| Author | Message | ||
| John S Brown (Brownyjs)
New member Username: Brownyjs Post Number: 4 Registered: 2-2002 |
James and Verell...knocked me over when you mentioned Gordon Bell. I met him on a number of occasions. I joined DEC in 1969 in Perth Western Australia. Worked on the PDP6 (serial#4) installation and debugging for the Uni of Western Australia. Olsen used to say it was his toughest sale. Stayed with them for 20 years in Australia. Spent a lot of my time in Mass. as a result..great company great times Nice to be reminded about Gordon. | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 341 Registered: 6-2002 |
Verell has corrected HenryK's summary perfectly. The alternator supplies current for the various loads and for charging the battery. The only time that battery supplies current to the various loads is when the alternator is unable to - as when idling with lights, windshield wipers, radiator and ventilation fans, etc. At this moment one appreciates the real function of the battery - it is a storage device to be used for short periods when the alternator output is insufficient. Jim S. | ||
| Steve (Steve)
Member Username: Steve Post Number: 298 Registered: 2-2001 |
Randall if you can adapt another alt. then go ahead and do it. This is not a big deal all you need to do is make sure you wire it up right. I have done this many times with Jags. | ||
| Randall Booth (Randall)
Junior Member Username: Randall Post Number: 66 Registered: 1-2003 |
I hate to repeat my original question, but what about an aftermarket alternator? I have added the brighter lights and larger stereo and feel that a higher output alternator will do less work and hopefully last longer. BTW some cars can run without the battery. Once they are started the alternator can power everything, but it varies from vehicle to vehicle. | ||
| Mike Procopio (Pupz308)
Junior Member Username: Pupz308 Post Number: 68 Registered: 10-2002 |
James and Verell, Excellent posts with some additional depth that I hand't considered. Thanks! | ||
| Henryk (Henryk)
Member Username: Henryk Post Number: 450 Registered: 8-2001 |
Thanks guys; I now understand this much better than I did, the same time yesterday. | ||
| Verell Boaen (Verell)
Member Username: Verell Post Number: 544 Registered: 5-2001 |
HenryK, Not quite. When running & the battery has reached full charge again, the voltage regulator sets the alternator's output voltage so that it's a tiny bit higher than the battery's internal voltage. Just enough higher so that the battery's always being trickle charged. Meanwhile, the alternator is also supplying the current to the car's load. Under these conditions, you could put an ammeter between the alternator connection & the battery & you would only measure the trickle current. The voltage regulator is only 'cut out' while the dash light is on. The rest of the time it's varying the current being supplied to the rotating field coil. (It's moot whether you say the regulator varies the field coil's voltage or current as they're directly related by the field coil's resistance.) | ||
| JRV (Jrvall)
Member Username: Jrvall Post Number: 842 Registered: 11-2002 |
James S, good explanation of system function. | ||
| Henryk (Henryk)
Member Username: Henryk Post Number: 448 Registered: 8-2001 |
Let me get this straight. ALL the amps neaded to supply the car's accessories, etc., comes from the battery. This draw will eventually bring down the voltage of the battery, which kicks in the voltage regulator, and starts the alternator to produce amps to charge the battery. When the battery is charged to a specified voltage, the regulator kicks off, and charging by the alternator is stopped. This then goes on and on, until the car is stopped. This makes a lot of sense to me. I recall that one can drive, for a while, with the alternator belt broke. But this would only be to the time where the draw on the battery brings down the voltage, to a level that cannot support the requirements of the ignition system, thus the car can't run anymore, and it stops......hopefully you made it home!!!!!! Practically speaking, therefore, the alternator only supplies the battery, and NOT the accessories. Can one jump start and run a car on the alternator alone, with a dead battery?.......hooked up, of course, so there is ground connection. | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 340 Registered: 6-2002 |
Mike - at the risk of confusing some of the readers some of the time, perhaps I can clarify a few things that you mention. The output of the alternator is indeed related to RPM, but it is also related to field voltage. I believe you have not considered this voltage in your explanation. The field voltage is regulated by, you guessed it, the voltage regulator. The steady state scenario is zero charge to the battery - load (current requirements) is supplied by the alternator output. The battery is charged, and no (or miniscule) current supplies the battery charge. One does not want "excess current" (as I believe you mentioned) going to the battery, as it will, indeed, lead to over-charging and the consequent battery failure. Quoting your thread - "My point in this, is that if you go up to a 120A alternator, that alternator