# Tag Info

31

Yes, the total mass of a battery increases when the battery is charged and decreases when it is discharged. The difference boils to Einstein's $E=mc^2$ that follows from his special theory of relativity. Energy is equivalent to mass and $c^2$, the squared speed of light, is the conversion factor. I would omit the scenario I. If the lithium is leaking from ...

29

Sometimes it is easier to understand circuitry in the context of water. What you're imagining is two tanks of water of equal size linked together by a pipe that has been sealed off. If one tank holds 5% water and the other holds 35% water, when you remove the seal, the tanks equalize and you end up with 20% in both tanks. What you're forgetting is that ...

24

The key here is the voltage of both the batteries. The battery in the phone is generally at a voltage of 3.7V. The battery pack has a higher voltage or a circuit which gives a voltage of 5V to your phone. So, as long as the voltage with which you charge the phone is higher than that of the battery, the percentage of power in it doesn't matter and the phone ...

23

Connecting your phone to the battery pack doesn't directly connect the cells in parallel. I assume this is where your guess of an equilibrium with equal voltage -> equal charge percentage comes from. Shorting lithium-ion / lithium-polymer (LiPo) cells together like that would likely cause one or both to literally catch fire from the high currents, or from ...

16

Yes Sam, there definitely is electric field reshaping in the wire. Strangely, it is not talked about in hardly any physics texts, but there are surface charge accumulations along the wire which maintain the electric field in the direction of the wire. (Note: it is a surface charge distribution since any extra charge on a conductor will reside on the surface.)...

15

There are many reasons for this situation. Power produced is non-adjustable. The battery produces power at nearly constant rate (slowly decaying with time). It cannot be increased and if not consumed (or stored) the power is lost. (Mentioned by DumpsterDoofus) low power density. ${}^{63}\text{Ni}$ for instance produces ~5 W/kg (and kg here is just mass of ...

12

A battery generates a voltage by a chemical reaction. There is a class of chemical reactions called redox reactions that involve the transport of electrons, and you can use the reaction to drive electrons through an external circuit. This is the basis of a battery. The battery will continue to provide power until all the reagents have been used up and the ...

11

First, your camera is not designed to work with batteries below a certain voltage. When it detects an excessively low battery voltage it turns itself off. That circuit stays in the "off" state until voltage is completely removed from the circuit. When you operate your camera, the current required by your camera varies according to what you do with it. So ...

11

In ideal circuit theory, the parallel connection of two voltage sources results in an inconsistent equation, e.g., a 3V and 2V source connected in parallel, by KVL, gives the equation: 3 = 2. In the real world, batteries are not ideal voltage sources; batteries can supply a limited current and the voltage across the battery does, in fact, depend on the ...

11

Heat. Batteries have internal resistance and so produced heat when current flows through them (Joule heating). Also, the heat generated increases by the square of that current. E.g, doubling the charging current causes the heat produced to be increased 4 times. Ultracapacitors are a different technology that can be used like batteries--they have very very ...

10

You should not connect different batteries in parallel. If you do, the battery with the highest voltage will discharge into the other one, until they end up with equal voltages. If the second battery (the lower voltage one) is a rechargeable, then it will be charged by the first one, again until the two have the same voltage. In this case the end voltage ...

10

Mostly the problem is that in batteries, current flow is not by electrons as in something like a copper wire, but by physical movement of ions. Only so many ions can migrate to the right place and perform the right chemical reaction over some fixed time.

9

I've just sacrificed an AA manganese alkaline battery to the cause of physics. When I first shorted the battery it produced a current of about 9.5 amps, which I thought was actually pretty impressive. However over the course of 30 seconds the current dropped to around 5 amps. The battery got pretty warm, though I don't think it would have set fire to ...

9

The resistance of water, even with ions and minerals and such, is still fairly high. So, a tiny current flowed through the water, but not very much. Additionally, the heating effect that often destroys them when short circuited would also be nullified by the cooling water.

8

Once the battery is fully charged it will not accept any more energy (current) from the charger, since all the energy levels that were depleted when empty are now at their highest level. For example in a Lithium ion battery when all the ions have arrived at the proper electrode the resistance to more current becomes very large, but not infinite since there ...

