18650, charge and supply at the same time.

Question

I am planning to use a NodeMCU with a couple of encoders to determine the position a telescope is pointing at. While running, the device will be connected to wifi all the time since it needs to interact with an app (or maybe on AP mode so a tablet/phone connects to it).

In the future I may also add a barometric pressure (BMP280) and temp/humidity (DHT11) sensors, gps, and a touch lcd. Probably wont have enough pins for this, but maybe using a MCP23017 could work. Anyway these are all just ideas, to start I will only use the 2 encoders. I haven't even received any of the devides yet, so in the meantime I'm reading and thinking of stuff I could do.

The device will have to be mobile since it may be used in places where there's no electricity, so I am thinking on powering it up with some 18650 batteries I have. I want it to work for about 4-6 hours before having to recharge it, and the device itself must have a charging (usb) port. I also need a 5v line since the encoders work with 5-24v (I choose to use the min 5v since it would be easier to use a logic level converter for input in the nodemcu). So I thought of doing something like this: https://i.imgur.com/qYO3LEL.png

Pinout is not important right now, just the concept.

Would it even work? What will happen if I turn the switch on while the charging usb port is plugged in? Any simpler solutions? The charger protection board would be this one.

michaelkellett · Answer

The basic idea is OK but you need to consider several factors:

1) can the usb supply manage the current - the up and down converters will be less than 100% efficient and you will need to provide enough power to charge the battery and operate the system at the same time.

2) you need to properly control the charging of the batteries - the protection board does not do this - it only (if it works at all) isolates them if they are overcharged or discharged. It is hard for the charger to know how much current is going into the battery and how much into the load.

3) make sure the 5V down converter can cope with 9V of input.

If you do the wrong things to lithium batteries they can explode or catch fire - if you buy unbranded electronics via 'alice1101983' please test it very carefully !!!

The same applies to the charger and converters if you buy them from the same kind of sources.

MK

genebren · Answer

You can charge the batteries and power your device at the same time given an appropriate design. Given that you have 5 volts devices and 4.2V (4.2 to ~2.5) battery, you will most likely need a boost circuit to produce the 5volts. If you add a dual schottky diode inline with battery and the +5V (usb line), you can steer power into you circuit without inferring with the charging of your battery. In the schematic fragment above is a circuit that allows charging and usage simultaneous. V+ is the USB charging voltage and VBAT is the battery voltage the dual schottky diode (label D1G$1) steering the higher of the two voltages on to C4. The voltage across C4 would be feed into the boost and other power regulators to power up your electronics. Keep in mind that the USB input will have to provide enough power for both charging and normal operation. I was managing this by switching in different resistors to idle back the charge when the rest of the circuit was active. I hope this helps. Gene

18650, charge and supply at the same time.

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