is ADDING a fixed CURRENT into the system at a given RPM. It does NOT just produce "just enough" current to meet the draw. The energy from the engine has to go somewhere--hopefully not heat. If you only accept a fixed current, the voltage MUST get higher, since power (V*I) is fixed. But you can't let your voltage get higher--bad things will happen to the battery. (Hence the VR.) " In fact, the alternator does not provide a fixed current. It supplies a current dependent on the field voltage (as defined by the voltage regulator). Here is how it works. The voltage regulator senses system voltage. As the system voltage goes down (lights, fans, radio, etc.) the voltage regulator increases the field voltage (what we engineers refer to as a feedback loop). This causes the alternator to generate more current, which supplies the necessary load and charges the battery. As the battery voltage rises as a result of the charging current, the voltage regulator senses the rise in system voltage, and decreases the field voltage, which leads to lower alternator current output and less battery charge. This is a closed-loop feedback control system. The error signal is the difference between the system voltage and 13.5 volts (typically). When the system voltage falls below this value, the alternator is encouraged to produce more current (field voltage goes up), which charges the battery and raises the battery/system voltage. As the battery/system voltage rises above the 13.5 value, the regulator says, "Hey, too much, let me lower the field voltage." Thus, the voltage regulator is CONSTANTLY monitoring the system/battery voltage, and adjusting the alternator field voltage. In steady state (driving down the highway with whatever electrical load you care to imagine), the alternator is generating just enough current to supply these loads and maybe a few hundred milliamps more to charge the battery. Adding extra capacity to the alternator (125-amp versus standard 80 amp, for example) will not help much at all IF the total possible load does not exceed 80-amps. The consequence of 75-amp load (lights, fans, etc.) with an 85-amp alternator is that it will take longer to charge the battery after starting, or after idling at a traffic light, but it will charge the battery. The difference between an 80-amp and 125-amp alternator is the number of windings that make up each coil, and the gauge of the wire. (And diodes). The heat generated by an 80-amp alternator at maximum output is likely similar to a 125-amp alternator at 80-amp output. Long winded, but I hope it helps. Jim S. | ||
| Verell Boaen (Verell)
Member Username: Verell Post Number: 542 Registered: 5-2001 |
Mike, Your overall tutorial is pretty much conceptually right up to the point where it assumes that an alternator constantly puts out it's maximum power rating. The voltage regulator does actually control the alternator's total output power, gradually reducing it as necessary to prevent overcharging the battery. ALLTERNATOR THEORY 101: An alternator generates power by the drive pulley rotating an electromagnet (called the rotor) whose magnetic field is coupled into the stationary coils (called the stator) that actually supply the alternating output power. The diodes then convert this output power into unidirectional direct current. The amount of output power actually generated is controlled by varying the amount of direct current flowing thru the rotating electromagnet. When the magnet's current is reduced, the magnet's not as strong & less energy is coupled into the stator coil, thus reducing total output power. The voltage regulator senses the battery's terminal voltage & adjusts the rotor's current accordingly. While the battery's terminal voltage is below the regulator's set voltage, the regulator supplies full current to the rotor. This in turn causes the stator coil to produce full output current. As the battery's terminal voltage reaches the regulator's set voltage, the regulator begins to back off on the rotor current just enough to ensure that the battery's terminal voltage never goes above the regulator's set voltage. While the battery is charging, it's terminal voltage will be below the regulator's set point, & things behave just as Mike describes. The alternator's full output current will be going into the various loads of the vehicle, & whatever's left over will be charging the battery. However, as the battery becomes charged, it's terminal voltage rises until there is little or no current going into the battery & the voltage regulator has reduced the alternator's output to just enough current to power the vehicle loads. BTW, in the BOSCH alternators, the voltage regulator doesn't begin to put current into the rotor coils until the alternator is spinning at a preset rpm. When the regulator senses this, it stops powering the alternator light on the dash & puts current into the rotor. It will continue to power the rotor even when the rpms drop below the revs needed to turn on rotor power. However, if the rpms do drop sufficiently low, the regulator will again shut off rotor power to prevent damage to the alternator coils. Sorry if I butted in..., just thought this might help. | ||
| Mike Procopio (Pupz308)
