Charging a li-ion cell?

Hi guys. I recently bought a new laptop battery, since my old one had 3 mins life after being disconnected from the power. So I decided to rip the old one apart. They seem to still have a 4V charge under no load. They look like a oversized AA, and seem to have built in protection circuitry. Anyone know how I could build a charger that would safely charge these things? I know they are slightly dead, but they re still holding a charge pretty well. Here is a pic of it next to a normal AA
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safely charging those things is beyond normal experimenter techniques, you need a special charger.

if you want something you can safely charge try the LiFePO4 chemistry:

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Hi.
Apply not more than 4.2000 Volts to the cell at any current rate that does not warm them up. ---> 4.1V is fine !
(Use a fixated voltage regulator) That's it.
Do not charge several in series. You do can charge several in parallel.

There is fancier ramping methods to charge them to extreme 99.9% capacity but do not get into that specialized circuitry for general purpose use. You do not need any 'special' chargers

Miguel

You shouldn't charge Li-ion:-

When below -10 deg C
When above +40 deg C
Faster than 1 hour
Faster than at the 100 hour rate if the voltage is less than 3.2V

as well as the suggestions from Externet

Charging,

I'm at the moment charging a Li - ion battery from a camera from which the charger got lost.

I usually stick with a charging current of between 5 and 10 % from the Ah Capacity.

At 5% you can leave it safely on charge for 24 hours.
At 10% rate I would check after 1 hour or so that the cell is not excessively warm otherwise reduce the charging current.

For current below 50 mA , I often put a LED in series which gives a visual indication of charging.

I use a normal DC power supply and use a series resistor in line 56 or 100 hm: 1/2 watt is fine.