Originally Posted by Alan

Yes to most all of this.

A highly discharged battery will consume a lot more current at the initial ~13.8-14v alternator output - possibly enough to cause a voltage drop until the charge current drops. The charge current will drop from its peak quite quickly but will still consume significant current for quite a long time to replace the lost charge - this can stress an alternator. A well charged battery will take much less current and will taper off more quickly - but it doesn't ever go to zero. The voltage from the alternator drooping with temp is deliberately intended to provide a time constant for the peak charging period and also compensates for high ambient temps (batteries don't like very high charge rates when they are hot). These are fixed and dumb mechanisms - the time constants are ~the same for a fully charged battery and a fully depleted battery - because the alternator can't tell the difference & it always does ~the same thing. Actually a depleted battery with higher initial current draw will heat up the alternator faster so will tend to get less time before the voltage droops... so it's worse than that. The net result is that fully depleted batteries take a long time to fully recharge (much better to charge them off the car) - yet they may still be overstressing the alternator as they charge, and a healthy fully charged battery will get overcharged every time you drive the car. Fortunately lead acid batteries are very forgiving and alternators are usually also fairly robust.

Now when the System Voltage drops below ~12.6v-12.7v we are exploring the interesting domain you get to when the alternator and battery are both contributing to System Current (and therefore System Voltage). I'd like this to never happen in my vehicle - but today it does under some challenging conditions (both high summer loads and high summer heats combined).

At this voltage the battery stops charging and is neutral - it neither consumes nor supplies current. Drop below this point and the alternator and battery are now both supplying System Current... This voltage drop could be driven by reducing alternator RPM lowering its power output, or by additional loads being switched on. As the System Voltage drops below this point the battery current will go up, but it becomes a balancing act between the battery & alternator and load. For a fixed load configuration the total System Current load will go down as the System Voltage goes down and the battery will support relatively a higher % of the System Current as the System Voltage goes down. It is tempting to think that the System Voltage has become the Battery Voltage again - but it hasn't. If the alternator fully turned off the battery current would go up - as it has to supply the missing alternator current and the voltage would drop further as a result.

It's an interesting hybrid case - but it's not really a good place to be, certainly not for long.

Modern alternators have smarter regulators that monitor battery current and will cycle the alternator voltage to ~float levels when the battery charge current drops far enough, they may also do a better job of compensating for these recharge requirements.

Very smart regulators (like boat battery regulators) can sense battery voltage & current and temperature remotely, often the alternator case temperature too. This allows accurate transitions from the initial Bulk Charge phase (max allowed constant charge current) to the Absorption Phase (constant high voltage) and then to the Float Stage (low voltage charge - current ~<1% max charge rate). The transitions are controlled by voltage/current sensing for Bulk-> Absorption and usually by an absorption timer for Absorption->Float. The voltage in each mode is different - bulk is variable for ~constant current (@~Ah/4), absorption is high (~14.4v) and float is low (~13v)

Our alternators look like complete dinosaurs compared to this (they do Bulk at huge huge rates - ignore absorption altogether and then float way too high) - But they most still work OK for most usage models. But not really for mine... Many of you out there have smart battery chargers/maintainers that do all these steps for the battery off the vehicle. Ideally you'd have exactly the same recharging & maintenance steps on the vehicle while running too, best way to keep your battery happy & healthy for longer. I think it would really help in Phoenix - the constant overcharging really heats up batteries here... excess heat hurts eventually.

Alan

Originally Posted by Carl Fausett

I do not interpret this data the same way.

The alternator produced 14v shortly after starting the car to replenish the heavy draw that had just occurred by the starter motor. OK.

On the highway - the alternator is not producing 14v (and shouldn't be) because the battery is now charged full again and is not calling for it. This is the voltage regulator doing its job. You would not want 14v all the time, you will boil the water right out of that battery. 13.6 to 13.8 is the ceiling (depending on where you read), and then, only for a short time. Neither the battery or the alternator can survive max output for extended periods of time. Fortunately, that also is not required.

Originally Posted by mark kibort

Also good points... thanks.... im thinking one of the reasons the batteries don't last as long as they should , is the systems are always overcharging the batteries. as you said, they are pretty resilient, but in the end, I think this contributes to killing them off.....

Originally Posted by mark kibort

Also good points... thanks.... im thinking one of the reasons the batteries don't last as long as they should , is the systems are always overcharging the batteries. as you said, they are pretty resilient, but in the end, I think this contributes to killing them off.....

Originally Posted by dr bob

Much more likely is that the resistance in the wiring and connections increases over time, to the point where most cars aren't able to keep the battery fully charged. For fun, next time you visit your local parts store or even WalMart, grab one of those cheap (~$1) testers for the specific gravity in the battery. The one with the floating ***** is fine -- easy to read, cheap, and good for this exercise. After you get home and the battery has had a chance to settle for a while, go ahead and test the electrolyte in the cells. With luck, you won't have many ***** snking to the bottom. Realistically, you'll see that the specific gravity is low. That's either from undercharging, or from a battery that's sulfated to the point where the acid is no longer at "full strength".

Then, next time you replace the battery, take a reading when you buy it, and track the numbers as the battery ages.

I went through a few batteries that just never seemed to be that good. Initially I blamed the batteries, but later figured out that they just weren't getting fully charged by the alternator. Resistance in the ground straps and connections let the alternator voltage look good to the regulator, but was slightly low at the battery terminals. Charge the battery with a 'smart' charger, and the specific gravity was good.