Junior Member Username: Pupz308 Post Number: 67 Registered: 10-2002 |
Henry, No problem! I'm enjoying this discussion. You've got the basic idea correct with current draw. Your thinking is correct given the assumption that you're running off a battery and there is no SOURCE of current in the system. For example--running the radio with the car off and ignition key in the "Accessory" position. A given electical component will DRAW (take) a fixed current at a fixed voltage (uses a fixed amount of power). If you wire up this electical component to a standard AA battery, you are correct: the battery will SUPPLY, from its internal energy store, whatever current asked of it (assuming it can) at its specified (1.5v) voltage. So you see where your thinking is correct here--current provided by the battery is dependent on the draw. So, we have a neutral system (no current flow). Then we attach a power-draining component (1A, 12V). 1A of current is provided until the battery gets drained, and dies. Enter the alternator. It's just the opposite of of a power-drawing device; it's a power-SUPPLYING device (it gets this energy from your precious internal combustion V8!). No matter what, it's feeding current into the stream. What you refer to as current draw is more accurately called "Net Current." Net_Current = (Current_Input - Current_Draw). So, you're right on: current IS dependent on the draw from electrical components. Assuming there is NO current INPUT, the battery will supply the current requested of it at its rated voltage (now we're talking car batteries, 12V)--for a finite time. But, by that same reasoning, net current is also dependent on the current INPUT. With the alternator supplying (+) more than the draw (-), we end up with a surplus in the system. The battery needs that surplus current (+) to charge it, since starting the car (before the alternator kicks in) drains it somewhat. You're right--I believe that batteries must have an acceptable range of current that they can handle to be charged. However, given that they are rated to put out 600A to 700A in some cases, I believe they can handle that on the input side of things, too--and of course, your alternator could never put out that much! --Mike | ||
| Henryk (Henryk)
Member Username: Henryk Post Number: 446 Registered: 8-2001 |
Thanks for the explanation. This is a learning curve for me. I can see where I am wrong in thinking the voltage regulator kicks in and out. If the 120A alternator is pushing out a constant 120A, with the excess going to charge the battery, it would seem that this would result in an over-charge, thus a bad battery in short order. I always thought that current was dependent on a draw for the amount it needed, and no more. When a fuse blows, due to a short, the current draw becomes much greater, due to the greater demand (the short), thus the fuse blows. | ||
| Mike Procopio (Pupz308)
Junior Member Username: Pupz308 Post Number: 66 Registered: 10-2002 |
Henry, Current is "directional," and can have a positive (charging) or negative (draining/drawing) flow. The idea is for your alternator to be able to supply current for all accessories, with a fair amount of room to spare. That is, we're looking to ensure that the alternator "keeps up" with the average load. Oh yeah, you'll need some current to charge the battery--can't forget about that! I'm trying to really get my arms around this, and haven't measured (YET!) myself, but I suspect that stock 308 alternators, at say 55A, don't leave a lot of extra room for charging. For example, throw in *any* sort of Amplfier/stereo system, and some (brighter) aftermarket headlamps (high wattage = high power = higher current consumption at fixed voltage, as regulated by the VR), and you're drawing more current than you think. An engine is spinning the alternator constantly, generating power. The voltage needs to remain fixed (your electical components expect it), ensured by the VR. The alternator produces fixed POWER (based on its draw from the engine) at a GIVEN RPM; the voltage remains the same according to the Voltage Regulator. Therefore, the alternator is putting out constant power, constant voltage, and constant current--hopefully--at a given RPM. My point in this, is that if you go up to a 120A alternator, that alternator is ADDING a fixed CURRENT into the system at a given RPM. It does NOT just produce "just enough" current to meet the draw. The energy from the engine has to go somewhere--hopefully not heat. If you only accept a fixed current, the voltage MUST get higher, since power (V*I) is fixed. But you can't let your voltage get higher--bad things will happen to the battery. (Hence the VR.) Now, if your CURRENT DRAW stays the same, say 40A, with the alternator adding 120A, there's a surplus of 80A current in the loop. That current charges the battery. Electrical components are designed to work at a FIXED (basically) voltage, within a certain current range. Too much of either will be bad. That's why you count on your VR to work, and you count on your fuses to blow if too much current enters in to the scene. Upgrading your alternator's current output ensures you can power all of your accessories (oh yeah, and ignition coils, too!) and maintain a charge on the battery. So my question is, Henry, why would the "Voltage Regulator" kick in and out frequently? My understanding of its operation is that it is an "instantaneous" adjustment in the electrical (not mechanical) domain. | ||
