Discrete Lithium Ion Charger

!! Disclaimer !!

Build this circuit at your own risk. I assume you know how to work around Lithium Ion/Lithium polymer cells if you proceed with this project. I will not be held responsible for any of your mistakes. My prototype works fine for a couple years now, yours may or may not depending on your understanding of everything involved.

A Lithium Ion Battery is rather picky on how it is charged. It requires about four charging steps from empty to full.

  1. First stage: If the cell voltage is below 3 Volts, it has to be charged at about 0.1C or less. (ex. 1C = 1000mA if batt is 1000mAh)
  2. Second stage: when the cell voltage is between 3 Volts and 4.20 Volts, charging should be done with constant current. at not more than 1C.
  3. Third stage: When the cell reaches 4.20 Volts, cherging will move to Constant Voltage mode. Voltage accuracy should be 1% or better.
  4. Fourth stage: Charge termination. Do Not float charge a Lithium Ion cell. Cell damage will result or worse, the cell could explode.

Charge termination can be determined by time (2 hours or so after CV mode is reached) or a minimum current (about 0.02C to 0.07C) is reached.

If this charge algorithm is not followed strictly, battery capacity will suffer. Worst case is that the battery burns up and explodes.

The charger presented here follows the four charging stages. This charger is designed to accept only three cells in series for a nominal voltage of 11.1V. I decided to change the trickle to normal charge rate threshold from 9V (3x 3V) to 10V (3x 3.33V) to allot for unbalanced cells making sure all cells have gone above 3V/cell before moving to normal charge rate. Charge termination is determined by minimum current.

Schematic diagram can be found here: Charger; Balancer. <-- do not use this circuit... see update below.

Basic charger circuit blocks will be explaned below: (all part naming are from the charger circuit)

IC1 is a basic three terminal adjustable regulator. It is used as a pass transistor without needing a driver transistor. At CC mode, its output voltage is determined by Q3 which senses the voltage drop across the total resistance of R4 to R7. at CV mode, the voltage is limited by the three optocouplers in series in the balancer board.

IC2A senses the voltage across R4 to R7 to determine when to terminate charging when it reaches minimum current set by R3.

IC2B senses the threshold voltage of the trickle-normal charge mode. When 10V is reached, it will turn Q2 on (Q2 should be a low Rds On device like IRFZ46 or IRF3205, I just used '540 for the package) When the Mosfet turns on, it essentially connects R4 and R5 in parallel to R6 and R7 to increase charge current.

Q1 is used to alter the imposed voltage across the shunt resistors (R4-R7) to select low or high charge current modes.

The balancer circuit is a basic TL431 based circuit I found from the internet. I added optocouplers in series with the LED's so that the charger board goes to CV mode when all three balancers reached 4.20V. Q7 is there just to make sure that the charger will only charge the battery if both the charger and balancer wires are plugged in properly.

The complete charger accepts 15-18VDC at 1.5A input from a laptop brick charger. Charge current can be selected to be around 1.3A (Hi) or about 1A (Lo). Trickle current(V cell < 3.3V) is 18% of the normal charging current.

It is manual start (using push button to start charging sequence) and stops automatically when battery is full.

Pictures of my prototype.

Led lights from left to right:

(Top: 'power', 'stopped', 'hi current mode', 'trickle mode'; Bottom: 'cell3', 'cell2', 'cell1', 'check connectors').

18 Apr 2011 I redid the circuitry. Added a 2 hour timeout circuit after a close encounter with the charger failing to stop with a Li-poly battery and it started to bulge. The balancer circuit used is the same.

           Circuit operation is the same with the 4-step charging process but now it terminates charging in two methods. Minimum charge current and 2 hour timeout whichever comes first. The timer circuit has been tested to turn the charger off in 2 hours 20 minutes after going to CV mode. It might vary depending on the brand of CD4060 you use.

            New schematic diagram of the charger is here,same balancer circuit is used.

           Same circuit explanation above still applies as I did not change much of the original circuit. I just grafted the timer circuit into the existing timer. Q4 is added to the charger-balancer link to detect when the CV mode is initiated. Once the balancer circuit is limiting output voltage, Q4 will conduct. Q4 will turn Q5 on and pull RESET pin of IC3 low starting countdown. Once the counter starts running, LED 5 will blink once every 10sec or so indicating the timer is oscillating. Once the last output goes high (Q14 of IC3) it turns the mosfet on shorting IC2A's input to ground fooling IC2A that the minimum current is reached terminating the charging cycle.

            R22, D8 and C5 simply supplies IC3 with a regulated 12V as the 15V input from the laptop brick is on the limit of the CMOS IC.

           The high-low reference has also been changed. Instead of using a schottky diode, a lower reference voltage was achieved using a resistor divider R19 and R21. This offsets the 0.65V Vbe drop of Q3 by approx 87mV. If Q1 is on, the voltage across the current sensing resistors R4-7 is equal to the Vbe drop of Q3 at 0.65V. When Q1 is turned off, it will be 0.65V - 0.087V = 0.563V lowering the charge current.

           I-stop current is also offset by the resistor divider. The threshold is at 140mV in the prototype (measured) so with the 87mV offset, charger currents are therefore:

Mode Trickle charge current (calculated) Normal charge current (calculated) Turn off current (measured)
Low 0.240 Amp 1.366 Amp 0.13 Amp
High 0.276 Amp 1.576 Amp 0.34 Amp

           I will post pictures of the added timer circuit later.

23 Apr 2011

Here are the pics of the graft-on 2 hour timeout module.

It's basically piggy-backed onto the main board. Connections are inserted between the links of the charger and balancer boards.

My supplier doesn't have the CD4060 chip in SOIC so I bought the DIP version and used it instead.

Page created and copyright R.Quan © 23 Apr 2011.