A Lithium polymer battery or simply a lipo battery is an advanced breed of the more popular lithium ion battery, and just like it's older counterpart is specified with stringent charging and discharging parameters.
However if look at the these specifications in detail we find it to be rather lenient as far as the rates are concerned, to be more precise a Lipo battery can be charged at the rate of 5C and discharged even at much higher rates, here "C" is the AH rating of the battery.
The above specs actually gives us the liberty of using much higher current inputs without worrying about an over current situation for the battery, which is normally the case when lead acid batteries are involved.
It means the amp rating of the input could be ignored in most cases since the rating may not exceed the 5 x AH spec of the battery, in most cases. Having said that, it's always a better and a safe idea to charge such critical devices with a rate that may be lower than the max specified level, a C x 1 could be the taken as the optimum and the safest rate of charging.
Since here we are interested in designing a lithium polymer (Lipo) battery charger circuit, we'll concentrate more on this and see how a lipo battery may be charged safely yet optimally using components that might be already sitting in your electronic junk box.
Referring to the shown Lipo battery charger circuit diagram, the entire design could be seen configured around the IC LM317 which is basically a versatile voltage regulator chip and has all the protection features built in. It will not allow more than 1.5 amps across it's outputs and ensures a safe amp level for the battery.
The IC here is basically used for setting up the exact required charging voltage level for the lipo battery. This may be accomplished by adjusting the accompanied 10k pot or a preset.
The section at the extreme right which incorporates an opamp is the over charge cut off stage and makes sure that the battery is never allowed to overcharge, and cuts off the supply to the battery as soon as the over charge threshold is reached.
The 10 k preset positioned at pin3 of the opamp is used for setting the over charge level, for a 3.7 V li-polymer battery this may be set such that the output of the opamp goes high as soon as the battery is charged to 4.2 V (for a single cell). Since a diode is positioned at the positive of the battery, the LM 317 output must be set to about 4.2 + 0.6 = 4.8 V (for a single cell) for compensating the accompanied diode forward voltage drop. For 3 cells in series, this value will need to be adjusted to 4.2 x 3 + 0.6 = 13.2 V
When power is first switched ON (this must be done after connecting the battery across the shown position), the battery being in a discharged state pulls the supply from the LM317 to the existing level of its voltage level, let's assume it to be 3.6 V.
The above situation keeps pin3 of the opamp well below the reference voltage level fixed at pin2 of the IC , creating a low logic at pin6 or the output of the IC.
Now as the battery begins accumulating charge its voltage level starts rising until it reaches the 4.2 V mark which pulls pin3 potential of the opamp just above pin2 forcing the IC's output to go instantly high or at the supply level.
The above prompts the indicator LED to light up switch ON the BC547 transistor connected across the ADJ pin pf the LM 317.
Once this happens the ADJ pin of the LM 317 gets grounded forcing it to shut off its output supply to the lipo battery.
However at this point the entire circuit gets latched in this cut off position due to the feedback voltage to pin3 of the opamp via the 1K resistor. This operation makes sure that the battery under no circumstance is allowed to receive the charging voltage once the over charge limit is reached.
The situation stays locked until the system is switched OFF and reset for possibly initiating a new charging cycle.
In the above proposed 11.1V battery pack, there are 3 cells in series and the battery poles are terminated separately through a connector.
It's recommended to charge the individual batteries separately by locating the poles correctly from the connector. The diagram shows the basic wiring details of the cells with the connector:
UPDATE: In order to achieve a continuous automatic charging of a multi-cell Lipo battery, you may refer to the following article, which may be used for charging all types of Lipo batteries regardless of the number of cells included in it. The circuit is designed to monitor and automatically transfer the charging voltage to the cells which might be discharged and needs to be charged: