Question about changing the battery of Ipod

battery Qs

little bit of ipod modifying here.

first off, is it possible to damage a device by underpowering it? I want to replace the 3.3v li-ion battery with 2 1.2v AA rechargeable ones and want to make sure dont damage it.
I may end up using 4 AAs and a 3v regulator to power it anyway.

second is if the 2/4 AAs were stepped up to 12v and connected to the same pins as the 12v charger was in, would it provide the same usage time? transformers were never my thing....

lastly how come the original 800mAh battery costs about £50 whereas 2 AA batteries that add up to 4000mAh cost £5 out of a standard DIY shop? The original battery is only about the size of a slightly flat AA so its not too much of a size issue

ill be leaving the original one in and splicing a switch and connector to the outside of the ipod btw....

Re: battery Qs

Q - Can you damage a device by underpowering it?
A - It depends on the device. Lower voltage for some electronics may cause oscillators to stall which can cause direct current to flow in the connected circuit and cause overheating. Some switching circuits may fail to properly switch causing indefinite states for connected circuitry, etc. You really need to look at what circuits are being powered to answer the question.

Q - Can I use batteries to replace the charger input?
A - Sure, but you probably won't run very long. Batteries discharge according to their chemistry and milliamp-hour rating. The terminal voltage of the battery drops off along a curve (called the characteristic discharge curve) as long as you are drawing current from it. A charger transformer maintains a relatively steady voltage and current output into the load as long as it is plugged in.

Q - How come the 800mah battery costs more than the AA battery?
A - It is probably because the 800mah battery was custom made to fit a particular space/holder. It may also be one of the more expensive chemistry types that keeps a flat output voltage longer as it discharges. Another factor would be the number of charge/discharge cycles it can tolerate. Still another factor might be the design to deliver the required voltage and current within certain weight design requirements for portable equipment. The typical rechargable Nickel Cadmium AA battery is pretty rugged, but the terminal voltage begins to droop almost immediately as it is discharged. The AA batteries are also probably larger and heavier than the battery you want to replace.

There is much more that can be said; however, more details about your exact circuitry are required.