the battery is very close to 13.5 volts (fully charged).

[rick-l]

Originally Posted by mysocal911

The point, which was obviously missed, is that under those conditions (that can occur on occasion) the alternator can exceed it's rated output and destroy itself. And having a large battery, e.g. 80-100Ahr/1000CCA, that can provide high output currents can also draw high charging currents exceeding the rated alternator's output.

The point you missed is that the battery alone is not going to make the alternator exceed its rated output. The voltage on the lead acid cell is going to determine the current sourced.

If the battery is shorted or equivalently someone has hooked up a huge load to the electrical system that is another mater entirely.

[Strygaldwir]

I believe what causes the battery to go "Dead" is a process called sulfation. i.e. the coating of the plates falls off and no chemical reaction generating electricity can occur. This process will occur over time and is sped up by discharging the battery either too fast or too deeply.

Some batteries are constructed to be more tolerant of deep discharge, they call these Deep cycle batteries. (like the Optima Yellow top). You can pull more current from one, more times, before it loses efficiency and/or dies. (They typically have thicker plates, but that discussion will just distract us). Bottom line is with a yellow top, you can leave your lights on more often without killing the battery.

The disadvantage of a yellow top is it is optimized to give up smaller currents over a longer time, vs a LOT of current very quickly. This would be an issue if you were trying to start your car by continually turning over the starter. A deep cycle battery would not be as good at this "abuse" as a regular "starter"/"automotive" battery. With the larger, deep cycle batteries this is not as much of an issue because they may have LOTS of current they can deliver.

The other issue deep cycle batteries have is something called "charge acceptance rates". This is just how much juice you can put back in them how quickly. Deep cycle batteries "tend" to have lower charge acceptance rates than automotive batteries. So, an "automotive" battery is fairly optimized for the way we used to use cars. We start them (Very high, quick discharge), then we drive them a short distance and turn them off. We would want to put what we took out, back in very quickly (high charge acceptance rate). Once again, the auto battery is optimized for this.

Back to the Optima. it is constructed differently than a normal automotive batter. It uses something called Dry Cell technology. (the plates don't sit in liquid acid, the acid is contained in a mat that is wrapped around the plates, a type of Absorbed Glass Mat). The bottom line is the battery can be charge quicker and discharged more frequently without wearing it out. This is principally why the Optima's have longer lives than most "automotive" batteries. They can be discharged more deeply more frequently than normal batteries, and they can tolerate sitting without being recharged for longer periods.

On the alternators, the rating of the alternators basically state the maximum amount of current they can deliver. "Typically" higher is better. Having said that, higher is bound by how much you can use, need. A battery has a "charge acceptance rate" You try to exceed this and the battery breaks (sulfation of the plates). To counter this problem, alternator have something called regulators. The regulators limit how much charge (current) the alternator will put out. This means even if you have a 100 amp alternator, it may only be putting out 25 amps because that's all the battery can accept. Now if we have things like very high output stereo systems and high current lights we have more demand for the current and the regulator can allow more output from the alternator.

Thus if your regulator is working correctly, it will keep the alternator from burning up your battery. Higher output on the alternator may be useful, if you have something that needs it. If you just turn your starter over, and your car starts immediately, you may not need a "high" amp battery or alternator.

The spiral wrap AGM type batteries (Optima) eliminate many of the disadvantages of the flooded cell lead acid batteries. Thy overcome several of the disadvantages the deep cell batteries had verse the starter batteries. There only disadvantage may be their cost (maybe, given the life span of the battery) and the fact they don't stand up very well to broken regulators. Essentially, they break down faster when abused by over charging or consistent undercharging. The AGM can sit in your car longer without breaking (low internal discharge rate). They can be charged faster (higher charge acceptance rate) and they don't discharge acid that eats your car! I'd only use AGM batteries for automotive and marine needs. Not sure about solar, but that's another discussion.

[mreid]

Originally Posted by rick-l

Stress is a physical force or pressure. Stress over time is something different that relates to reliability.

According to SAE J56 to obtain the rated current of the alternator you run it at rated speed (nR = 6000 RPM) connected to a battery in parallel with a resistive load and vary the resistance to get the specified test voltage (Vt 13.5 V ± 0.1 V). You also have to run it for 5 minutes after it has reached thermal equilibrium in this state. After it has stabilized the current into the load is the rated output current IR.

Wouldn’t this imply that the size of the lead acid battery in parallel with the load would have nothing to do with the magnitude of the current the alternator outputs if 13.5 is the charging Voltage?

Basically, no.

In the test, you vary the load (resistance) to achieve a consistent test voltage. In effect, you are compensating for differences in available current due to different alternator capacities through a variable load (resistance) to achieve a fixed voltage. That's in the lab.

In the real world, the load placed on an alternator varies significantly. The battery is a critical part of this load. The battery in parallel with the load of your critical and optional electrical components forms a "tank" circuit that both draws and controls current from the alternator (have you ever wondered why you shouldn't use your battery cutoff switch while the engine is running? It's about the current limiting function of the battery).

Just like you shouldn't use your alternator to charge a discharged battery, you shouldn't use an underpowered alternator to manage the charge on a high capacity battery.

