lt2duffer

Anyone have any clue what would cause my truck to discharge the battery when either the A/C or Lights or both are on ? I have changed the altenator and checked the altenator output. It is approx 13.9/14.1 v when not connected to the battery. The fuses all seem to be good. I have removed all aux items from the wiring to isolate. I also checked the battery cables for ground. Any Ideas ?

Midnight Tech

You may want to pull the alternator and have it tested off the truck...13.8 is actually normal battery voltage.
Also, disconnecting that unit either at the alternator or at the battery while it's running creates a surge that is very destructive on anything electronic...it could be cooked something.....

qldit

Good Morning lt2duffer, nominally a lead acid battery is threoretically 2.2 volts per cell, but generally a fully charged battery will only show slightly over 12 volts in static state.

So a static theoretical voltage on a properly charged 12 volt battery is the amount of cells times the voltage or 6 X 2.2 = 13.2 volts, but as a general rule this is not seen.
Usually it will show in the order of 12.5 volts or something like that.

To charge a lead acid battery properly, an increase over the static cell voltage of .2 volts is needed, so this is where the previous 13.2 volts is added to, by the 6 X .2 for that charge voltage, so you appreciate this will equal an extra 1.2 volts and when added to that 13.2 volts then equals 14.4 volts.

This is why you will see 14.4 volts mentioned commonly in 12 volt battery charging circuits and how it's importance applies.

Now to make it slightly more interesting, the battery elecrolyte in most vehicles is dependent on where they are used and the atmospheric conditions etc.

So in the tropics a battery acid SG in a fully charged battery will be in the order of 1220, whereas in a colder climate the SG will commonly be found as high as 1300.
(Checked using a hydrometer)

This SG electrolyte selection has bearing on the voltage regulators used in alternators and the selected voltage that they regulate at, normally you can expect to see regulators using alternator voltage outputs in the order of 13.8 to 14.4 volts.

Most modern vehicles use alternators with amperage output capacity greater than 35 AMPs. Typical current family sedans have 75 or 100 amp alternators fitted.

When the vehicle is first started the alternator output is maximum until the battery voltage increases and then virtually "float" charges at a much lower current but maintains a relatively steady voltage on it.

If extra power is needed to support accessories, that voltage should remain in the higher state and the current from the alternator should simply increase to support the extra demand.

You can expect a slight voltage drop in the cable from the alternator to the battery under heavy load conditions but you will appreciate even with all accessories running and reasonable engine revs you should not see a discharge situation such as you are mentioning.

If we consider that charge cable and it's terminations has good integrity with it's connections between the alternator and the battery terminal, and your drive belt is in good connection, then the problem you describe should not happen.

It sounds like your alternator is defective, check the voltage both on the battery terminals, and at the alternator output terminal under running and loaded conditions.
It is not uncommon for one or more of the diodes in the alternator to fail and cause a similar symptom. (no guts!)

Under loaded operating conditions you should see no voltage apparent between the engine and the chassis.

A high resistance connection in the negative battery line to the engine or chassis can also give this symptom.
I prefer to have the negative battery cable directly connected to the engine and this ensures better reliability, but a lot of vehicles use the negative lead to the chassis and rely on some pithy cable to go from the chassis to the engine.
This is usually a source of problems in those designs.

Cheers, qldit.

lt2duffer

Midnite Tech,
Thanks for responding.
The unit was tested off the vehicle, it tested within normal operational outputs. The reason the alternator was changed was because this same problem existed. I am having no problems with starting. All other systems are acting normally, the only thing I have noticed under the hood, is that when the A/C compressor is running, (serpentine belt) the belt jumps up and down between the spring tension pulley and the Alternator. Could this be a weak cell in the battery ? I Didn't seem to have a problem with the lights this afternoon with a fully charged battery.

Any help would be appreciated

lt2duffer

Thanks qldit!
The vehicle is a 1 ton truck with a large(00) cable going from the negative battery terminalto the engine block. I guess I need to find someone with a more strenuous test of the alternator than O'Reilly's uses. In the mean time, I will check for voltage in the negative cable and check some of your other suggestions as well. I have trouble believing that a NEW delco alternator has a bad diode, but I guess all things are possible with electronics.

Midnight Tech

Is that tensioner doing a constant bounce too? If it is, it could be that the tensioner is bad and not keeping proper tension on the belt to get full output.

qldit

Good Morning Chaps, certainly looking like an alternator drive problem, belt or tension.
Cheers, qldit.

Joshua Prosser

I would check the headlight grounds or the a/c pump ground to make sure that they're not grounding out and drawing off the battery.

lt2duffer

Well, A small bit of history, I have always used Gates (serpentine)gatorback belts, Now I am questioning this. The gatorback kept me from being able to trun the waterpump pulley ( to see if it was slipping). Hence I thought there was not a tensioning problem. However, as a mechanical engineer, I deal with strength of materials on a frequent basis, once the yield or tensile strength of a material is exceeded, it breaks, over time during wear, this Y&T diminishes. My belt broke a few days ago, I grabbed my spare, put it on, and I be dang if the alternator started charging. good thing for my wifes checkbook, it was on the way to the parts store to buy the idler pulley and the belt tensioner assy.

Thanks all for your input!!

qldit

Good Evening lt2duffer, thanks for the feedback, I did suspect the "multi-Vee" section was worn or damaged. With most of those belts the friction is in the "multi vee" not so much with the tension, it is quite amazing what actually happens when an area on the belt actually wears, the frictional surface-area is lost and they run erratically, I did strongly suspect that was the actual problem, an interesting problem and a good solution.
I have also struck worn drive pulleys from constantly slipping belts, but this was more often with single "V" section.

I don't like running belts excessively tight, they overload component bearings, the serpentine idea is excellent and normally only has a simple idler tensioner.

The early VW beetles had a unique way of adjusting tension, it was with an actual placement of spacers on the generator, the "V" section was actually altered in diameter, I always thought it was an incredibly clever idea, it was the smaller diameter pulley with the most load.

Well done!

Cheers, qldit.