EE Question

[Dualfuel]

Here is something I have been wondering about for a while....its kinda mathy but whatever.
Lets say the battery bank has a resting voltage of 12.4 vdc and suddenly you start charging at 14.7vdc at a 100amps. How many watts are you actually charging at? I have been thinking for years its 14.7X100 or1470 but it occurs to me that up until the charging voltage is over the resting voltage. So would the actual charging wattage be (14.7-12.4) X100 or 230 watts?

What be the deal, EE dudes?
BPJ

[TimSR2]

Actual charge volts x actual charge amps = input charge watts.  x charge efficiency% equals actual charge.   Less all other losses equals output load capacity. 

[Dualfuel]

Dear Tim,
Actual Charge volts = what? 14.7vdc? Or what ever the meter says when the alternator is spinning?
Actual Charge Amps= what? the 100amps the meter says whilst the alternator is spinning?
Charge effciency is defined as what? Is this where we subtract the 12.4 resting voltage from the 14.7?
Help me out here, I can spew formulas too, got thousands of books full of them, but I'm looking for the meaning behind the curtain.

Let me use the water analogy.... we are pumping water up hill to a sealed tank. The pump has to build up pressure enough to move the water up the hill to the tank, work is being done but it isn't filling the tank. Once the pressure has built up enough for the water to reach the tank, it starts filling. The tank resists filling because its sealed so the air above the water is being compressed, continually raising the pressure needed to put water in the tank.
There is a difference in the amount of work the pump is doing, and the actual water being put in the tank. Not sure how thats defined. Pumping loss, head pressure. Dunno, same with the actual quantity of electrons shoved into the batteries by the alternator. Clear as mud right?
DF

[mike90045]

Actual Charge volts = what? 14.7vdc? Or what ever the meter says when the alternator is spinning?
Actual Charge Amps= what? the 100amps the meter says whilst the alternator is spinning?

Thanks DF, for typing the first part.

We don't care what the battery voltage is,
(as long as we have the proper setup, and are not charging a 6V battery with 15V)
since no amps will flow, until the charger voltage is higher than the battery voltage, so the amps you read off the meter
(clamp-on meters are real handy, no mucking about with meter leads
and 'gator clips and "whoops, wrong setting, room full of magic smoke)
combined with the voltage you read off the 2nd meter, are the watts going into the battery.
  But we seldom measure battery charging by watts.

And to measure state of charge, you need a hydrometer to measure it NOW, but if you let the batteries idle
(no charge or discharge for 3 hours) you can get pretty close with a voltage table (see the battery faq)

http://jgdarden.com/batteryfaq/carfaq4.htm

4. HOW DO I TEST A BATTERY? INDEX: 4.1. Inspect 4.2. Charge 4.3. Remove Surface Charge 4.4. Measure State-of-Charge (SoC) 4.4.1. Specific Gravity vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Battery Table How Do I Use a Hydrometer? Electrolyte Freeze Points Table 4.4.2. Open Circuit Voltage vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Battery Table 4.4.3. Open Circuit Voltage vs. Temperature at Various States Of Charge (SoC) for a Wet "Maintenance Free" (Ca/Ca) or VRLA (AGM or Gel Cell) Battery Table 4.4.4. Interpreting the SoC Measurements

[Ronmar]

Dual Fuel   Are you actually measuring the current going into the batteries?  Because if you are putting 14.7@100A into the bank, 14.7 X 100 is indeed 1470 watts and that would be(minus the charge efficiency) the energy you are putting into the battery. Some of this energy is altrering the chemical composition of the electrolyte(charging the battery), and some is making heat.  The ratio of these two activities is your charge efficiency.

Resting voltage dosn't really come into the picture other than you need to exceed this voltage in order to force electrons to flow into the cells...

To simplify things battery capacity and state of charge are ususlly refered to in amp hours.  SO if you have an 800AH bank, and it is discharged to 80%(600AH remaining), you need to put 200AH back in, plus whatever is required to overcome the charge efficiency.  So if the charge efficiency is only 50%, you actually need to put 300AH back in to fully charge it...

[BruceM]

I'm pretty sure Ronmar meant 400 ah to charge 200ah at 50% efficiency.

[mobile_bob]

if resting voltage is 12.4 there will be no current flowing from the alternator to the battery until you go over that threshold,  therefore 12.4 x 0amps = 0 watts

when you start to charge at 14.7volts at 100amps you then get all the watts delivered
to the battery, and as Ronmar has stated part will go to heat and part will go to doing actual charging, but you account for it all which is 1470watts.

if that makes sense

bob g

[Dualfuel]

Thank you all...I think I got my head around it....for example if I tried to charge at 12.5 volts or a .1 volt over resting, then I won't be getting a 100 amps in there, at least not with the same charging setup.
We charge our bank in the morning for an hour through the Magnum inverter and the read out gives the voltage and amperage. Its usually 1450 to 1470 watts.
I do get it about the amp-hour rating, but seldom use those units because I find it easier to convert to horsepower with watts. I realize its shameful, and klunky, especially when I had an excellent engineering education in metric, and lived in a metric country for years but....I just always revert back to horsepower.
Come to think of it A-hrs is one of those units with the time taken out of it while horsepower is a rate. So I guess I am saying I think in rates of work done and not quantity of energy. Must be because I am always doing work and the gas cans are always empty!
anyhow, thanks and I'll keep working on it.
yours,
BPJ