Reminder to folks at home that MK uses a small 'racing' battery, so his results may be slightly different from ours.

Originally Posted by Carl Fausett

I do not interpret this data the same way.

The alternator produced 14v shortly after starting the car to replenish the heavy draw that had just occurred by the starter motor. OK.

On the highway - the alternator is not producing 14v (and shouldn't be) because the battery is now charged full again and is not calling for it. This is the voltage regulator doing its job. You would not want 14v all the time, you will boil the water right out of that battery. 13.6 to 13.8 is the ceiling (depending on where you read), and then, only for a short time. Neither the battery or the alternator can survive max output for extended periods of time. Fortunately, that also is not required.

Originally Posted by j.kenzie@sbcglobal.net

Would it be better to cool the alternator or the battery?
Dave

Originally Posted by mark kibort

I think if you put a meter on a charged battery , after a long drive, its still being charged at 13.8 volts.. ive never seen this change on any car ive ever owned.

Originally Posted by mark kibort

the thing that wears out the batteries i have, is the slow discharge that i forget to monitor when the kill switch isnt activated. from 0 volts, always brings back a weaker battery. 2 or 3 of these cycles will kill most automotive batteries.

Originally Posted by dr bob

As interesting as it is to find less temperature correction to build charging voltage while hot, the Phoenix situation is a case where you might want to go the other way, ie: less charging voltage while hot, if battery life is truly the #1 issue.

Originally Posted by dr bob

Carl and Alan both have 17AH 'aux' battery options. Alan uses it instead of his AAA card to get when the primary battery suddenly abandons him. His is mounted in the front right fenderwell under the headlight and next to the horn brackets. Maybe the use for that is to be the primary, with the rear one isolated (by relay/contactor) unless needed, so they could both stay relatively cool.

Originally Posted by Carl Fausett

I like Dr Bob's answers above even more than my own. He's got an excellent grip on what's really going on. As they use to say in British Parliament: "Hear him! Hear him!"

Originally Posted by Carl Fausett

PS: I say again, dump the wet lead-acid battery and put in an Optima Red 24R. It really does make a difference.

Originally Posted by mark kibort

Ill take that as pointed to my comment to you regarding the voltage charge level, for a fully charged vs half charged battery.. Ill say it again, there should be no difference and all batteries are being over charged by 1 volt , when on a normal charger. this doesnt hurt them much, but still hurts them if its done for extended time. this is why car batteries last only a few years and then start to die. the charging voltage is always near 13.8 volts, charged or not. even the trickle chargers , wont be moving much current to the battery, but the voltage is near 13.8 as well..... over time, it takes its toll, just as it does in the car.. add some heat and there you go

Originally Posted by dr bob

The alternator and voltage regulator have no clue about your plans for a particular drive. Rumbling over to the corner store, 4 hours of cross-country cruising, either way it works to quickly restore sufficient charge to start the car again. Charge current quickly drops as battery charge is restored and terminal voltage rises. With your mini-battery, you should be particularly aware of this. The voltage regulator setting is a compromise that allows for the fairly quick restoration of the charge lost from starting, is current-sensitive only where it comes to the capability of the alternator to maintain the target regulator voltage (including the derate for temperature).

What many seem to be concerned about is a battery that doesn't have enough capacity to maintain 13.xx volts when the alternator falls off that capacity curve. So there's a conundrum -- charge the battery hard enough (higher voltage equals higher charging current) to have the desired capacity to support any/all driving loads at 13.xx Volts, while quickly restoring the charge lost to cranking, .and. charge the battery slowly/gently enough that excessive heat and plate-warping are avoided no matter what, with no loss of electrlyte to boiling/outgassing, and don't ever exceed 12.6-13.xx Volts in the process. Hint -- those cases don't have anything in common.

There's alwsy the option to try and regulate current going to the battery, but it has to be done without any high-speed switching technology, since the regulator can't deal with that. You need to isolate the battery from loads while charging, or measure and add the load current to the target charge current so the battery only gets what you want it to get. In effect, you'd adjust regulator voltage based on the net current you want to go to the battery. Of course, since the alternator capability may not include that much current at idle, you'd be back to looking at the depletion when idling, and calculate how much current is needed to restore the battery's charge --during your current drive cycle--. So yes, we can estimate the charge efficiency curve of the battery at any 'normal' temperature, put in a deterioration allowance as the battery ages/cycles, add a smart charge computer to manage the voltage from the alternator needed to do everything. All you'd need to do is manually input the total time you will drive before you need to crank the starter again.

Or buy a new battery every three to five years, ignore the low hot-idle charge voltage so long as actual battery terminal voltge stays above 12.small, and be a lot less excitable on those days. The battery is intended to support the car loads when the engine is at hot idle. If the load demand pulls the terminal voltage down to barely 12 volts, it's really OK. The battery really is doing its job.

MK has a small battery and no AC, so the cooling fan and HVAC blower duty is a lot less. He seldom drives the 928 for casual use in the hot weather anyway with no AC, so fewer cases of battery discharge for him unless he forgets to open the disconnect switch. He can get by with the mini-battery. I probably can't, since I run the AC on warm days, drive with at least the parking lights on at all times, do drive in slow traffic sometimes, etc. And I expect the car to start quickly after I get my change at the drive-up window.

Maybe we need a simple Zener diode and resistor circuit at the voltmeter, and snip..... the Holy Grail of charging regulators.