Battery Charging Circuit - Prototype

[digitaldanny]

Describe the solution you'd like

• Implement CC and CV circuits. These should just be some linear voltage regulators with appropriately placed resistors.
• Be able to switch between CC and CV circuits when battery reaches the voltage threshold.
• Be able to disconnect the circuit from the battery if CV stage current reaches minimum current threshold.
• Include LEDs to show the charging mode (CC, CV, OFF).

Additional context Research for this circuit was completed in #25.

[digitaldanny]

Instead of using voltage regulators as constant voltage and constant current sources, I could use a current mirror for the constant current source.

[digitaldanny]

Parts list

Part Number	Description	Quantity
PRT-13853	3.7V 110 mA Li-ion Battery	1
LM339N	Quad Linear Comparator	1

[digitaldanny]

• The PRT-13853 battery's datasheet shows that there is a max charge current of 1C = 110mA. May want to drop this to 0.5*C just to be safe, but I will tune the current mirror in the CC circuit to provide this 1C or 0.5C of current.
• The datasheet also shows that the end-of-charge current is 0.01*C. So I need to disconnect the circuit at the 1.1mA threshold.

[digitaldanny]

Originally, I was planning on using a direct comparison with the battery voltage and a 4.2V reference for switching between CC and CV circuits. The problem with direct comparison for a slowly changing voltage is that the comparator output will probably bounce around between low/high when the battery voltage gets to 4.2V. The Schmitt trigger action uses a comparator to produce stable level-crossing switches in contrast to the action of a straight reference comparison.

Using a Schmitt trigger, once the battery crosses the 4.2V threshold, it will switch to the CV circuit. The battery would have to drop to a much lower voltage level before the CC circuit is activated again (I'm thinking around 3.9V??). As shown in the simulation below, I have found values for R_offset, R1, and R2 on the op-amp's positive terminal that creates a Vutp = 4.2V and Vltp = ~3.9V. Ideally, I can use this circuit to control whether CC or CV circuits are connected to the battery.

[digitaldanny]

I'm testing the Schmitt trigger on a breadboard using a UA741CP general purpose op-amp. I am using potentiometers for R_off and Rb (in the LTSpice schematic above). Messing with those potentiometers, I can produce Vutp = 4.2V and Vltp = 3.9V. However, I'm having trouble getting Vout to swing between min and max output voltages. I'm assuming the positive feedback is keeping the high internal gain (A) from swinging the outputs high and low.

I would really like to be able to implement the Schmitt trigger with 1 op-amp since I think I will need 2 of them. However, I can probably solve this problem using an additional op-amp as a direct reference comparison. The output could swing high when the Schmitt trigger's Vout > 3V.

Vin (yellow) vs Vref (blue)

Vin (yellow) vs Vout (blue)

[digitaldanny]

Building a CCCV circuit with linear regulators

[digitaldanny]

Current Sense Amplifiers - YouTube

• Low-side current sensing (shunt resistor below the load) is usually avoided because the load voltage drop is floating. Because of this, the voltage above the shunt resistor will vary, so the ground reference seen by the load will be off.
• Current sensing with op-amps might be problematic since most op-amps don't actually swing from rail-to-rail. This means the negative terminal would need a slightly negative voltage to swing all the way down to 0V.

[digitaldanny]

Spice model seems to be working correctly. The Schmitt trigger correctly senses 3.7-4.2V thresholds, controlling some npn transistors to switch between the CC and CV circuits. The termination current sensing is working in simulation, but I am a little skeptical that it will work in practice. I did a low-side shunt resistor to remove the possibility of Constant Mode Rejection Ratio (CMRR). This StackExchange thread does a good job of explaining this issue. I will try breadboarding the circuit for high-side sensing to avoid the change in ground reference for the battery, but I will change back to low-side if my high tolerance (~5%) resistors are causing CMRR issues at the op-amp input.

[digitaldanny]

HW Prototype Parts List

Part | Quantity | Ordered?

• LM324N 4-Channel General Purpose Op Amp | 10 | yes
• [LM741CN 1-Channel General Purpose Op Amp]() | 10 | yes
• Shunt Resistor 0.1 Ohm 3W 1% | 5 | yes
• LM1117T-5V Linear voltage regulators | 5 | yes
• PRT-13853 SparkFun Li-ion battery 3.7V 110mA | 2 | yes
• 2750 Adafruit Lithium battery 3.7V 350mA | 1 | yes

These parts may not be needed, but I'm ordering them just in case.

• PN2222A NPN BJTs | 10 | yes
• PN2907ABU PNP BJTs | 10 | yes
• OPA350PA Rail-to-Rail Op-Amp | 2 | yes
• Low tolerance resistors for high-side sensing?

[digitaldanny]

Modified the LTSpice simulation to show the battery voltage increasing with a logarithmic curve like it will in the real world. This has shown me that I have 3 issues with my current design.

