18650 in series while charging using TP4056 and connecting two buck regulators LM2596 in parallel, is it safe?

I'm building a robot which contains a battery system. The 3x18650 batteries are connected in series and are charged using TP4056 boards. The voltage is then fed into the circuit shown below. The voltage dividers and power resistors assist in obtaining the voltage and current of the system.

My questions are:

1. Is connecting the batteries in series while charging a bad idea?

2. The buck regulators have a maximum current output of 3A, that is why im thinking of connecting two of these boards in parallel to share the load, is it safe to do this?

3. Any thoughts of improving the system.
4. What is the simplest way to get the Arduino to know if the batteries are being charged or not?

• @Nick Alexeev Good to know that two buck converters in parallel are safe. Thank you sir – Ibrahim Sep 16 '18 at 22:58
• Thank you for the datasheet link. What else is on the charge controller board, besides a TP4056 and associated discrete components? The picture shows what looks like a micro-USB connector. Is it powered by USB? – Nick Alexeev Sep 16 '18 at 23:02
• @Nick Alexeev the TP4056 are popular for charging 18650 batteries. They have a micro usb port where you plug in your cable just like charging your phone. It has some resistors and indicators, its working i dont fully understand – Ibrahim Sep 16 '18 at 23:09
• Related threads about connecting buck converter outputs together: Effect of using buck converters in parallel and Is it ok to connect the output of buck regulator in parallel? – Nick Alexeev Sep 16 '18 at 23:24

The batteries can NOT be connected in series when charging. This will short out the battery. This is because the ground is not floating on USB hubs. Gnd will short out the lower V+ connected to it.

You would need 3 isolated USB wall chargers capable of 5W or 1A or more to charge in series.

You may add 2 LEDs with the anode (+) to 5V to indicate charging with the cathode (-) to 330 ~ 1K series R to CHRG low when active and STDBY low when done.

An actual current of 10mA is adequate which is V/R=I

The charge current depends on the onboard PROGramming resistor into a current amplifier. $I_{BAT}=\dfrac{1V*1200}{R_{PROG}}$ thus 1.2k = 1A

The battery chargers are linear regulators and only need 4.2V in but 5V is ok but gets warmer with 1000mA and may need heatsink from 3V discharged cell. (5-3)*1A=2W

If your USB Hub cannot drive 1000mA then it is slow to charge up.

Your LM2596 will not generate split +/-5V supplies. Yes, it is true the intermodulation or having each Buck supply interference with simultaneous charge currents is not a good idea.

Although it is a good idea to decouple the motor power from logic Vdd and Vss but not with this PS design. Go to ti.com and register your email and choose a triple output design from their "Webbench" online in the browser with Java approval or Analog Devices.

Often a 3 secondary winding forward converter works best on a custom coil form. By tight mutual coupling of coils, the ratios of voltage are maintained while regulating off one for feedback. This is how PC PSU's generate all their voltages.

• Thank you sir. The batteries are connected in series for the circuit but each battery positive and negative terminal is connected to its individual TP4056 boards. Can you emphasize more. I will add 2 leds to indicate with the resistors range you defined, thats a good idea. – Ibrahim Sep 16 '18 at 23:06
• the regulators are buck converters which use switching to regulate voltage. They give 5V output. I did not choose linear regulators because they waster power which is not good for battery powered system. Because the buck converters work by switching, i was wondering what if i used two in parallel, because some say its not safe – Ibrahim Sep 16 '18 at 23:12
• I was referring to the TP4056 linear regulators/ – Sunnyskyguy EE75 Sep 16 '18 at 23:20
• Thats true, i understood why i dont connect the batteries in series. I will use a relay to isolate the batteries during charging. The indicate idea is well understood, i will implement it too. Im still a beginner, somethings you mentioned, i did not understood. Thank you Tony – Ibrahim Sep 17 '18 at 2:04
• make sure the Relay contacts are rated for the DC current being switched. and it might be best to use 12V motors and MOSFET full bridge shields unless going with low power 5V motors. Start power is always 10x rated power at full voltage unless current regulated by PWM. – Sunnyskyguy EE75 Sep 17 '18 at 2:35

[ We are missing a schematic for the little charger boards.
In lieu of that, I'm going to assume that this charger board doesn't have galvanic isolation (or something equivalent) between the USB and charger output. ]

The proposed charging arrangement isn't going to work, because the USB ports aren't floating1 with respect to each-other. The grounds of the USB ports are connected together at the hub. The battery ground of each charger board is connected to the USB ground. That's okay when you have only one cell. When you have two cells each with its own charger board like that, you will be connecting the node between the cells to ground. So, you will be applying a short across one of the batteries.
At best, a circuit breaker on the hub will open. At worst, you will toast the charger chip, or the USB hub.

By the way, do your cells have built-in protection?
I would recommend to abstain from using unprotected Lithium-ion and Lithium-polymer cells.

1 Isolated USB hubs do exist, but they are an exotic.

• Now i understand why its not good to connect them in series. How can i use one TP4056 board to charge all the 3 batteries together and at the same time feed the total voltage of the batteries to the circuit? – Ibrahim Sep 16 '18 at 23:38
• Yes they are of protected type – Ibrahim Sep 16 '18 at 23:40
• That gave me an idea, i will isolate the batteries during charging mode. I will use a relay with normally close for batteries in series, when charging, the power from usb power supply will turn on the relay, thats making it open for batteries in series and closed for charging. Im still wondering what to do with buck converter in parallel. If you know, do let me know. Thank you Nick – Ibrahim Sep 17 '18 at 2:01

It won't work.

TP4056 modules connect ground between input and output, so they cannot be "stacked".