The article explains a simple circuit which can be used for charging at least 25 nos of Li-Ion cells together quickly, from a single voltage source or a 12V battery. The idea was requested by one the keen followers of this blog, let's hear it :
The Circuit Request:
Helo mr swagatam...please can you help me design a circuit to charge 25 li-on cell battery(3.7v- 800mA each) at the same time.my power source is from 12v- 50AH battery. Also let me know how many amps of the 12v battery would be drawn with this setup per hour...thanks in advance.
The Circuit Design
When it comes to charging, Li-ion cells require more stringent parameters compared to lead acid batteries.
This becomes especially crucial because Li-ion cells tend to generate considerable amount of heat in the course of the charging process, and if this heat generation goes beyond control may lead serious damage to the cell or even a possible explosion.
However one good thing about Li-ion cells is that they can be charged at full 1C rate initially, contrary to lead acid batteries which doesn't allow more than C/5 charging rate.
The above advantage permite Li-ion cells to get charged at 10 times faster rate than the lead acid counter part.
As discussed above, since heat management becomes the crucial issue, if this parameter is appropriately controlled, the rest of the things become pretty simple.
It means we can charge the Li-ion cells at full 1C rate without being bothered about anything as long as we have something which monitors the heat generation from these cells and initiates the necessary corrective measures.
I have tried to implement this by attaching a separate heat sensing circuit which monitors the heat from the cells and regulates the charging current in case the heat starts deviating from safe levels.
Controlling Temperature at 1C Rate is Crucial
The first circuit diagram below shows a precise temperature sensor circuit using the IC LM324. Three of its opamps have been employed here.
The diode D1 is a iN4148 which effectively acts as the temperature sensor here. The voltage across this diode drops by 2mV with every degree rise in temperature.
This change in the voltage across D1 prompts A2 to change its output logic, which in turn initiates A3 to gradually increase its output voltage correspondingly.
The output of A3 is connected to an opto coupler LED. As per the setting of P1, A4 output tends to increase in response to the heat from the cell, until eventually the connected LED lights up and the internal transistor of the opto conducts.
When this happens the opto transistor supplies the 12V to the LM338 circuit for initiating the necessary corrective actions.
The second circuit shows a simple regulated power supply using the IC LM338. The 2k2 pot is adjusted to produce exactly 4.5V across the connected Li-ion cells.
The preceding IC741 circuit is an over charge cut off circuit which monitors the charge over the cells and disconnects the supply when it reaches above 4.2V.
The BC547 at the left near the ICLM338 is introduced for applying the appropriate corrective actions when the cells begin getting hot.
In case the cells begin getting too hot, the supply from the temperature sensor opto coupler hits the LM338 transistor (BC547), the transistor conducts, and instantly shuts off the LM338 output until the temperature comes down to normal levels, this process continues until the cells get fully charged when the IC 741 activates and disconnects the cells permanently from the source.
In all 25 cells may be connected to this circuit in parallel, each positive line must incorporate a separate diode and a 5 Ohm 1 watt resistor for equal distribution of charge.
The entire cell package should be fixed over a common aluminum platform so that the heat is dissipated over the aluminum plate uniformly.
D1 should be glued appropriately over this aluminum plate so that the dissipated heat is optimally sensed by the sensor D1.
Automatic Li-Ion Cell Charger and Controller Circuit.