Alternator Capability versus Loads

[Maxs Dad]

Rick - Hit the nail right on the head with the capacitor/surge protector analogy. An interesting item that you brought up - Disconnecting the Positive lead from a battery first. I always thought that you disconnected the Negative lead first so that if the screwdriver/wrench/zipper-or all of the above-came into contact with any of the metal parts we call a truck it wouldn't immediately go into arc welding mode. PS - Ever watch one of these batteries self distruct under a dead short across the terminals with a conductor big enough to handle all of the amps? It is truly awesome -especially with all of that acid boiling out through the split in the case.

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Jim E. - Newbie to Diesels w/2001-2500Quad-4X4-LB-Bronze Beauty

 

[illflem]

Jim,removing the negative cable first is the way I learned it should be done for the same reasons you state. Several places in the Dodge service manual also say to remove the ground cable before performing different types of work. I don't see how removing the positive cable first could make any difference to the computer. MY 2¢

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95' 25004X4,AT,Driftwood,Banks Stinger,Warn fender flairs and running boards(work truck)
95 3500,5 speed 3:54,Driftwood,Banks&Psycotty,34,000 GCVW apple and tractor hauler(works harder truck)

 

[fest3er]

Originally posted by illflem:
Seems I recall one of the options available when I bought my trucks was a larger alternator for higher power needs like wenches etc.

Huh. I thought wenches required low power, but high maintenance. #ad


As stated in a few other posts, the batteries really only smooth out the rough voltage in the system. During operation, the alternator supplies all necessary current *until* the current draw exceeds the alternator's capacity. At this point, the voltage starts dropping and the batteries start supplying current in addition to the current the alternator is supplying.

In theory, you can run the vehicle on either the battery or the alternator. In practice, the voltage control systems can be so poor that they can allow the voltage to climb too high, thus damaging voltage-sensitive equipment.

One time, many years ago, I was testing a CPU chip that wanted current injection (constant current, but varying voltage). So I set up the big HP power supply to the specified current and proceeded to test the CPUs. Well, on one of them, I neglected to flip the little switch that powered down the CPU before removing the chip from the ZIF socket.
The power supply, seeing the current falling, faithfully increased the voltage to maintain the current, all the way up to 600VDC. By that time, the chip was out of the socket. And fried. And I learned something. #ad


A similar thing likely happens when cars blow out electrical and electronic thingys. The voltage regulation system isn't fast enough to drop the voltage fast enough. Thus, if you happen to disconnect the battery from a running vehicle when the alternator is heavily charging the battery, you could well see the alternator output voltage jump very high. Why?

When the alternator is matching the battery with no load, it is, in effect, set to produce about 14VDC. When a load is put on the system, the system voltage drops, and the regulator tells the alternator to increase its voltage to compensate. When the load is removed, the voltage jumps up, and the regulator tells the alternator to decrease its output voltage. During the periods of slight voltage sag, the battery kicks in, smoothing out the sag. During the short periods of over-voltage, the battery acts as a load, again smoothing out the 'spike'. But remove the battery from the circuit and have a poor, slow, regulator, you could see the voltage jump all over the place as loads are added and removed. *Especially* as a 50A or 100A load is added or removed. The moral? If you must remove the battery from a running vehicle, disable the voltage regulator first, or first remove all heavy loads from the electrical system.

As someone stated elsewhere, if you have computer equipment, the best thing you can do for it is to install a battery-backed UPS. One battery can absorb huge voltage spikes, just as it fills in the long voltage sags and drop-outs.

Fest3er

 

[Steve St.Laurent]

Fest3er, a little known thing about UPS's is that in order to get the protection you are talking about you need an online style UPS, not a switched one. The majority of UPS's out there are switched, when they see a voltage loss (or drop) they then switch from the line current over to the battery (switch time is usually measured in milliseconds). On an online UPS the computer is always drawing it's power from the battery and the UPS is constantly charging the battery, just like our alternator systems. Online UPS's are 2x-3x more expensive than switched ones.

-Steve