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  #1  
Old 03-09-2009, 06:39 PM
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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?

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


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

I really don't know the answer and I'm asking.
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  #2  
Old 03-09-2009, 06:43 PM
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In Parallel you are correct.
In series you would have 12 and 50 amps.
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  #3  
Old 03-09-2009, 06:52 PM
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When using batteries in parallel, always install a fuse between them. Otherwise, a shorted cell in one battery can cause a fire.
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  #4  
Old 03-09-2009, 07:57 PM
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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.....
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  #5  
Old 03-09-2009, 08:04 PM
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keep this in mind as well, amperage and voltage are input measurements, watts are a measure of output...2 different things....
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  #6  
Old 03-09-2009, 09:17 PM
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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)
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Old 03-09-2009, 10:07 PM
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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

Last edited by tinypanzer; 03-09-2009 at 10:16 PM.
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Old 03-09-2009, 10:35 PM
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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.
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Old 03-09-2009, 11:01 PM
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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
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Old 03-09-2009, 11:23 PM
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"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.
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  #11  
Old 03-09-2009, 11:34 PM
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no, watts are the draw, not the push
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Old 03-09-2009, 11:45 PM
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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....
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Old 03-10-2009, 12:09 AM
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Quote:
Originally Posted by cliffmac View Post
Why is your electric bill measured in KiloWATT hours instead of KiloAMP or KiloVOLT hours? Explain that one to me
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.
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  #14  
Old 03-10-2009, 12:44 AM
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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?
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Old 03-10-2009, 02:59 AM
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Quote:
Originally Posted by cliffmac View Post
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?
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