8

Real batteries have a finite energy storage capacity. Adding additional cells adds additional capacity (this is why I would add them, you haven't really specified any context so it's hard to say what you are looking for). Also, it's worth noting: I'm assuming that all the cells are identical, and internal resistance is negligible. This is an important ...

8

Electrons that reach the positive terminal indeed remain there. The potential difference between the two terminals pushes electrons from the negative anode toward the positive cathode. When an electron reaches the cathode, it stays there to equalize the original charge imbalance between the two nodes. When electrochemical redox reaction sustaining the ...

6

One alkaline AA cell has about 11 kJ of energy. For a laptop battery, it is 360 kJ. Chevrolet Equinox Fuel Cell has 58 MJ of energy. One kilogram of TNT carries about 4.184 MJ of energy. Divide the numbers from the previous paragraph by this constant to see that the AA cell, laptop battery, and electric car battery have 2.6 grams, 86 grams, or 14 kilograms ...

6

Typically batteries involve using the energy stored in some chemical compound (for example, we have batteries of type Lithium-ion). So what happens is we use the potential energy stored in the chemical as an electromotive force to power our device. Now what happens when allt he potential energy stored inside that compound runs out? We have to recharge it. ...

6

Your original text admitted three interpretations, and I'm leaving the answers here: 1: What happens with a toy model when there's a circuit with an ideal battery and no resistance? All the charge moves around the circuit at one moment in time (infinite current). The energy must leave the system as Electromagnetic radiation - accelerating charges radiate, ...

5

Crazy Buddy's answer and related comments have made the point that you could indeed use a capacitor to charge a battery, but the amount of energy stored in capacitors is generally less than in batteries so it wouldn't charge the battery very much. However there is a new generation of capacitors called ultracapacitors that are being developed with electric ...

5

I am going to assume what you are thinking about is some sort of power pack that uses AA batteries like this one. There is no physical reason that you can not use AA batteries to charge a cell phone, as long as you have the correct adapters. What might be confusing the issue is that Li-ion batteries have definite charging issues. If overcharged, they can ...

5

What you are describing is called a series connection. Think of the batteries as pumps, with each pump generating 1.5 PSI. If you connect the outflow of one pump to the inflow of another, then overall the two pumps are going to generate 3.0 PSI. Note that the current capability is not doubled. If the two pumps are each rated for 1 gallon/minute, then the ...

5

I apparently cannot post images, so I apologize but you'll have to open this link in a new window to see my atrocious diagrams :) Diagrams -> http://i.imgur.com/Lxfu1e2.png EDIT: Here are the diagrams, sorry about my lack of artistic skills, haha. Voltage is an electrical potential difference, which is essentially a force caused by electrons wanting to ...

5

In the left image, the vehicle is prevented from moving downwards as the ground is exerting a force upwards equal to the force of gravity on the vehicle. However, in the case of the electric circuit, there is no such opposing force. The orientations of the conductors attached to the battery terminals doesn't have an effect on the flow of electrons (unless ...

5

For low current applications that are sensitive to voltage, like modern electronics, the resistance of the contacts can be significant. So when you reinsert the batteries you clean off any dirt, moisture or corrosion on the contacts, the resistance drops. So when a current flows the voltage drop accross the contacts is reduced and more of the battery ...

5

Consider for a moment, a cell that is not connected to a circuit, i.e., there is no path for current external to the cell. The chemical reactions inside the cell remove electrons from the cathode and add electrons to the anode. Thus, as the chemical reactions proceed, an electric field builds between the anode and cathode due to the differing charge ...

5

I understand also that there would be a tiny minuscule resistive loss through the wire, but really it's not enough to say anything about. On the contrary, it's crucial. Assuming an ideal voltage source (can supply unlimited current) of voltage $V_S$, an ideal resistor of resistance $R$, and an ideal uncharged capacitor of capacitance $C$, are suddenly ...

5

As drawn, the circuit, assuming ideal circuit elements, is problematic for the reason you've deduced (KVL gives a contradiction). One interpretation is that there is infinite large current for an infinitesimal time which instantaneously charges the capacitors to their final steady state voltages. To gain some insight, add a resistance $r$ in series with ...

5

For an iPhone the battery voltage is a nominal 3.8 V and the battery pack would probably replicate the 5 V output voltage of a USB power supply. So the battery pack would be discharged as it was driving current into the positive terminal of the phone battery and thus recharge the phone battery. So only when the battery pack voltage was less than the ...

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