| Henryk (Henryk)
Member Username: Henryk Post Number: 445 Registered: 8-2001 |
Randall: Isn't current determined by the draw in a system? That is, if you go to a 120A alternator, and your accessories only draw 40A total, then the new alternator will only pump out 40A, (with the regulator kicking in and out very frequently, thus shortening it's life?....not sure). Why waste the money in something that you don't need. | ||
| Randall Booth (Randall)
Junior Member Username: Randall Post Number: 64 Registered: 1-2003 |
What about just replacing the alternator with an aftermarket one? Maybe something with a lot more output. | ||
| Verell Boaen (Verell)
Member Username: Verell Post Number: 532 Registered: 5-2001 |
re: Alt rebuild The rotor had to be replaced, & an exact replacement came from BOSCH germany. Rebuild cost was: $85 basic rebuild (bearings, tear-down, cleanup, & reass'y). $200 rotor, $45 regulator w/brushes. (There's a bit more detail in the archives.) There were no changes to the diodes or stator wiring. The shop said that some alternators just do test out better than spec. There was no special work done, or special parts used. 90 A is what the shop's machine measured when all was done. (Actually there were 4 meas'ts made at increasing RPM, with 90A being the highest.) re: When @ DEC? I hired on in early Jan 1970 in the mill as a new grad EE. Worked in HW develpment until early '80s. (Gordon Bell was there for most of the time, worked with him directly a few times. Was in his HW organization most of the time.) Then switched to direct precursor to my present job. Have survived both aquesitions as the VMScluster Communications Architect & software team leader. | ||
| Mike Procopio (Pupz308)
Junior Member Username: Pupz308 Post Number: 64 Registered: 10-2002 |
Approx $130 to rebuilt alternator and upgrade to 85A, including a new voltage regulator. It seems to be pretty much universally held that the bosch alternators in 308s are specified for a maximum of 85A. I wouldn't go for 90A--it's asking more than the alternator was designed for, and it may not be possible at all. | ||
| Greg Rodgers (Joechristmas)
Member Username: Joechristmas Post Number: 663 Registered: 3-2001 |
How much did they charge to rebuild your Alt.? I got a quote locally and it was $150. Is that about right? Is it safe to go up to 90A for a 308? | ||
| John_Miles (John_miles)
New member Username: John_miles Post Number: 32 Registered: 7-2001 |
Instead of interrupting your battery circuit, I'd recommend measuring the resistance of the positive battery cable from the battery to the solenoid, measure the voltage drop under starting (and other) conditions, and use I=E/R to calculate the current. Should be good to at least 2 significant digits, I'd think. Or more if you have an HP34401A with Kelvin connections in your garage.  | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 338 Registered: 6-2002 |
Verell - when were you at DEC? Maynard? Gordon Bell was my first professor of electrical engineering (visiting professor at Carnegie Tech). Were you there during his day - early 60's. Jim S. | ||
| Verell Boaen (Verell)
Member Username: Verell Post Number: 529 Registered: 5-2001 |
Hmm, I used to have a 0.01 Ohm 100A shunt from my EE days at DEC when I designed a power supply for the PDP-11/55. Maybe I can find it. It's beefy enough so I could measure the starting current for a couple of seconds! Of course the shunt drop won't be negligable either. BTW, I have the receipt from my alternator rebuild: 90A @2000 alternator rpm. | ||
| Mike Procopio (Pupz308)
Junior Member Username: Pupz308 Post Number: 62 Registered: 10-2002 |
Let's evaluate the health our charging systems. Are you up to the challenge? I was thinking that it would be very helpful and interesting to do a quick survey and document some voltage/current readings on our 308s. This information would be particularly helpful in getting a feel for how our electrical and charging systems our doing, and especially in establishing a reference to compare against should something seem not quite right. This shouldn't take too long if you were interested, and only requires a Voltmeter/Ammeter (say, Multimeter). These can be had for $20 to $40 at Radio Shack. If you have one that can measure up to 100A current, that would be helpful, but most common multimeters that I'm familiar with max out around, say, 10A. If this is you, DO NOT perform any current readings on your battery--you'll blow the fuse or fry your multimeter). Also, a helper (son/daughter, wife/husband, buddy) will be very helpful. All readings done at the battery should be done at the actual battery TERMINALS themselves, and not the clamps on the terminals. ANOTHER WARNING: Don't measure the current draw at the battery when STARTING your car, regardless of your multimeter. Your starter will pull large amounts of current (not easy to turn that engine!), on the order of 200A to 400A (that's a lot of power). A Radio Shack multimeter will not be able to handle this--you'll blow a fuse (if you're lucky), or... fry your multimeter. If you only get Voltage measurements, that's fine as well--Ohm's law will give us the relative change in current assuming a fixed or negligible resistance. Take these measurements on a 308 that's actively used (e.g. hasn't been sitting for too long), ideally the day after a good run, and after the car has sat overnight. OK, onward. Here's what would be useful to know: 1. 