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I am a mechanical engineering student and not that familiar with electronics.

I am currently trying to establish a photovoltaic (PV) system that can charge a 3.7V 100mAh lipo battery and deliver power to a load (1.4V, 0.9mA) consistently (when there's sufficient input from PV, the battery charges and the load will be supplied by the PV <- charge mode; when there's insufficient input, the battery will discharge to supply the load.)

I have browsed some previous discussions and understand that if I want to charge a lipo battery safely according to the suggested charging algorithm, I should connect the solar panel, for example, 6V PV panel to a charging module like TP4056, and then connect the load and TP4056 with an LDO regulator to regulate the output voltage to 1.4V. However, people are suggesting that the load of the TP4056 should be disconnected while charging otherwise the TP4056 will not work properly. (https://www.best-microcontroller-projects.com/tp4056.html).

Is there any method to achieve my smart loading goal? (Providing consistent power to load either by battery/PV depending on the availability of input source.)

In this link, there is a method of using a 5V relay but as long as my input is a solar panel and is preferably a tiny system, the method should not be applicable. Furthermore, James suggested adding a pair of diodes in the circuit. However, I am a bit confused about "Add one diode between the TP4056 IN+ and load VIN+." Where exactly should I add it in the schematic? I am a bit confused about the principle behind this method.

This BQ24074 chip BQ24074 seems can achieve the goal but I am not so sure if it suits me as the maximum charging current can only be lowered to 500mA which is still much higher than 1C (100mA) of the battery I chose.

enter image description here

As an electrical engineering noob, I might have asked some silly questions, but I really hope someone can give me some insight into constructing this system.

updated photo for O'ring circuitenter image description here enter image description here

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    \$\begingroup\$ If you choose a solar panel that can only source 100mA then current-limiting won’t be a problem for charging. \$\endgroup\$
    – Frog
    Apr 23, 2021 at 5:51
  • \$\begingroup\$ @Frog Thx! Do you mean ideally if I choose the right panel, I can use bq2407 for battery management and charge control together and connecting a 1.4V regulator to the load? \$\endgroup\$
    – BennyJohn
    Apr 23, 2021 at 20:35
  • \$\begingroup\$ Not quite - you’ll need a way to make sure the load doesn’t discharge the cell below 3V. \$\endgroup\$
    – Frog
    Apr 23, 2021 at 22:41
  • \$\begingroup\$ Oooo I see, I thought the bq34074 board will also act as a charge controller which regulates the charging V, I value, and also avoid the battery to discharge when it is drained. Can I just add a diode between the battery and the bq24074 board such that only when the battery is 3V up can discharge, but will this disable the charging function? Thx a lot! \$\endgroup\$
    – BennyJohn
    Apr 24, 2021 at 2:01
  • \$\begingroup\$ Yes that looks it it would do the job - the BM chip will prevent over-or under-voltage, and if the panel is incapable of supplying excessive current or voltage then you’re good. \$\endgroup\$
    – Frog
    Apr 24, 2021 at 3:22

3 Answers 3

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To switch between battery and PV array based on availability, you can use an active OR'ing controller. TI, for example, has a wide variety of those and some designs using discrete components have been discussed on this forum as well.

This will allow you to have a battery charging stage and a PV stage. When there will be sufficient power going from the PV panel, your load will draw it from there and not the battery; when there will be no PV power the load will draw power from the battery.

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  • \$\begingroup\$ Thank you so much for your reply! It gave me some insight! I have looked into the links you provided and understand that this controller can switch between voltage sources accordingly. But I am a bit confused about how the PV charging battery function is established in this case. I assumed the connection to the O'ring controller is as shown in the updated photo. Then how should I connect the whole system with TP4056 to perform the charge controller function. Thanks a lot! @Mu3 \$\endgroup\$
    – BennyJohn
    Apr 23, 2021 at 20:35
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The feature where the system load is prioritised over charging the battery, and the battery acts as a backup, is often called PowerPath by charging IC manufacturers. You'll find a number of battery charger chips that have it. If you can find one that has suitable specs, it's a matter of interfacing with the solar panel (there are even a few chips that do powerpath, charging and some basic form of MPPT on the input), and lowering your output voltage to 1.4V (most of these chips assume that the system voltage is normally close to or above the battery voltage).

