Battery Math
 

This is not a brain teaser, it's a serious question involving batteries.

I have a diesel MB, an old one. Originally, they were powered by two standard 6 v separate batteries that were connected in SERIES to make a 12V battery.

This got me thinking.

If I had a 6V battery producing 50 amps = 300 watts and another 6 V battery producing 100 amps = 600 watts

If I put them in PARALLEL, they would be a 6V system producing 150 amps, right? :confused:

If I put them in SERIES, would they be a 12V battery producing 75 amps? :confused:


After all, 900 watts is 900 watts is 900 watts, right? :confused:

I really don't know the answer and I'm asking.

In Parallel you are correct.
In series you would have 12 and 50 amps.

When using batteries in parallel, always install a fuse between them. Otherwise, a shorted cell in one battery can cause a fire.

I think it's 600 watts...the 2 six volt batteries are wired in series, not parallel to provide 12 volts, the only reason for that was space (MGB's had the same goofy thing). So, depending on the output of your altenator/generator(regulator) and battery capacity you should, on a rated 50 amp 12 volt battery equivelant would be 12X50 which would be, of course 600 watts.....

keep this in mind as well, amperage and voltage are input measurements, watts are a measure of output...2 different things....

Batteries are rated in Ampere/hour - i.e. the maximum amount of current available for a given time.

Are you confusing 50AH and 100AH with pure current load?

In general use we never put mismatched batteries i.e. with different AH rating in parallel.

Batteries conform with the rules of physics. i.e. the bigger the battery - (the larger plates) will give a larger capacity (measured in Ampere hours)

Exactly what Ivan said.

Part of your math is theoretically correct, however connecting two dissimilar batteries is series would have a few bad consequences. The first is that the lower current battery would not be able to pass the full current of the higher current battery. In fact, no battery will be able to pass more than its current rating. If you take two 100A 6V batteries and connect them in series, you get 12V@100A. Current stays the same, voltage doubles. In parallel, current doubles and voltage stays the same.

The formula you used is the one used for resistors......

If you connected two dissimilar batteries in series, the smaller one would cook. Charging them would be an equally dangerous proposition.

Cliffmac sez: "keep this in mind as well, amperage and voltage are input measurements, watts are a measure of output...2 different things...."

Not quite.... Watts are a measure of power (the symbol for watts is P), which requires you to know two things in order to calculate. Knowing any two of the following variables allows you to calculate Watts. They are: Voltage, Current, Resistance of circuit. Also, Watts are equally applicable to input and output. It's just a power measurement and can be applied to any current.



-tp

What tp said.

In series, you will not completely charger the larger capacity battery, but will overcharge the smaller cap. battery.

Discharging at a high rate (cranking) will possibly create over-discharging of the smaller battery.

Bad idea. Series batteries should be matched exactly, I always choose quality batteries from the same production run.

exactly...wattage is the resultant consumption of amps and voltage...watts are an indicator of consumption, not output....believe me, I worked for CommonWealth Edison for years. Why is your electric bill measured in KiloWATT hours instead of KiloAMP or KiloVOLT hours? Explain that one to me

"Why is your electric bill measured in KiloWATT hours instead of KiloAMP or KiloVOLT hours? Explain that one to me"

I can't tell if you're being sarcastic or not, so here I go.....

Since your household service typically comes in on two phases, and gives you a choice of 120V or 220V, measuring just amps consumed would not be enough information. 10 amps consumed for one hour @ 220V would be 2.2 Kliowatt hours. Consuming 10 amps at 120V for an hour would only use 1.2 KWh.

If your entire service consisted of one voltage, than amp hours would be enough information since voltage is known and constant. But, that just means you're measuring Watts since voltage and current are known in this scenario.

Measuring kilovolt hours wouldn't tell you anything becuase your voltage is always present. You'd need to know how many amps are being consumed. Once you know that, you're now measuring Watts again....

Watts can be a measurement of consumption, and usually is used for that. However, you could describe the voltage and current traveling across a wire as Watts, and not be mistaken. Even though it is the device at the other end of the wire that is consuming the power, the power is still traveling across a wire on its way to consumption. So, Watts is a perfectly usable term to describe the amount of power being either consumed or transmitted.....

As such, watts can be directly converted to horsepower. If you use time as a factor as in KWh, you can directly convert to Joules.

So, KWh is a superfluous term and could be replaced with Joules..... Joules=Watts X Seconds........ So, the number would be much bigger but just as accurate.

no, watts are the draw, not the push

what the heck are you talking about....amperage and voltage are constant in a circuit...it's the usage of said circuit that constitutes wattage consumed, they draw off of the circuit. That's why there are these things on power lines/poles called transformers, they step down voltage and amperage to a more usable form so that you don't blow your light bulbs out. power plants do not put out wattage, they produce voltage and amperage, not watts...the watts thing is the way the electric companies measure your consumption in order to bill you....

Because KWH is a measure of ENERGY. KW is just a measure of power. Power x time = energy. Volts or amps alone doesn't tell you what you need to know if you are producing and selling the stuff.

But this has nothing to do with the OP's question. It is unclear to me if the OP is just curious or really wants to do this.