308 type and year. 2. Battery type and age. 3. Alternator--original, rebuilt, or new; if rebuilt or new, how long since the change. 4. Rated current output of your alternator (stock I believe for early 308s is 50A to 55A; rebuilds often involve upgrades to 85A, the listed maximum for our alternators). Now, the measurements. Feel free provide any and all that you can--if you don't get a particular measurement, don't worry. 5. BASE BATTERY VOLTAGE--CAR OFF Car engine off, ignition off--voltage measured at battery. Expect approximately 12.5V, +/- 0.3. 6. BASE CURRENT DRAW--CAR OFF Car engine off, ignition off--current draw measured at battery. This should be very low. Your clock would be drawing current, and perhaps any Automotive Voltmeter gauge you have hooked up. Maybe a dome light. Figure less-to-much-less than 50mA. 7. VOLTAGE AND CURRENT DRAW AT IGNITION "RUN"--NO ACCESSORIES Car engine off, ignition in RUN (not accessory) position. Measure Voltage and current draw at battery, all accessories off (What is the impact of the electric fuel pump, and possibly other stuff?). 8. VOLTAGE AND CURRENT DRAW AT IGNITION "RUN"--ALL ACCESSORIES Same as above, but turn on EVERY accessory. High beams, dome light, radio, air conditioning, fan/blower, rear defrost, etc. Measure current draw and voltage at battery. The current draw could be considerable (make sure your multimeter can handle it). 9. VOLTAGE AT BATTERY DURING START--NO ACCESSORIES Turn all the accessories back off. Start the car. Measure voltage at battery terminals when starting car--record the lowest number you see. (How much does starting discharge our batteries? Figure voltage drops to 80% of value measured in #5 during starting for a healthy battery.) 10. VOLTAGE AT BATTERY WITH ALTERNATOR CHARGING--NO ACCESSORIES Car idling right after start, engine RPMs blipped up to ensure alternator is charging the battery, with the "generator" light off. Measure the voltage at battery. We want to see the alternator charging the battery. A good range would be 13.8V +/- 0.5V. Too much over, say, 14.5V is overcharging the battery for most automotive batteries that I'm familiar with (e.g. Optima Red Top). My alternator's voltage regulator went bad, cooking the battery with a toasty 17V. Didn't last too long (both the battery and the voltage regulator). In general, we want to see: 12.5V base charge in battery; Approx dip to 10V when starting; jump to 13.5V when alternator has kicked in. This is a basic pattern; would be interesting to see how closely ours conforms. Should be close. 11. CURRENT PRESENT AT BATTERY--CAR RUNNING--NO ACCESSORIES In the same scenario as #10, car running with all accessories off, measure the current draw at the battery. This could be 40A to 80A (Multimeter warning!). (What is the net current at the battery, that is, power available to your car under normal circumstances? Normal Power in Amps = (Alternator current output) - (Net current draw from engine ignition and other required accessories). 12. VOLTAGE AT BATTERY--CAR RUNNING--ALL ACCESSORIES Turn on all accessories, as in #8. Bring the engine RPMs up to 2000 (picture a nice, easy cruise in 5th through the neighborhood). Measure the voltage at the battery. We're looking for at least 12.5V. (Can our alternator support, that is, continue to charge, our battery while driving with all of the accessories on?) 13. CURRENT PRESENT AT BATTERY--CAR RUNNING--ALL ACCESSORIES Same as #12, measure the current at the BATTERY with all accessories on. (Careful with your multimeter; expect somewhat less than your alternator's rated output current by at least 15A.) 13. CURRENT PRESENT AT ALTERNATOR--CAR RUNNING--NO ACCESSORIES (Optional and a pain--only if you already know or really want to know.) Car engine running--measure current output at ALTERNATOR, i.e. what is the BASE current output of the alternator before drawn from by all accessories? Should be close to your alternator spec (55A or 85A), minus, say 5A? The difference in this and the current values in #11 and #13 will give you an idea of the current draw of your system ranging from no accessories to all accessories. You'd like a little to spare, you know, to charge the battery and all! Well, that's it. I didn't intend on this becoming a dissertation, but that's where it ended up. If you feel up to it, whip out that spare tire, remove that battery cover, and do some exploring... This'll be a good service to the 308 community (and maybe uncover some alternator gremlins while we�re at it). �Where�s my specs,� you�re saying? I�ll be providing them ALL as soon as I pick up my car from the alternator rebuild (this week!). All the best, --Mike 1978 308 GTS, US, Carbed, #23785 Albuquerque, NM | ||
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