Here's some tech info from the yellow top battery folks. Float charge minimum is 13.2 volts. A lot of places use that as a rule of thumb (2.2 volts per cell) to bring battery to full charge and keep it there. In my D1, it hits 13.28 when everything I can turn on is activated. With no load on battery, fully charged, it should be between 12.6 and 12.85. If you only supply 12.9 volts to recharge it, may take a very long time, and if you make a lot of short trips, you never get the battery recharged.

Alternators are three-phase AC electric generators, to be more efficient due to the mechanical rotation. That AC is rectified by the diode array into DC, and the regulator keeps that fairly steady even though RPM varies a great deal. When regualtor fails open, or brushes that contact the rotating coils wear away, output is zero. If regulator fails "closed", the output can hit 17 volts or more (is the stereo smokin' or what). If a diode fails open, the output volts stay up, until amp draw is increased. Then the maximum is reached at 33% or 66% of the regular output of the alternator. So if it is a rainy summer evening, lights on, AC on, wipers on, radio on, and alternator not so strong, you will cruise along, but not enough amps to keep battery charged. Battery may be supplying part of the load. Battery won't crank truck after extended period of this.

While I don't have a video, I did post this back a few months ago, with pix and readings of voltage. See for yourself. https://landroverforums.com/forum/ge...ter-pix-44317/

OPTIMA Commercial YellowTop Charging Information

YellowTop Type: D31A & D31T
These batteries are dual purpose. They are designed for engine start and deep-cycle applications for use in vehicles with large accessory loads.
Recommended charging information:

Alternator:

• 13.65 to 15.0 volts, no amperage limit.

Battery Charger:

• 13.8 to 15.0 volts, 10 amps maximum, 6-12 hours approximate.

Cyclic Applications:

• 14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C). When current falls below 1 amp, finish with 3 amp constant current for 1 hour.

Rapid Recharge:

• Maximum voltage 15.6 volts (regulated), no current limit as long as battery temperature remains below 125°F (51.7°C). Charge until current drops below 1 amp.

Float Charge:

• 13.2 to 13.8 volts, 1 amp maximum current, time indefinite (at lower voltage).

All limits must be strictly adhered to.

Safety Information:

• Always wear safety glasses when working with batteries.
• Always use a voltage regulated battery charger with limits set to the above ratings. Overcharging can cause the safety valves to open and battery gasses to escape, resulting in premature failure. These gasses are flammable! You cannot replace water in sealed batteries that have been overcharged. Any battery that becomes very hot or makes a hissing sound while recharging should be disconnected immediately.
• Failure to fully charge a battery can result in poor performance and a reduction in capacity.

 

Now going by those pictures posted, everything is as I stated it should be (and what you said it should be also). A car that's off should have a voltage of about 12.5-12.8, a car that's idling with nothing on should be at around 13.5 volts. When the car is idling and everything is on, it should be over 13 volts. If you're over 13 volts with everything on, at idle, the alternator is okay. If you're dropping down to 12.5, there's a problem. I can't say anything less than 13 volts means there's a problem because, in addition to the possibility of the beginnings of alternator failure, it is also possible the person has something else hooked up to the car that the engineers did not plan on when deciding what alternator to use, like adding an additional power amplifier and subwoofer, or even upgrading the factory speakers. With added electrical devices aside, and all else being equal, it should be over 13 volts.

Did your truck have an alternator problem (the one used to take the pictures in)? Because I didn't see any of those numbers sitting at 14+ volts.

You would have to replace the alternator with a more powerful one in order to pump out more than 14 volts with everything on. I would really like to see a Discovery with everything on and still able to crank out 14+ volts using all stock components. I could be wrong, but I have to see a video, or picture, or something.

 

My radio and the M+S lights let me know that my alternator was beginning to crap out on me. The radio would momentarily cut off when the ABS system kicked in or hi-beams turned on. The M+S lights would kick on when I ran the AC fan above the second-lowest setting... only revving the engine for 15-20 seconds before shifting into gear would keep the lights from coming back.
I dealt with this crap for 6 months before finally replacing my alt.

 

re: If you're over 13 volts, you're good, if you're below 12.5 volts, there may be a problem with the alternator.

I believe this to be the difference of opinion point. I believe that if you are below 13,2 then the electrical system is not supplying enough, because of non-factory loads, etc. I certainly don't believe that 12.75 at idle will provide satisfactory operation and fully recharge the battery.

The 13.8 - 14.4 range is a general spot. On mine, it does 13.98 at idle, bare minimum, rebuilt alternator from big box store. Gets up to a hair over 14 at freeway speed, also in that post, I just don't like to take photos while driving. Mine does 13.28 with everything turned on. So, mine is above 13.2 worse case, and within 13-8 - 14.4 best case. So I believe mine to be "nominal." Wifey's Kia Sedona does 14.4 at idle. Eats low beam head light bulbs.

Could you get a bigger alternator to provide more amps - sure, I've got a 250 amp stock in an E350 so I could run a large inverter. Ambulances can have a 350 amp alternator. But in a D1, it is usually a 100 amp, and in a D2 130 amp.

 

That comment was regarding the car at idle with everything on full blast, not simply at idle. I do admit the 12.5 is very low. I wouldn't say that 12.5 automatically means an alternator problem. I look for extras like power amps, video systems, and anything else that may put an extra load that was not calculated when the car was built first. If there isn't anything added to the electrical, then 12.5 means there is something wrong, even with everything running. Even at 12.5, you're not killing the battery yet (but just barely). I had someone with that condition and I told him not to drive around with the radio blasting until he had the alternator replaced. He didn't have an additional amp, but it was a loud system.

I'm in agreement with everything except that an alternator should be able to put out 14+volts with everything at full blast, while at idle. Just remember, we are talking about at idle, with everything on and at full blast, not simply at idle.

 

I too agree that at idle, 14 volts and above is possible, but only with minimal amps, just what it takes to run the engine. I'll be posting in a few minutes on another thread more about this.