As an overview of what I am trying to achieve. I want to create a circuit that will trickle charge 8 x NiCd batteries (2200mAH) so that in the event of an “occasional” power loss my 5v circuit is provided power for around 12 hours (duration tested @ 16 hours on fully charged batteries). If the voltage of the NiCd batteries becomes too low to support the circuit then the power to the ESP32 is cut via the MOSFET. When the AC power is restored the batteries start to recharge and at a suitable voltage the power to the ESP32 is also restored. The ESP32 also monitors the NiCd batteries for Temperature and Voltage only for reporting purposes at the moment.
I testing everything on the breadboard with the exception of the Battery Monitoring (LM35’s and Voltage Divider), everything seemed to work perfectly. Power to the ESP32 was cut when the battery fell to 8.81 volts, then after applying the AC powerpack back to the circuit the battery would recover to 8.95 volts and switch the ESP32 back on which would boot correctly and control my device again. Everything seemed good :) - Refer to Picture 1.
However, adding monitoring to the NiCd batteries (LM35 and Voltage Divider) gave me some strange results when the battery hit the low voltage condition and the Q1 MOSFET turned off. While I did not hang around before removing power I believe D1, R8, and D3 started to get very hot. At this stage I’m not sure if the ESP32 was damaged also.
I think the issue is regarding the grounding of the battery monitoring components? What I think I need to do is take the grounding from the Drain side of the MOSFET so that when the battery is in a low voltage condition the battery monitoring is also disconnected. – Refer to Picture 2
Does anyone see any issues or could offer me any advice?
Note: I do intend to review the type of charging that I am doing, maybe using one of the dedicated IC’s (i.e. MAX713) but for now I would like to get the basics working. I assume currently this circuit would take several weeks to recharge the batteries…
As an update this has worked perfectly (referencing the MOSFET to drive the GND feed) for a number of weeks now in bench testing with figure 2. When the battery voltage drops the ESP32 (a 32bit micro-controller similar to arduino) switches off and on battery recovery it switches back on. I have played around with the hysteresis values though in figure 2 and the Anode side of D3 now connects to the Anode side of D1 (diagram error).
The next step is to have the ESP32 monitor the level of the batteries and switch on and off the charging current as necessary just to save trickle charging all the time - why not since I have spare GPIO ports and the code is simple.
I need to ask another question though because I really am stuck now trying to use 3.3v logic to break the charge feed into the battery while allowing it still to power the circuit.
I have simplified the circuit below in the hope someone can tell me what I should really be doing with Q1 and Q2 before I subject anymore transistors to the magic smoke.
Thanks in advance