This is also why you should use a maximum 10amp battery charger to test the wiring in your newly built baby rather than simply hooking up the battery and hoping for the best. I've seen newly completed projects significantly damaged by fire created by an amperage surge from a fresh battery through a miswired project, where a 10 amp charger would have produced a whisp of smoke at worst.

[tirwin]

Originally Posted by mreid

This is also why you should use a maximum 10amp battery charger to test the wiring in your newly built baby rather than simply hooking up the battery and hoping for the best. I've seen newly completed projects significantly damaged by fire created by an amperage surge from a fresh battery through a miswired project, where a 10 amp charger would have produced a whisp of smoke at worst.

I have never heard that suggestion before. So hook up the charger to the battery terminals (no battery) and do what? See if something shorts? If you were going to do that, wouldn't it be a good idea to start with all the fuses pulled and add them one at at time? That way if you had a problem you'd at least know what circuit it was on.

[mreid]

I wasn't attempting to provide the detailed approach, just a better source of restricted power. However, your approach would work. Assuming Porsche fused every circuit (we know that's not true), you could simply hook up the battery and hope the fuse blew first. Every car I've built uses the 10 amp charger approach hooked up to the battery cables without the battery installed.

[rick-l]

Originally Posted by mreid

In the test, you vary the load (resistance) to achieve a consistent test voltage. In effect, you are compensating for differences in available current due to different alternator capacities through a variable load (resistance) to achieve a fixed voltage. That's in the lab. .

No In the test you vary the load (resistance) to determine how much current the device will put out at the voltage of a typical lead acid cell as it would be used in a road vehicle for charging and auxiliary loads. SAE J56 and ISO 8854 Alternators with regulators Test methods and general requirements are active test standards and not just lab notes.

Originally Posted by mreid

In the real world, the load placed on an alternator varies significantly.

Can’t disagree there.

Originally Posted by mreid

The battery in parallel with the load of your critical and optional electrical components forms a "tank" circuit that both draws and controls current from the alternator.

You’ll have to elaborate there as a tank circuit is typically defined as L-C circuit that can serve as a resonator.

The chemical cell in the battery determines the system voltage.
The regulator controls the current out of the alternator. Typically full on until the battery is fully charged.

Originally Posted by mreid

(have you ever wondered why you shouldn't use your battery cutoff switch while the engine is running? It's about the current limiting function of the battery). .

No it is about the voltage on the system. Since the alternator is a current source the voltage is determined by the load. That chemical cell battery does a nice job of that. Without the battery the alternator will let the voltage get very high before the regulator (and stored energy in the field coil) can react.

Originally Posted by mreid

Just like you shouldn't use your alternator to charge a discharged battery, you shouldn't use an underpowered alternator to manage the charge on a high capacity battery..

I haven’t seen any compelling reason not to do either.

[tirwin]

Originally Posted by mreid

I wasn't attempting to provide the detailed approach, just a better source of restricted power. However, your approach would work. Assuming Porsche fused every circuit (we know that's not true), you could simply hook up the battery and hope the fuse blew first. Every car I've built uses the 10 amp charger approach hooked up to the battery cables without the battery installed.

Couldn't you add an inline fuse (10A? Or less?) between the charger and the battery terminal? If you had a short on an unfused circuit that would hopefully prevent damage to something. The only circuit that I know of that is normally unfused is the instruments. Here is a great thread that discusses a simple mod to fuse that circuit.

Fire under dash!

[mreid]

Semiconductors are heat sensitive and life is impacted by heat cycles and intensity. The diode matrix in your alternator is the problem.

As for the tank, without getting too complex, your battery acts as a capacitor (low frequency, charge/discharge against the flow of current as the alternator turns on and off) in parallel with load (stator windings of motors and resistance of other components) to form a balanced circuit that must not exceed the capacity/capabilities of the alternator.

In summary, I'm just trying to offer insight to help people make an informed decision. You are free to do as you please.

[mysocal911]

"Since the alternator is a current source the voltage is determined by the load."

rick-i

Again, that's not correct. The regulator on alternator is called a "voltage regulator"
and NOT a current regulator because it monitors and regulates the alternator's
output voltage. Just test an alternator's regulator and you'll find that it's just
an electronic switch that switches the field voltage 'on' or 'off' based on whether
the output voltage is less than or greater than a preset voltage in the regulator.
The voltage regulator does NOT monitor the alternator's output current and
just attempts to maintain a fixed output voltage.

"Just like you shouldn't use your alternator to charge a discharged battery, you shouldn't use an underpowered alternator to manage the charge on a high capacity battery.."

mreid

That's the totally correct and stated as simple as one can!

[rick-l]

Originally Posted by mysocal911

Again, that's not correct. The regulator on alternator is called a "voltage regulator"
and NOT a current regulator because it monitors and regulates the alternator's
output voltage. Just test an alternator's regulator and you'll find that it's just
an electronic switch that switches the field voltage 'on' or 'off' based on whether
the output voltage is less than or greater than a preset voltage in the regulator.
The voltage regulator does NOT monitor the alternator's output current and
just attempts to maintain a fixed output voltage.

It controls the voltage by controlling the current out of the alternator.

Voltage less than set point (13.6 typical) put current through the field winding and turn it hard on - that is maximum current it will produce at a given RPM. Old school regulators typical have very high gain in the control loop around typically a zenner diode setpoint..

EDIT: more explicitly it controls SYSTEM voltage by controlling alternator current.