The charge and discharge of the pack of rechargeable batteries in series (every chemistry) is a repetitive process that accentuate the differences between the cells. In practice the cells with lower capacity will deeply discharge than the higher ones, and during the charge the higher capacity cells will be overcharged. For the NiMh and NiCd cells is important to equalize each cell in order to prevent an eccessive difference of charge stored: for the LiPo cells is mandatory a balancer in order to avoid an overvoltage of some cells during the charge. In fact the LiPo cells of different capacity will discharge in different way and will reach a "discharge voltage" quite different: if charged in series the total voltage can be under a fixed (safe) threshold when a single cell voltage rises over the threshold with potential destructive effect. The LiPo balancer will limitate this voltage under a safe threshold bypassing a variable amount of charge current.



The LiPo balancer is composed by an "element" to be put in parallel to each LiPo cell or parallel of cells. You need a number of LiPo balancer elements equal to the number that preceed the "S" in the LiPo batteries terminology: for 3S2P pack (series of 3 sub-packs composed by a parallel of 2 cells) you need 3 LiPo balancer elements.
The LiPo balancer cell acts like a big and very accurate zener diode; when the cell voltage is under a threshold fixed by the user the system is in "idle state" and watch only for the voltage with a equivalent resistance of some KOhm and a negligible current shunt. When the voltage reaches the threshold the balancer element will start to shunt current from the cell with an internal power resistor: the amount of current sunk by the element is proportional to the cell overvoltage with respect the user "safe" threshold. If the voltage reaches a second (destructive) threshold the element communicate with a separate line to the charge the immediate charge stop in order to avoid any severe failure (at this point, from the point of view of the balancer element, the voltage is out of control). The last element is a power thermal control that avoid the over-temperature for the balancer element and prevent any over heating failure.



The J1 connector is directly connected to the LiPo cell, the pins 3-4 to the positive pole and the pins 1-2 to the negative. The cell voltage is measured by the PIC via the variable partitor R2/R15/R3 with the filtering of capacitor C2. The A/D process within the PIC uses as a voltage reference the D1 (LM336-2.5) that is a low cost very stable reference "zener" diode (this is not a normal zener but a quite complex IC, but the function is the same). The signaling to the user is performed by the LED D2 and the temperature sensor (in thermal junction to the power element R7) is done by a general purpose NPN transistor, sampling the Vbe voltage. The power shunt is composed by a power mosfet (IRL520) and a power resistor (R7) that in pratice "shorts" the LiPo cells with a very low resistance. The "ALARM" functions is performed by the optocoupler U1 (4N35) that saturate the output transistor when the cell voltage is out of control (note: the catode pin of the optocoupler is shared with the input voltage and the anode with the A/D Vref, because the PIC has only 8 pin. In non-ALARM usage the voltage across the diode is about 0.5V and the diode is an open circuit, when the ALARM start to operate the pin 7 become an output and shall be sets to 0V , with current in the optocoupler diode and consequent saturation of the output transistor).
The possible modification to the schematic and components are below summarized:

• The LM336 can be changed with another 2.5V reference, the IC shall operate correctly at 3V and shall have a low thermal coefficient
• The power element sets the maximum shunt current: with the chosen values (0.27 Ohm logic level power mosfet and 1 Ohm resistor, the maximum current will be 4.2/(0.27+1) = 3.3A)
• If you want to have an higher peak current, change the mosfet with an IRL540 and a 0.8Ohm resistor (5A current)
• If you want to have a lower peak current, change the mosfet with an IRF540 and a 1 Ohm resistor (2A current)
• If you want to change the 2N2222 with a similar one please recalibrate the tempearture law for the transistor (see the firmware)
• The optocoupler is a very general purpose 6 pin DIL optocoupler and can be changed with a similar one