Short answer: don't do it, as has been said. When assembling batteries into arrays of any kind, keep them as precisely matched as possible.

well, that doesn't make any sense either...when a power plant can produce 17K killowatts that means 17,000,000 watts of capacity. The power plant doesn't actually store that much, it just means that the draw could be 17 million watts from the grid....just as if your altenator can produce more power than your car needs, what does the altenator do with this excess/unused power? Watts are the demand from an electrical
source, amps and voltage are the source. Houses are 120 or 240 volt source, that's the power coming in. Fuses are of various amperage designations, simply to stop overload. Now, watts are what actually USE the electricity...get it? Ever hear of a 30 watt fuse? No, 30 amp fuse is something you hear about. Ever hear of a 60 amp light bulb? No, you hear of a 60 watt light bulb, get it?

What specifically is it that doesn't make sense?

So, you thinking of converting your 170 to 12v?

Okay, this is all screwed up.....

Let's pick this apart bit by bit and then I am going to leave it alone...

"what the heck are you talking about....amperage and voltage are constant in a circuit.."

That statement can be true, but isn't necessarily. First, amperage varies with load and so does voltage to a lesser extent. Ever crank up a stereo so loud that the bass notes cause lights to dim? That's an increase in the amperage demand, and it results in a voltage drop that you can visibly see.

"it's the usage of said circuit that constitutes wattage consumed, they draw off of the circuit."

No, it's not the usage necessarily, though it is often used to describe such. Watts is merely power. Go look it up, the symbol for power in electrical engineering is P and Watts is the unit of measurement for P. All you have to know to calculate watts is two of the three variables I outlined earlier.

"That's why there are these things on power lines/poles called transformers, they step down voltage and amperage to a more usable form so that you don't blow your light bulbs out."

That statement is just flat out incorrect. The function of a power transformer is to simultaneously step DOWN voltage while stepping UP current. Notice how the wires going from the transformer to your house are several times thicker than the wires supplying it? That's because the current on those wires is very high while the voltage is relatively low. The transformer trades the high voltage for current so that you can have 200A service in your house at a relatively safe 120V. For example: A transformer with a winding ratio of 100:1 would give you 10V@100A out if you supplied it with 1000V@1A. That's how you can get so much power (watts) out of such skinny wires on a phone pole. Wires need to be fat to carry current, but voltage doesn't care as much. So you can put tens of thousands of volts (high tension lines can go up to a half-million or more volts) down a skinny wire and still convert that into current that would burn up a wire 30 times thicker.....


"they step down voltage and amperage to a more usable form so that you don't blow your light bulbs out."

Again, voltage is stepped down, current is stepped up. The lights won't blow because at a fixed voltage, it's the resistance of the light bulb's filament that determines how much current the circuit will draw. I could put the light bulb on a 1000A circuit, it's still only going to draw 1 amp if it's a 120 watt bulb.

"power plants do not put out wattage, they produce voltage and amperage, not watts...the watts thing is the way the electric companies measure your consumption in order to bill you..."

Again, this is just wrong. Watts = Volts X Amps. So, if they are producing voltage and amperage as you just stated, they are also producing watts, aren't they? And no, watts is not for billing purposes, that's KWh or kilowatt-hours. A kilowatt hour is different from a kilowatt because it adds the factor of time. As I said before, KWh is directly convertible into Joules.

"Now, watts are what actually USE the electricity...get it? Ever hear of a 30 watt fuse? No, 30 amp fuse is something you hear about. Ever hear of a 60 amp light bulb? No, you hear of a 60 watt light bulb, get it?"

Sorry, but it's you that doesn't get it..... Watts are not a measure of electrical use, they are a measure of power. Go read the literature if you refuse to believe me. The formula for watts is P=I*E in electrical engineering abbreviations or W=A*V in standard notation.


Go look it up!!

For a DC circuit, P = E*I. But not for an AC circuit unless it has a power factor of unity.

Watts = volts * amps * PF, where PF is the cosine of the phase angle between the voltage and amperage. If there is no phase difference, PF is unity (1).

A (fictional) purely capacitive or inductive load will show a PF of zero. No matter what the current flow or voltage, there is no power consumption.

I'm fully understanding the past-tense now.

Yes, that is correct, Matt L. I think you can understand why I decided not to complicate my explanation with that tidbit.

For all intents and purposes, what I said is perfectly adequate since we were using a resistive load in our examples. There is no reactive power, and therefore P=I*E

Strelnik, where are you???

His eyes lit up like 40 watt bulbs due to overload current after reading these technical posts, the heat generated from his eyes has temporarily fried part of his brain and at the moment he's still confused - he'll be back. ...... Now that's power factor for you. :)

You have the patience of a saint to explain that. Very well done. I have 30 years in the power generation field generating power with conventional rotating machinery. Your explanation is correct.

OK fine, then explain to me why both my Mercedes and BMW owners manuals refer to the Alternator as 14 V/80 A and 14 V/80 A respectively and the Starter Motor as 12 V/1.7 kW and 12 V/1.1 kW, again, respectively? Alternator produce power, starters draw power, why only kW rating on starters only? What, does the starter produce electricity, that would be great if they did....

German electric motors use metric horsepower [ PS] measures for mechanical power capacity/output.
Power is PS , and 1 PS =735.5 watts.
US electric motors are stated in HP...like a "3/4 HP motor...."
And 1 HP = 746 watts.
So 1 HP is close to a Metric PS.

So , your 1.1 kw starter is approx 1.5 PS [ ISO Horsepower]
You alternator has ZERO mechanical output. But it has 80 AMPS electrical output capacity.
One is a Mechanical power measure and the other is an Electrical measue ..and completely different...
Many German starters will be stamped right on the case " 1.4 SP ..or 1.8 SP, etc "