Battery Chargers‎ > ‎

Basic equalizer

General description

This equalizer discharge the battery packs with individual action on each cell.

The battery equalizing is necessary for the not perfect matching between the elements that composes a battery pack. During the successive battery charging and discharging there is always a cell that has a capacity slightly different from the others and after multiple cycles is deeply discharged; this cell has a big probability to break.

In order to avoid this problem all the cells should be discharged individually with a particular circuit capable to individuate the end-of-discharge moment for each cell.

This hard "reset" for the cells clears the integral charge/discharge difference accumulated in several work cycles. The equalizer is connected at each cell in the pack and discharge with 6 different mosfets, up to 6 different cells.

The user can define the cut-off threshold for each cells, and the discharge current. The realistic values are 0.5V for the cut-off in a deep discharge and a current of 0.5A or similar.

Main features listing

1. Cut off voltage from 0 to 999mV per cell
2. Current from 0.1 to 1A per cell
3. Displaying of the individual cell voltage during the discharge
4. Up to 10 consecutive discharge cycles for better equalizing and charge removal

Like all the others instruments also the equalizer is powered with 12VDC supply.


Schematic description

Description of the schematic diagram for Basic Equalizer.

The Basic equalizer schematic is built around a PIC 16F876 microcontroller. The microcontroller check the adjustement key, controls the LCD display and switch on the single cell discharge. The power supply (12VDC typical) is filtered and reduced to 5V using a 7805 general purpose linear regulator. From the 5VDC is created, using a small DC/DC converter a 15VDC power supply for mosfet gates biasing, in order to turn on the gate of the "last" cell discharger is necessary at least a potential of 3-4 volts above the last cell voltage or 10V+4V=14/15V. If the user connects a power supply of 15VDC the small DC/DC converter can be not mounted, but the user must shorts the LM7805 pin 1 (input) to the DC/DC converter pin 3 (output) in order to replace the DC/DC converter. A supply voltage above 20V can over-bias the mosfets gates with possible transistor damage.
The voltage from the cell pack is routed to the PIC A/D converter using an ADG508 analog multiplexer and 6 buffers (OP290 or LM358). The PIC calculates the differences from each battery tap to the following for single cell voltage controlling and for cut off voltage checking.
The discharge process is controlled by the PIC microcontroller using a series of mosfet driven by an individual optocoupler for gate voltage shifting (this barrier of optocoupler is not strictly necessary, the voltage shifting can be also achieved with a bjt transistor...).
The discharge current is controlled by the 6.8 Ohm resistor that give a (1.2/7=0.17A) typical current that decrease with the cell voltage decreasing. The LCD display plus the leds are for cell voltage visualization and for cell discharge status observing.


PIC program description

PIC source code remarks and description

The Basic Equalizer source code is very simple because there are no concurrent or real time operations to perform. The main program loop reads the voltage on the six battery taps and calculates the single cell battery by successive subtractions. The single cell cut off voltage (set at the start of the operations) is stored in the eeprom memory for successive discharge easy restart.
The interrupt routine is only a timer for cyclical battery voltage conversion in digital, but the code could also work with no interrupt because the operations can be performed in a sequence without concurrent interactions.
The display shows the cell voltage and the cut off setting using a very simple set of routines. The theory of operations is the following:
  1. The user starts the discharge with the "start key".
  2. The user can modify the threshold (with select key) or wait for start of discharge.
  3. The discharge on all the cells is started and each led signals the discharge of the individual cell.
  4. When the cell reaches the cut off voltage the led shuts off and the cell wait for the end of discharge of the other cells (battery recovery time).
  5. The cells that have more charge stored are the last to end the discharge.
  6. The single cycle of discharge finish when all the leds are off (all the cells have reached the cut off voltage).
  7. The single cycle discharge is repeated 10 times in order to extract all the charge from the cells, and the last cycles are very shorts and about "simultaneous": all the cells starts and ends the discharge at the same time to signal that the battery pack is completely equalized.

Basic Equalizer building notes

Tip and tricks for basic equalizer building.

The building of the basic equalizer is very simple and fast. The user can start the building with the lower height parts (resistors, ceramic capacitors and ICs sockets). Then should be solded the DC/DC, the electrolytics capacitors and the TO-220 (transistors and regulators) components. The last phase is the wiring of the keys, the display and the connectors.

Placement of polarized components

The TO-220 are placed with the small metallic tab as shown in the assembly file. The polarized capacitors have the positive to the board edge (shown in the assembly). The dual in line integrated circuits have the pin 1 marker shown in the assembly scheme. The DC/DC converter cannot be inverted because the pinout is not mirrorable.




Ċ
Oloap,
Nov 5, 2013, 1:03 PM
Ċ
Oloap,
Nov 5, 2013, 1:03 PM
ċ
EQU_material list.txt
(4k)
Oloap,
Nov 5, 2013, 1:03 PM
ċ
EQU_pic.zip
(21k)
Oloap,
Nov 5, 2013, 1:04 PM
Ċ
Oloap,
Nov 5, 2013, 1:04 PM
Ċ
Oloap,
Nov 5, 2013, 1:04 PM
Comments