I've got some spare phone batteries and I'd like to be able to charge them without a phone. However, it seems like I can only charge it with my phone: I attach + pin to 5V DC, - pin to GND, the third, middle pin disconnected. My battery charges only if it's in a telephone, why? How can I charge it? Do I need to connect somethin to the middle pin? The battery is Samsung Galaxy Mini 2's own.
I have designed several products incorporating lithium ion batteries and chargers. Part of the qualification involves abusive tests of the battery. Generally, charging batteries in a mildly abusive manner (for example charge to 4.3V or 4.4V instead of 4.2, or charge at very high rate) do not result in immediate damage to the cell. Also, most battery packs have built-in battery protection which will shut down charge current at 4.3 or 4.4V. What this means is that what you are doing may seem to be working fine. But it is still not a good idea. I hope you will trust my opinion and advice, since I have nothing to sell to you, and have no vested interest.
Do not charge your lithium ion or lithium polymer batteries in an uncontrolled fashion. During charging, the current needs to be controlled to something like 1C or 0.5C until the voltage reaches 4.2V. Then hold at 4.2V. At that point, the current will taper down. When it gets to something like 0.1C, charging must be halted. If the cell does not reach 0.1C (or similar limit) within a reasonable time, (say 2X or 3X the normal charge time) then the charge must be halted. This latter case would be a sign that the battery is at the end of its service life. You may wish to consider discarding a battery that does not taper to 0.1C after a long time.
If you fail to follow this program, the cycle life of the battery will be reduced (which is a good enough reason not to do it), and the probability of catastrophic failure will increase, although it is hard to say by how much.
I know some people say "lithium batteries explode when you over-charge them." Then you try it once, and it doesn't explode, so you think they are just full of BS. The reality is that over-charging can lead to irreversible chemical changes inside the battery that increase the chances of dangerous failures. It is not a recommended practice, and the faster you charge the battery, or the higher you over-charge it, voltage-wise, the larger the chance of problems. Just for completeness, I will also mention that lithium cells should not be charged at all at temperatures colder than 0C, or warmer than around 55C. Fast charging at temperatures below 0C is especially harmful to the cell.
A lot of this information is available in the JEITA document on laptop batteries (URL below). If the link is dead just google for JEITA charging and you will find it. Full title is: "A Guide to the Safe Use of Secondary Lithium Ion Batteries in Notebook-type Personal Computers."
A safe way is to buy a duplicate phone, but with no network contract. It will not be usable as a cell-phone, but it will charge duplicate batteries safely. Usually a green LED comes on when the battery is charged.
You can take it out and put it into a "charged" bin where it will hold its charge for weeks or months, allowing you to charge up more than one spare battery safely. If your phone offers a 'charge percentage' display option, turn it ON.
There are many cheap chargers on ebay which contain the appropriate charging circuit to charge your battery correctly (many will even use the correct polarity whichever way around you connect it).
It is possible to charge the battery with 5V directly but it is not advised as it is very possible to damage the battery or even cause fires or explosions. (I have done it in an emergency and was sure to only apply a small amount of the battery's capacity).