[edit] There's actually an Adafruit part based on the MCP73871 that might fit your needs.

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  • \$\begingroup\$ Thank you so much! It is really useful for me to have the keyword 'PowerPath' to search for charging IC, I end up finding a chip called MAX77734 (mouser.co.uk/new/maxim-integrated/maxim-max77734-pmic), I don't know if it's suitable or not as there's only a 5V input, also it's is only a PMIC chip which needs extra sophisticated board design such that it can be used properly? (I am not sure if this is right or wrong as I can't see any existing charging board of MAX77734 in the market) \$\endgroup\$
    – BennyJohn
    Apr 24, 2021 at 21:32
  • \$\begingroup\$ Also, it seems that the Adafruit bq24074 board is the successor of the Adafruit part that you have mentioned. And it seems it might fit my needs as long as I can add deep discharge protection for the battery. If that's the case, I am wondering if it is possible to create a Simulink model to evaluate the system performances (e.g. simulate a varying irradiance condition with a constant 0.9mA current drain from the load to evaluate wether the system can be sustained). Would this be possible? \$\endgroup\$
    – BennyJohn
    Apr 24, 2021 at 21:42
  • \$\begingroup\$ I tried to follow this video youtube.com/… to create the model, but it seems a bit different to my situation as in his case the MPPT boost converter boosts the PV voltage to above 48V to charge a 24V battery and supply a 48V load, whereas in my case I have a 6V PV, 3.7V Battery and1.4V load, the bi-directional buck-boost battery controller doesn't fit as mine is some sort of buck-buck? Thank you! \$\endgroup\$
    – BennyJohn
    Apr 24, 2021 at 22:25
  • \$\begingroup\$ One issue with the MAX77734 is, it's a tiny smartphone chip that's going to be difficult to solder if you don't have access to relatively advanced PCB manufacturing. \$\endgroup\$
    – DamienD
    Apr 26, 2021 at 14:48
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I am currently trying to establish a photovoltaic (PV) system that can charge a 3.7V 100mAh lipo battery and deliver power to a load (1.4V, 0.9mA)... for example, 6V PV panel to a charging module like TP4056, and then connect the load and TP4056 with an LDO regulator to regulate the output voltage to 1.4V.

With a 6 V panel and 1.4 V load there is plenty of voltage 'headroom', so you can simply use diodes to feed the regulator from the highest voltage source.

A '6V' solar panel typically puts out ~7.2 V max, so it should be safe to power the TP4056 directly from it. The Lipo battery should not be discharged below 3.0 V, which means the LDO will get at least 2.4 V from it when connected via a suitable silicon diode, providing 1 V or more of headroom. The solar panel (when illuminated) should deliver at least 5.4 V through a diode to the LDO.

Alternatively you could just connect the solar panel to the TP4056 and the LDO to the battery. The TP4056 is designed to stop charging when battery current falls below 10% of the set charge current. At 50 mA (C/2 for a 100 mAh battery), it will stop charging when current goes below 5 mA. If the LDO draws much less than this it will not prevent the TP4056 from working properly. If your load only draws 0.9 mA and the LDO's quiescent is low then it should work fine.

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  • \$\begingroup\$ Thank you so much for the reply! May I know how the 2.4V for LDO is calculated 1.4V load+ 1V headroom?), so one 3V diode between Battery, TP4056 to prevent deep discharge then where should another diode be placed to choose the highest voltage source? And will the diode prevent the battery from charging? For the second method, is the LDO connected between battery and Tp4056? I originally thought that the LDO is connected between the load and TP4056 instead. Thank so much! \$\endgroup\$
    – BennyJohn
    Apr 24, 2021 at 21:11

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