Li-Ion cells are probably the most complex when it comes to charging them, because these cells are quite vulnerable to overcharging, and tend to get hot at unfavorable conditions. The following post explains a couple of simple yet a safe way of charging a Li-ion cell which can be easily constructed at home by any new hobbyist.
Advantage of Li-Ion Battery
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.
1) Using LM317 as the Controller IC
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.
Using Temperature Detection as the Parameter
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.
Employing an SCR for the Cut-Off
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.
- The basic criteria that needs to be maintained for any battery are: charging under convenient temperatures, and cutting off the supply as soon as it reaches the full charge. That's the basic thing you need to follow regardless of the battery type. You can monitor this manually or make it automatic, under both cases your battery will charge safely and have a longer life.
- The charging/discharging current is responsible for the temperature of the battery, if these are too high compared to the ambient temperature then your battery will suffer heavily in the long run.
- Second important factor is never allowing the battery to discharge heavily. Keep restoring the full charge level or keep topping it up whenever possible. This will ensure that the battery never reaches its lower discharge levels.
- If you find it difficult to monitor this manually then you can go for an automatic circuit as described on this page.
2) Li-Ion Battery Charger Circuit Using IC 555
The second simple design explains a straightforward yet precise automatic Li-Ion battery charger circuit using the ubiquitous IC 555.
Charging Li-ion Battery Can be Critical
A Li-ion battery as we all know needs to be charged under controlled conditions, if it's charged with ordinary means could lead to damage or even explosion of the battery.
Basically Li-ion batteries don't like over charging their cells. Once the cells reach the upper threshold, the charging voltage should be cut off.
The following Li-Ion battery charger circuit very efficiently follows the above conditions such that the connected battery is never allowed to exceed its over charge limit.
When the IC 555 is used as a comparator, its pin#2 and pin#6 become effective sensing inputs for detecting the lower and the upper voltage threshold limits depending upon the setting of the relevant presets.
Pin#2 monitors the low voltage threshold level, and triggers the output to a high logic in case the level drops below the set limit.
Conversely, pin#6 monitors the upper voltage threshold and reverts the output to low on detecting a voltage level higher than the set high detection limit.
Basically the upper cut off and lower switch ON actions must be set with the help of the relevant presets satisfying the standard specs of the IC as well as the connected battery.
The preset concerning pin#2 must be set such that the lower limit corresponds to 1/3rd of the Vcc, and similarly preset associated with pin#6 must be set such that the upper cut off limit corresponds to 2/3rd of Vcc, as per the standard rules of the IC 555.
How it Works
The entire functioning of the proposed Li-Ion charger circuit using IC 555 takes place as explained in the following discussion:
Let's Assume a fully discharged li-ion battery (at around 3.4V) is connected at the output of the below shown circuit.
Assuming the lower threshold to be set somewhere above the 3.4V level, pin#2 immediately senses the low voltage situation and pulls the output high at pin#3.
The high at pin#3 activates the transistor which switches ON the input power to the connected battery.
The battery now gradually begins charging.
As soon as the battery reaches full charge (@4.2V), assuming the upper cut off threshold at pin#6 to be set at around 4.2v, the level is sensed at pin#6 which immediately reverts the output to low.
The low output instantly switches off the transistor which means the charging input is now inhibited or cut off to the battery.
The inclusion of a transistor stage provides the facility of charging higher current Li-Ion cells also.
The transformer must be selected with voltage not exceeding 6V, and current rating 1/5th of battery AH rating.
If you feel that the above design is much complex you could try the following design which looks much simpler:
How to Set up the Circuit
Connect a fully charged battery across the shown points and adjust the preset such that the relay just deactivates from N/C to N/O position....do this without connecting any charging DC input to the circuit.
Once this is done you can assume the circuit to be set and usable for an automatic battery supply cut off when fully charged.
During actual charging, make sure the charging input current is always lower than the battery AH rating, meaning if suppose the battery AH is 900mAH, the input should not be more than 500mA.
The battery should be removed as soon as the relay switches OFF to prevent self discharging of the battery via the 1K preset.
IC1 = IC555
All resistors are 1/4 watt CFR
IC 555 Pinout
Although the designs presented above are all technically correct and will perform the tasks as per the proposed specifications, they actually appear as an overkill.
A simple yet effective and safe way to charge a Li-Ion Cell is explained in this post, and this circuit may be applicable to all forms of batteries since it perfectly takes care of two crucial parameters: Constant-Current and full charge auto cut-off. A constant voltage is assumed to be available from the charging source.
Have further doubts? Please let them come through the comment box below 🙂