• (Fixed) The CC current is not constant as the battery charges up to 4.1V. Removing R3 fixed this issue, but I am not sure why.
• The CV current does not decrease as expected. Instead it stays at a minimum value. This is an important step to fix, which is also blocking a test to make sure the termination current is detected properly. The screenshot below shows the current I_battery (blue) during the CV state and the expected I_battery (dotted blue).
• The current sensor circuit does not work at all when plugged into the battery. Need to investigate further.

[digitaldanny]

I think it might be possible to get rid of the voltage regulator in the constant voltage supply by using a voltage divider and voltage follower.

[digitaldanny]

(1) Built the Constant Voltage circuit on the breadboard. Had to redesign this circuit since I was seeing too much voltage drop across the npn transistors to meet the 4.2V requirement. Also ended up having to use one of my 2 rail-to-rail op amps for the output buffer. Redesign for CV circuit is below.

I calculated what I believe the battery's min / max internal resistance will be in this stage by using Ohm's law on the max (55mA) / min (1.1mA) possible current draw. The formula I used is R = V/i.

R_batteryMin = 4.2V / 55mA = 76 Ohms. R_batteryMax = 4.2V / 1.1mA = 3.8k Ohms.

I tested the CV circuit's output with Rload ranging between the values calculated above. It is easily supplying 4.2V at anything above ~150 Ohms. Once the load drops to 76 Ohms, the output voltage is only around 4V instead of 4.2V. I don't think this will be a problem since the current draw should drop very quickly in this stage as the battery charges up.

(2) Also had to change the CC circuit design. The basic current mirror circuit would slowly rise over time (after ~10 seconds, it could rise by over 100 mV). Based on what I've read online, this is actually normal for current mirrors trying to output small currents. A possible fix for this is the Wilson Current Mirror, which uses 3 transistors. I went with another approach that uses 1 transistor, and it seems to be outputting ~500 mA for varying loads right now. Will do more testing tomorrow.

(3) I am concerned about 2 things in the current design..

• How will the CC circuit behave when the Schmitt trigger ranges from 0-3.5V instead of 0 - 5V (the Schmitt output will use the LM324, which can only swing from 0 - Vcc-1.5). I think the current mirror might not turn fully off in this case.
• Is it an issue to connect the CC and CV outputs directly? I'm not sure if the CC transistor and the CV FET will switch quickly enough to avoid breaking the lines. Maybe a small resistance or diode would be beneficial here?

[digitaldanny]

Seeing an issue I hadn't thought of before with the Schmitt trigger. The max output voltage for the LM324 is about 3.8V. When I decrease Roff1 enough to produce a Vupper = 4.15V, the output stays fixed (doesn't produce a Vlower). I think a possible solution is to divide the Vbattery voltage and sense for 2.1V-1.85V instead of 4.2V-3.9V. This allows the Roff1 value to be much larger.

[digitaldanny]

• How will the CC circuit behave when the Schmitt trigger ranges from 0-3.5V instead of 0 - 5V (the Schmitt output will use the LM324, which can only swing from 0 - Vcc-1.5). I think the current mirror might not turn fully off in this case.

In test, this was actually problematic. As expected, the current was not switching on/off fully. I fixed this by adding a MOSFET with the Schmitt trigger on the gate that allows current to flow through when the gate voltage is above the Vthreshold.

• Is it an issue to connect the CC and CV outputs directly? I'm not sure if the CC transistor and the CV FET will switch quickly enough to avoid breaking the lines. Maybe a small resistance or diode would be beneficial here?

Yes, this is problematic. The CC current would be max drawn by the OPA350's low output impedance. I was able to force the current flow direction with diodes D3 and D4.

New Concerns/Thoughts

• The OPA350 can output a max of +-40mA. My circuit is possibly getting away with this right now because my current battery charges at around 50 mA, so the voltage is only dipping during the first few minutes of the CV stage. However, I will be switching to a larger battery that needs to charge using a higher current. The OPA350 will not be able to supply close to this much output, so I need to think of another solution. Maybe look into using the LM386 if the dropout isn't too high?
• The PNP transistor and diodes in the CC circuit (Q4, D1, D2) need to have higher power ratings to support higher current sources. Might want to look into using the Wilson Current Mirror since, I do not think I will be able to produce much higher currents with my current circuit design (Iout = 0.7/Re).
• Use LEDs instead of D3/D4 to notify which mode the charger is in. Need to make sure the LEDs are appropriately power rated for the amount of current going through them (50 mA - 600 mA?).

[digitaldanny]

Updates:

• Upgrading the power supply to a 10V-DC wall charger. This will help with all the power related issues I dealt with before.
• The LM324n can operate with 10V-DC and has a 3V dropout on the output voltage. Changing the power supply eliminates the need for the OPA350 (rail-to-rail op-amp) for the 4.2V CV circuit and also removes an extra IC from the charger PCB.

[digitaldanny]

Circuit Truth Table	Vbat	Ibat		Schmitt	Thresh	Circuit Mode
<4.2 V	*	---	1	*	CC
4.2 V	>1.1 mA	---	0	0	CV
4.2V	1.1 mA	---	0	1	Disabled

[digitaldanny]

Updated CC-CV charger circuit schematic: