Batteries and Wires

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Hello!
I've been reading about electricity this evening and I was wondering if there was anybody out there who would be able to answer a couple of questions I have.
My first one is to do with batteries, when loading batteries into a device like a remote control, I have always been told never to use two 'different' batteries, as in two different makes, as I'm told one battery will 'zap' the power from the other one and so one will run out of power. Is there any truth in this? Is it to do with what the batteries are made out of? I'm trying to understand why in two different brands of batteries, e.g. one AAA duracell and one AAA panasonic, one battery would 'zap' the other one? Is this complete rubbish? Does it depend if one is rechargeable and the one not rechargeable?
My second question is to do with wires, now, again, is it true if you wrap wires around something, e.g. headphone wires around an MP3 player, this will damage the wires or the insides of the wires? Again, I don't particularly understand why this happens.
Thank you!

The batteries one is total BS as long as they're both alkaline 1.5v batteries there's not gonna be some weird zapping problem, now if you mix a rechargeable battery and an alkaline you might have a problem since alkaline are 1.5v and rechargeable batteries are 1.2v. As for the wires thing, it is possible to damage the wires by putting stress on them but inless you're wrapping them super tight it shouldn't be a problem.

The problem with mixing batteries is that if you let the batteries run down, it is quite likely that one battery will end up charging the other! This is really, really bad. If you draw a diagram with polarities, you may notice that the dead battery is being charged up backwards from normal!

This isn't a problem if you monitor all the cell voltages and shut down the device before any of the cells get too low, but most devices only monitor the voltage of the entire string, not the battery of each cell. Satellites are the exception--you do monitor the voltage of each cell, and have people on the ground taking corrective actions as necessary.

I've found that wires on headphones do break quite easily, especially on the cheap ones.

Well, the second part is that cheap headphone wires break easily. If you wrap them, they're bent more and break sooner. I always roll up my headphone wires as loosely as possible. But I'm not sure it matters too much, since they easily get a short in them anyway.

Mixing cells isn't as bad as people make out. As has already been mentioned if one cell has a lower capacity than the other, it will be reverse charged when it runs flat. Normal battery chemistry (alkaline, zinc carbon, nicd or nimh) is fairly tolerant to this and in most cases all that will happen is that your device will run out of power sooner than you expect as the weakest cell runs flat. Under very heavy load (say a device that runs batteries flat in less than half an hour) in some extreme cases the cells can heat up and it is theoretically possible for the weakest cell to overheat and leak or even burst. In practise you have to try hard to get this to happen - I've done some pretty nasty things to batteries in my time and have only once had a nicd pack start hissing and spitting at me. This was after some really severe abuse.

Be very careful when dealing with Lithium batteries--while these cells becoming quite popular, they represent a considerable fire hazard if abused.

http://www.ka7oei.com/nicds.html
When a NiCd cell is reverse-charged, a strange thing happens: Conductive metallic "hairs" (often called dendrites) begin to form - and they "grow" from one electrode to another. Eventually, this dendrite forms a short across the cell - one that can have a range of resistance from high to low, depending on the severity of the damage.

I was pretty careful to mention the battery chemistries. As you say, lithium batteries are a whole different ball game. Many popular lithium chemistries are just itching to go up in a ball of flame. I know of someone who lost his house due to getting slightly careless with some lipo batteries.

ya lithium cells are a complete in another ball field. With all the devices that use for example lithium Ion like Laptops, Tablets, mp3 player, Phones, etc. have a circuit in the charging system to protect and keep the battery from exploding from overcharging, if the circuits were built right. For example if you buy a battery for a laptop from china those are made cheap and those protection circuits are not up to par.