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LiPo Balancer

LiPo Balancer element

Description of the LiPo balancer element schematic.

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.


LiPo balancer basics

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.




LiPo balancer schematic description

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


LiPo Balancer base

Description of the LiPo Balancer base schematic.

The LiPo balancer base has two main functions: accomodate each LiPo balancer element with a connector for the battery pack and incorporate a power switch to stop the charge process when one cell voltage is out of control. This base is intended to work with all the "constant current" chargers giving a very high protection grade in LiPo cells charging. In fact this module is not specific for the Universal Charger but is a very generic device.
The ALARM signal coming from the LiPo balancer element is in a "AND" gate with the others using the paralleling of the optocouplers output transitors. When at least on of these transistor is saturated, the gate of the power switch transistor Q1 is forced at 0.4V stopping the current flow. The zener diode prevent the gate overvoltage when the charger is used for high number of cells (above 20V).
In the Universal Charger (like in the Advanced) the stop of the current flow will give to the user the "Battery Disconnected" message.
Using the LiPo Balancer system the user can also recharge safely the LiPo batteries with a constant current charger (like the Advanced Charger) with some limitations. The charge process should follow these rules:
  • choose the charge voltage greater than N(LiPo)cells*4.3V+0.5V
  • choose a charge current as stated in the LiPo batteries user manual
  • sets the delta peak to the maximum value
  • sets the discharge threshold TOTAL value to N(LiPo)cells*3.0V

In this way the Advanced charger will deliver a constant current to the pack until the LiPo balancer will stops the process because at least one cell voltage will reach the ALARM threshold. In this way (without the constant voltage charge phase) the user can store in the cells about the 70-80% of the total capacity.

The PCB for the LiPo balancer base is easily adaptable to the number of the balancer element that the user uses: the silkscreen ha predefinet break lines for different number of cells.


Building the balancer

Tip and tricks to easily build the LiPo balancer system.

The LiPo balancer has not particular criticalities in the mounting process. The user should, as usual, start the assembly with the resistor and IC sockets, the the condensators and LED: the last elements are the transistor, the reference diode , the MOSFET and the power resistor. Particular care should be taken in the assembly of the transistor 2N2222 that is located just below the power resistor and should be in good thermal contact with the resistor ceramic body: it is a good choice to place also silicon grease (thermal conductive paste) on the top of the transistor. Particular care should be taken in the soldering process of the board pins: it is necessary to right align these pins inserting gently the pins in a socket and, at this point, soldering the board. No additional cooling should be provided to the MOSFET, in general it is a better choice to pass to a logic level one with lower internal resistance.


The base is connected to the charger and to the battery cells with an high current cable (the two contacts in the PCB bottom part, with the switch) and with the flat cable connected to each battery pole.
IMPORTANT: there are two wires that links a junction point between two batteries and the balancer, one for the negative pole of a battery one for the positive pole. The voltage in these wires is the same but the current path for the balancing process is different for the two cells.











This photo shows the particular junction between the power resistor and the 2N2222 transistor that acts as temperature sensor.


















LiPo balancer firmware

Description of the LiPo balancer firmware

This section describe in details the firmware for the PIC 12F683. This software is developed in Mikropascal (www.mikroe.com) and this tool can be downloaded for free from this website. The free edition of the software has the only limitation that the produced code must be less than 2K word. The LiPo balancer software is under this threshold and for this can be freely changed and recompiled from everyone.

Voltage sampling, power shunt and LED drive

The main loop for the LiPo balancer firmware has a duration of 100 msec. For this the shunt current is calculated (starting from the battery voltage) 10 times per second. The first part of the loop (first 25 msec time slot) samples (and averages) the battery voltage giving an accurate (millivolt) reading. This value is compared with two threshold: the first one (point of start) is externally trimmed to 4.200V by the user, the second is 40 mV above the first one and sets the alarm point. Under the start threshold the balancer is in idle and do not shunt any current (only the PIC supply and few mA for the voltage partitors), between the two thresholds the shunt is active, and over the alarm threshold the system is shut down and the alarm signal is issued (stop of the charge). The active interval is between 4.200V and 4.250V: these 50mV causes a linear variation in the shunt current from 0 to the maximum value (the maximum value is set by the MOSFET and R7 resistor). The LED stay OFF when the system is in idle state,it will lamp in the active interval from a duty of 1/16 (start of shunt) to 15/16 (maximum shunt) and will stay continuously ON in alarm.

Temperature sampling

The temperature is sampled with a period of 1 minute for a better reading accuracy. The averaged value is compared with a threshold value of 120-130 celsius degrees on the transistor. If the temperature exceed this point the alarm signal is issued and the LiPo balancer waits for a new power on. If the user want to disable the temperature control it is sufficient to do not mount the transistor 2N2222 on the PCB.

ALARM function

This function is activated when the cell voltage is out of control or when the LiPo balancer is overheated. From this state (the optocoupler transistor is saturated) the program cannot exit and must be restared disconnecting the battery pack.


LiPo Balancer calibration

Description of the calibration process for the LiPo balancer

The calibration of the LiPo balancer is very critical because the measured voltage should be very precise in order to minimize the errors in the cut-off threshold calculation. An error in the voltage measurement could traslate the point of start with a potential destructive problem. For the calibration there is the need of a small and stable voltage regulator capable to give with high accuracy the point of start (4.200V) and to give at least the current for the idle state (10-15mA).


LiPo element calibrator building

The LiPo calibrator built is a very simple circuit based on the LM317, one resistor and one trimmer that can give a voltage from 4 to 4.5V. The trimmering range (around the point of start) should be minimezed in order to have a very good and easy adjustement with a standard multiturn trimmer. If the user has a very precise and stable power supply or another adjustable power supply (capable to be trimmered at 4.200V) it can be used to calibrate the LiPo balancer element. We built also a receptacle that accomodate the LiPo balancer element for a better mechanical stability and more reliable operations, but this accessory is recommended but not necessary for the calibration process.











Calibration process description

The calibration is very simple and is divided in two separate steps.

The first step is the calibrator voltage adjust (with an accurate multimeter) in order to have a power supply set EXACTLY to 4.200V. Then the power supply will be removed and the LiPo balancer element is connected to the calibrator. When the power is off the trimmer on the balancer should be turned counter-clockwise in order to prevent an unwanted operation start when the power will be applied. Then apply the power supply, re-check the voltage (stable at 4.200V). The LiPo balancer element will turn on the led at the power on, the turn off it to indicate the "idle" state. At this point the second step is to turn VERY slowly the trimmer on the LiPo balancer element until the start to flash for the LED. At this point stop the turning and the balancer should be calibrated. If the user don't stop turning the trimmer, the overall operation must be repeated, removing the power supply and turning the trimmer in a "idle" position.




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BAL_firmware.zip
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BAL_gerber1.zip
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BAL_gerber2.zip
(12k)
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