Charge LiFePO4 Batteries: Understand Balancing
Look around You! Do you see two equal things?
” Yes! “ – Will be an immediate answer…
Are you sure? They simply look like as the same. They need to be an equal on a micro level.
“They Are The same! “
No ! It is not true!
It is completely impossible!
In Nature haven’t two completely equal parts or things. Remember that!
There aren’t in the practice, existing of two things, which are exactly the same.
Today we will talk about the LIFEPO4 Batteries.
PIC. 1 LiFEPO4 Battery Example: A123 Systems 3.2V,
2500mAH, Internal Resistance < 6 mili-Ohms, ,
20C or 50Ampers – Continuous Discharge,
50C or 120Ampers – maximal Discharge Current for 10 Sec.
In the same way and the batteries are always more or a little bit different. Neither Chemistry quality and quantity (the batteries can be from different production series), their capacity or internal resistance are equal.
You will say then: “Let We use a single cell in the powering circuitry! ” O.K., it is a good suggestion, but, there is one large BUT, not every hardware design can be powered by 3.3V Source, is it?
Imagine the electrical bicycle at 3.3V Power Source and using 500 Watt brushless motor. O.K.
Even so you have the appropriate one, the peak current consumption at the full power will be:
Let we see … 500W / 3.3V = ~ 151 A.
As You see, the current is extremely high – about 150 Amps.
That is the current consumed by the starter-Motor in the moment of starting the middle volume gasoline engine.
Let’s return to our single cell design.
The losses in such design will bring you an efficiency – no more than 50 % or you just need to use super cooled conductors or very powerful, expensive and lossless boost circuitry, which isn’t a deal, is it?
Then, the developers prefer to use multiple batteries – in serial connection, that are assembled into packs.
Example 1: The Pack with 3 pcs. LiFePO4 rechargeable batteries have Up = [9.6V ÷ 9.9V] : It is suitable voltage range for electric screwdrivers, garden tools or laptops powering.
Example 2: The Pack with 4 pcs. LiFePO4 rechargeable batteries Have Up = [12.8V ÷ 13.2V] : The same voltage range as LEAD Acid 12 Car or UPS Rechargeable Battery.
Example 3: The Pack with 10 pcs. LiFePO4 rechargeable batteries Have Up = [32V ÷ 33V] : The typical Voltage Range for the electric PAB bicycle.
Using The multiple batteries in serial connection for example, to multiplying the operate voltage,and , minimizes the losses,there is hiding another problem. The batteries differences…
If the batteries are using the old, but reliable, Ni-MH or Ni-Cd technology, the over-voltage a single cell is not too compromising.
When a Hardware developers use in their designs multiple Lithium-Ionic based rechargeable batteries, their imbalances and just a single cell over voltage can result damage the whole pack, fire or even explosion.
Fortunately, the damaged LiFePO4 batteries are not accompanied flames and explosions.
To Avoid imbalance the serially connected pack’s batteries, the hardware specialists use balancing circuitry.
Many of them using partial balancing with MOSFET switches with the serially connected resistive load.
When the battery grows up to the limiting voltage :
Example 4 : Li- ion U charge Max = 4.2 V.
U nominal = 3.7 V.
Example 5 : LiFEPO4 U charge Max = 3.6 V.
U nominal = 3.3 V.
When the Li-Ion battery reaches 4.2 Volts or LiFEPO4 battery reaches 3.6 Volts, the bypssyng MosFet switch closes and Resistor grab the part of charging current – see next PIC.
That principle works fine, in case, the different battery members from Pack have very similar parameters.
In the opposite case, the balancing process can last days or months, or even never finishes.
In case of valuable differences between the batteries in just one quick iteration, we suggest and using the next principle:
The System works on a quite simple principle.
Firstly, Non-charged batteries working in Constant Current Mode (CC).
That means the charging current carry through an every battery cell.
When the one battery reaches the top allowable charge voltage- given by the manufacturer’s technical specs or just by the chemistry theory, the shunt regulator TL431 create on emitter and collector terminals of Q1 stable Voltage, like Power source – (CV) mode.
Note that Q1 is dissipating more power in process of charging and need to be cooled with a heatsink.
When both four groups are working in CV mode or MCU defined time after that point , the intelligent charger stops the charging process, because the batteries are fully charged.
If you are looking for a super quality Rechargeable LiFEPO4 Battery for your RC Car or Boat – see the author Choice:
Don’t Stop To Reinvent Yourself, Dear Explorers!