Electrical questions

[ogarcia_02]

How long does it take an alternator to give off its rated amp output? Like a 90 amp alt. gives off that much if you run your car for an hour?

Whats the maximum charge you can take off from the alternator to recharging a secondary battery without taking away from the starting battery?

[tencentlife]

The 90A alternator is rated at 90, but in real life will only produce about 70-75 amps of current.

Current is the actual volume of electrons flowing past a given point on a conductor (a wire), measured in amperes (amps). One ampere (named after some Italian guy) is one coulomb (named after some French guy) per second. One coulomb is something like 6 x 10^14 electrons, I can't find the exact figure right now, but trust me, it's a shitload of electrons. One electron is the smallest unit of charge, so add them all up and multiply by voltage (the electron pressure that makes the current flow down the wire, analogous to water pressure, and also called "electromotive force" or "potential") and you have power (watts). Multiply power by time and you have energy (watt-hours).

So, the alternator is able to raise 13-14 volts of electron pressure, and at that voltage can force a maximum of 75 amps into a load. A discharged battery is a load, just as a light or a fan is a load: a load will convert electric power into some other form, either photons escaping an incandescing filament, electromagnetic force that makes a shaft spin, or cause the disassociation of sulphur from lead that imparts charge to a lead-acid battery. They are all the same as far as the alternator is concerned. A load is a load is a load.

I burdened you with all that crap so you can see that power from the alternator is finite. The answer to your first question is, the alternator will put out 75A of current at 14V instantaneously once the engine is running at sufficient rpms, since amps is an instantaneous measurement of current. But in one hour, the alt can produce 75 amp-hours at 14V, which is obviously power multiplied by time. That is called energy.

So, here's a ridiculously simplified hypothetical to answer your second question:

Say your starting battery is discharged to a level where it would require 75 amp-hours of energy to get back to full charge. If that were so, and you had no other electrical loads whatsoever (which isn't true, because the engine's systems also use some power, but let's ignore that for now), you would have to run the alternator for one hour to fully charge the starting battery.

But as soon as you start the engine, your auxiliary battery isolator relay closes, so the aux battery is now also being charged, and let's say it is at an identical state of discharge as the first battery. So now the 75A of current will be split equally between both batteries. Each will now get only 37.5 amps of current, and it will take two hours to get both batteries fully charged.

But it's getting dark, so you have to turn on the headlights. You have the high-wattage lights and nifty relays installed, and you're a heedless sort who never dims his lights to oncoming traffic, so you're driving down the road with your lights using 400 watts of blazing retina-scorching power. Watts is volts multiplied by amps, so if we divide 400 watts by the system voltage, we get 400/14=28.5 amps. But there's only 75 amps available no matter what, so now there's only 75 - 28.5 = 46.5 amps left to charge the batteries. Since they'll split that equally, each will be getting 23.25 amps of charge current, so the full recharge will now take over 3 hours (75/23.25 = 3.22).
Hope you filled up your coffee mug, it's gonna be a long night.

This is a grossly oversimplified example, and ignores many factors that would alter these figures somewhat, but hopefully you get the picture. Your starting battery would be cooked if you actually discharged it by 75 amp-hours. A normal quick start might use less than one amp-hour. A decent deep-cycle cabin battery might have 40 amp-hours total reserve capacity, so if it was at 50% discharge you would have to put back 20 amp-hours of charge energy. In that case, the starting battery would quickly absorb its 1 amp-hour, and then only a trickle more, while the remainder of available current not being used by other loads like the ignition, fuel pump, fuel injection, stereo, lights, etc. would flow to the aux battery as fast as it could chemically absorb the current, until it was also fully charged. The alternator's voltage regulator would taper off the charge current as the total system voltage maxed out at around 14V, so the batteries would not be overcharged.

You can also see that while the engine is running, the batteries don't power the loads; the alternator does. It is the only actual source of electrical energy in the vehicle. The batteries are merely a place where some of that energy is stored temporarily as an electrochemical potential that can be used on demand.

[CF]

[Volksaholic]

Good answer, Professor! I couldn't have said it better myself... in fact, I couldn't have said it as well!

[Captain Pike]

freckin journyman
_________________
LEARN TO SELF RESCUE
59 Panel bus, 1966 Single cab. 73' 181. 73 Westy. 91' H6 Vanagon 3.3L.
.....................All Current.......................

[rockfish]

10cent - thanks for the detailed information! I am about to install the headlight upgrade (SA grill & lights) and was beginning to worry about whether alternator could handle the load. Especially if you're driving at night, headlights blazing, stereo blasting (w/ amp & sub-woofer) and perhaps the AC running all at the same time!

Have you heard of anyone upgrading their alternator to 120a? Would that be a fool's errand?

Thx.

[riceye]

Wade cautiously into this. You may need to increase the capacity (size) of the wiring to handle any extra amperage. Meltdown could ensue!

I'm not even sure a 120A alternator is available that would fit up.
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'87 Westy Weekender - daily driver on salt-free roads

There's gonna be some changes made.

“I find that things usually go well right up until the moment they don't.” - Ahwahnee

"Quality isn't method. It's the goal toward which the method is aimed." - Socrates, later quoted by R.M. Pirsig

[rockfish]

That's what I thought. But better to ask those in the know than to assume.

Thanks.

[tencentlife]

There was something echoing around here awhile back about some fancy alternators that could be used, but all I remember is I thought they were way expensive and not a direct fit. I honestly don't know, though. I think there may be some late-model Bosch units that are used in other VW's and Euro cars, but it's not a high priority for me. You can see that if your engine starts reliably, it takes only a very brief time to top off the starting battery, so then you've got 70+ amps to play with. That seems like plenty to me. The worst combination of loads would be running with hi-beams and AC full blast, but it's normally cool enough after dark not to need much, if any, AC, so in my part of the world at least, that kind of thing works out. And even with both those things running and a stereo power amp, there's still probably a small margin of unused amperage in reserve.

If you're a heavy user of your cabin battery, and would like it to get topped off on a very quick drive to the next campsite, more power may be something you want. But your small cabin battery may not be able to absorb such high current anyway (one of the factors I deliberately left out in my little lesson above, and the subject of a whole 'nother lecture). Me, rather than hunting for a bigger alternator, I'd just throw my extra 45watt PV panel in the van, and put it out in the sun whenever I'm camping. End of problem.

Riceye is quite right that if you added extra current, you may want to increase the conductor ampacity, but happily that would only require upsizing the short wire from the Alternator B+ pole to the positive pole on the starter. The rest of the charging circuit is the heavy starting cable from there to the battery, and it is able to carry hundreds of amps. Upsizing the motor and tranny ground straps would be the only other mod needed.

[Waldemar Sikorski]

Yup, that was me dreamin' 'bout Zena.
http://www.zena.net/htdocs/alternators/alt_inf.shtml

[ogarcia_02]

tencentlife from now on I will call you sensei

[tencentlife]

Call me whatever you want, just don't call me late for dinner.

Yes, Waldemar, that was them, the Zenas, warrior princesses of the hi-amp tribe. Pretty neat, but a bit rich for my taste. But if I have to build someone a custom charging station, I'll keep those in mind.