The main advantage with Li-Ion cells is their ability to accept charge at a quick, and an efficient rate. However Li-Ion cells have the bad reputation of being too sensitive to unfavorable inputs such as high voltage, high current, and most importantly over charging conditions.
When charged under any of the above conditions, the cell may get too warm, and if the conditions persist, may result in leaking of the cell fluid or even an explosion, ultimately damaging the cell permanently.
Under any unfavorable charging conditions the first thing that happens to the cell is rise in its temperature, and in the proposed circuit concept we utilize this characteristic of the device for implementing the required safety operations, where the cell is never allowed to reach high temperatures keeping the parameters well under the required specs of the cell.
In this blog we have come across many battery charger circuits using the IC LM317 and LM338 which are the most versatile, and the most suitable devices for the discussed operations.
Here too we employ the IC LM317, however this device is used only to generate the required regulated voltage, and current for the connected Li-Ion cell.
The actual sensing function is done by the couple of NPN transistors which are positioned such that they come in physical contact with the cell under charge.
Looking at the given circuit diagram, when power is applied to the set up, the IC 317 restricts, and generates an output equal to 3.9V to the connected Li-ion battery.
The 640 ohm resistor makes sure this voltage never exceeds the above limit.
Two NPN transistors can be seen connected in a standard Darlington mode to the ADJ pin of the IC.
We know that if the ADJ pin of the IC 317 is grounded, the situation completely shuts off the output voltage from it.
It means if the transistors conduct would cause a short circuit of the ADJ pin to ground causing the output to the battery shut off.
With the above feature in hand, here the Darlingtom pair does a couple of interesting safety functions.
The 0.8 resistor connected across its base and ground restricts the max current to around 500 mA, if the current tends to exceed this limit, the voltage across the 0.8 ohm resistor becomes sufficient to activate the transistors which "chokes" up the output of the IC, and inhibits any further rise in the current. This in turn helps keep the battery from getting undesired amounts of current.
However the main safety function that's conducted by the transistors is detecting the rise in temperature of the Li-Ion battery.
Transistors like all semiconductor devices tend to conduct current more proportionately with increase in the ambient or their body temperatures.
As discussed, these transistor must be positioned in close physical contact with the battery.
Now suppose in case the cell temperature begins rising, the transistors would respond to this and start conducting, the conduction would instantly cause the ADJ pin of the IC to be subjected more to the ground potential, resulting in decrease in the output voltage.
With a decrease in the charging voltage the temperature rise of the connected Li-Ion battery would also decrease. The result being a controlled charging of the cell, making sure the cell never goes into a run away situations, and maintains a safe charging profile.
The above circuit works with temperature compensation principle, however it does not incorporate an automatic over charge cut off feature, and therefore the maximum charging voltage is being fixed at 3.9V.
At 3.9V we cannot assume the battery to be fully charged.
To counter the above drawback, an automatic cut off facility becomes more favorable than the above concept.
I have discussed many opamp automatic charger circuits in this blog, any one of them can be applied for the proposed design, but since we are interested to keep the design cheap and easy, an alternative idea which is shown below can be tried.
Here, an SCR is used across the ADJ and ground of the IC. The gate is rigged with the output such that when the potential reaches at about 4.2V, the SCR fires and latches ON, cutting of power to the battery permanently.
The threshold may be adjusted in the following manner:
Initially keep the 1K preset adjusted to ground level (extreme right), apply a 4.3V external voltage source at the output terminals.
Now slowly adjust the preset until the SCR just fires (LED illuminated).
This sets the circuit for the auto shut off action.
How to Set-Up the Above Circuit
Initially keep the central slider arm of the preset touching the ground rail of the circuit.
Now, without connecting the battery switch ON power, check the output voltage which would naturally show the full charge level as set by the 700 ohm resistor.
Next, very skilfully and gently adjust the preset until the SCR just fires shutting off the output voltage to zero.
That's it, now you can assume the circuit to be all set.
Connect a discharged battery, switch ON power and check the response, presumably the SCR will not fire until the set threshold is reached, and cut off as soon as the battery reaches the set full charge threshold