# Understand voltage

Discussion in 'General Electronics Chat' started by LynneMarie, May 5, 2015.

1. ### LynneMarie Thread Starter New Member

May 5, 2015
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I am trying to understand voltage and my basic question relates to voltage of different watt batteries. If you have a 20 watt and a 30 watt battery, using an equal resistor, set at the same voltage... Wouldn't the power put out be the same? Is this even a logical question or am I completely misunderstanding voltage?

2. ### LynneMarie Thread Starter New Member

May 5, 2015
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Both batteries would be the same amp...

3. ### Ramussons Active Member

May 3, 2013
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What is a 20 Watt and a 30 Watt battery?

4. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Batteries are not normally measured in watts. They have an open circuit voltage that decays slowly with time. Batteries will deliver power to a load. How much power depends on the load, not on the battery. Batteries also have a capacity measured in Ampere-hours. Is that what you are referring to?

LynneMarie likes this.
5. ### Papabravo Expert

Feb 24, 2006
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Do you really think she has the answer to that question?

6. ### LynneMarie Thread Starter New Member

May 5, 2015
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Yes that makes sense. I am beginning to understand better now. When I stated watts... I meant the battery was capable of running watts vs volts at a given 'setting'. The amp of the battery would be the same, with a 'variable' setting on the voltage or wattage. I think my question was rather naive after reading more of currents and circuits, etc. thank!

7. ### ScottWang Moderator

Aug 23, 2012
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When you asking a question then you should provide a practical values of V/I, even better is that you have the link to the product or attach the pictures.

Remember that garbage in garbage out, if you want to get a good answer then more details infos is needed.

8. ### LynneMarie Thread Starter New Member

May 5, 2015
11
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Papabravo... I am learning..as I stated in original?, is this even a logical ?, apparently not. Yes, garbage in/garbage out. I have been fed garbage and am trying to learn the correct answer, which brought me to this forum.

9. ### MrAl Distinguished Member

Jun 17, 2014
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515
Hi there,

Batteries are rated in volts and also in ampere hours. They can also be rated in Watt Hours however, but that's a little more rare, being a more modern concept.

The old way, using volts and ampere hours, tells us about the main characteristics of a battery so that we can compare with other batteries and choose one that fits our application best.

First and foremost is the voltage. Most applications require a certain battery voltage. This is easy, 1.5v, 3v, 4v, 6v, 12v, etc.
You match it up with your application.

Second is ampere hours. This is a little more tricky, but if we estimate it becomes much easier.
If we have a 10 ampere hour battery (often abbreviated as "10 Ahr") that means that the battery can put out 10 amps for 1 hour, or 2 amps for 5 hours, or 5 amps for 2 hours, or 10 amps for 1 hour. As you can start to see the ampere hour rating is the multiplication of the current in Amperes times the run time in hours. So we have from above:
1*10=10
2*5=10
5*2=10
10*1=10

so we always get "10", which is the ampere hour capacity of this example battery.

If the battery was a 100 ampere hour battery, then we would have any combination of current and run time that equals 100 such as:
1*100=100
2*50=100
4*25=100
5*20=100
20*5=100
25*4=100
50*2=100
100*1=100

but these are just a few examples, as even 33.33*3=100 is possible.

So if you have a battery that is rated for 20 ampere hours and your circuit draws 2 amps, the circuit will run for 10 hours because 2 times 10 equals the rating of the battery, which is 20Ahr.
So here we divide, 20 divided by 2 equals 10, which is 10 hours run time.

There's a little more to it than that, but that's the basic estimation for batteries.

The watt hour calculations usually come in when you are dealing with a circuit that can work well with a more variable input voltage, so you should save that for after you learn about the voltage and ampere hour ratings.

studiot likes this.
10. ### LynneMarie Thread Starter New Member

May 5, 2015
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Thanks MrAl that explanation was helpful!

11. ### LynneMarie Thread Starter New Member

May 5, 2015
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So if I am understanding, batteries running at 3v, with 10Ahr rating, on a circuit drawing 1amp (the same current on all batteries), should run the same amount of time...10 hours. If the batteries are used on circuits running different currents at 3v they will last different amounts of time, but should still run at 3v.

12. ### studiot AAC Fanatic!

Nov 9, 2007
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Yes, Lynne that's absolutely correct, go to the top of the class.

10 amp hours would be huge for most three volt batteries, that is getting into the realm of a motorcyle battery.

A typical car battery is 40 amp-hours (12 volt)
A lorry battery might be 100+ amp-hours (24 volts)

Batteries for electronics are usually measured in milliamp-hours = amp-hours/1000

A 9 volt PP3 battery has a capacity of 625 milliamp-hours = 0.655 amp hours.
A little googling capacity of XXX battery will get you a long list.

13. ### LynneMarie Thread Starter New Member

May 5, 2015
11
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Thaks! Yes that was just an example, I am looking more at the electronics batteries, Measured in mah..
Posing my original question more knowledably now would be; two different brand of batteries with the same mah rating, same current, same voltage, should theoretically run identically? That was the original theory I believed but was told I was incorrect.

14. ### LynneMarie Thread Starter New Member

May 5, 2015
11
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Thanks to everyone for their input in helping me understand these basics. Much appreciated!

15. ### studiot AAC Fanatic!

Nov 9, 2007
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Theoretically all components of the same spec, batteries included, should run identically - whoever made them.

However even within a single batch of components from one manufacturer there will be a spread of characteristics.
That is what is meant.

So a brand new 9 volt battery will have a nominal voltage of 9 volts.
Actually it will have a voltage between about 9.1 and 9.5depending.
Depending?
Well some components like batteries age so if it has just come from the factory it will be nearer 9.3.
If it is 4 years old it may be as low as 8.9

Then in use this battery will only be at 9 volts for a short part of its service life.
A 9 volt battery starts off life at about 9.45 volts and ends at about 8.45 volts ( I gets down to about 6 volts in my wall clocks before they falter)

16. ### GentleRV New Member

Dec 18, 2010
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LynnMarie.,

Nice question about the Watts from a battery.

Watts is power what means how many heat or force you can produce in a element. The element can be anything that become warm or even hot. Force can be produced by an electromotor.
If you take a car battery (=12V DC) and mount a resistor from the Plus to the Minus pole from 1 Ohm then a current from 12 Amp will flow.
This is the law from Ohm I = U / R. Where I = Current, R=Resistance and U = Voltage
The same law say amount of Watts (P)= Current x Voltage in this case 12 Watt

The battery can not deliver that power endless, the voltage will go down and the battery becomes empty.
To measure the amount of energy a (rechargeable) battery can deliver a other way of presentation is needed, for this the Amps/ Hour is there (sort notation is Ah).
What does this mean for a battery who has 10 Ah ? that battery can deliver 10 Amps during 1 hour or 5 amp during 2 hours.
Any multiple to get the 10 Ah is valid.
The battery will not be empty after the 10 Hours but need to be protected for fully discharge.

More questions?

17. ### studiot AAC Fanatic!

Nov 9, 2007
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100 x 0.1 may well be too much for the battery as well as the circuit fuse.

18. ### GentleRV New Member

Dec 18, 2010
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As answer: NO it will not. Any idea what a startmotor use when you turn your ignition key? First moment is about 300 Amps.....

Nov 9, 2007
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20. ### GentleRV New Member

Dec 18, 2010
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It is not about pedantic. There is no way to be ever complete on a subject and even can explain why.
1) by purpose to make the story understandable and not make from a mouse a elephant
2) just forgot to mention it
3) we have a saying that go's: a foul can ask more questions then any wise man can answer (and that is referring to nobody here)

Hope you understand....