I have an arduino pro mini 3.3V and I have a 18650 3.7V li-ion battery I want to use to power it. One main part of my project is that I want to basically trickle charge the battery while it powers the arduino to see how much the battery will charge and how much I can extend battery life. I have solar cells that can harnest power from indoor lighting, one cell has about 2.4 - 2.7V when under light.
A good number of the solar projects I have seen seem to power the arduino primarily from the solar cell while the cell also charges a battery, including a AEM10941 evaluation board I have, but my cells cant drive enough current to power the arduino and I want it run on the battery primarily.
Please, what device or setup will you recommend for me to add here? Maybe a TP4056 but I'm not sure if it will need additional parts or if there's enough current here?
I do not have any components to solve this yet, I am unsure how to plan what I need to help with managing the charging of the li-ion battery so it doesnt overcharge or discharge too much. What will you suggest for managing current please?
That circuitry exists and is discussed here. I think it uses a TP4056 but also another chip. Many web sites get this wrong, there is a thread here that gets it right. There are actually several threads but I think this is what you want. You will need to read through the entire thread in order to get all other context and subtleties. ShermanP is in my opinion the expert in this field. https://forum.arduino.cc/t/at-last-18650-charge-boost-module-with-load-sharing/1164462/36
Here is general description from datasheet I saw:
The CN3791 is a PWM switch-mode lithium ion
battery charger controller that can be powered by
photovoltaic cell with maximum power point
tracking function with few external components.
The CN3791 is specially designed for charging
lithium ion batteries with constant current and
constant voltage mode. In constant voltage mode,
the regulation voltage can be fixed at 4.2V with ±
1% accuracy. The constant charge current is
programmable with a single current sense resistor.
Deeply discharged batteries are automatically
trickle charged at 17.5% of the full-scale current
until the cell voltage exceeds 66.5% of constant
voltage. The charge cycle is terminated once the
charge current drops to 16% of full-scale current,
and a new charge cycle automatically restarts if the
battery voltage falls below 95.5% of regulation
voltage. CN3791 will automatically enter sleep
mode when input voltage is lower than battery
voltage.
Other features include under voltage lockout,
battery over voltage protection, status indication.
CN3791 is available in a space-saving 10-pin
SSOP package.
Going through that thread some, it seems the goal is for the load to be powered by USB when it is connected instead of battery but I want mine to always be powered by the battery. Currently looking into the UPS modules being discussed in that thread.
Be sure they correctly identify what is a UPS and what is an SPS. An uninterruptible power supply and a stand-by-power supply. All low priced devices are SPS, but advertised as UPS! An SPS uses a relay to switch battery power to the AC power generation circuitry. But a real UPS always supplies the AC power using the batteries. A separate circuit keeps the batteries charged and ready to go at all times.
You will have to help me understand your thinking about how to combine the charging and the battery and powering the load without the charging also being connected to the load? The logic somehow evades my thinking.
I'm not sure but there looks to be "SYS OUT" that is seperate from the battery and solar input. My idea is basically like how we can use our phones while they charge unless our phones are also being powered through USB when charging
The CN3791 does not include any load sharing circuit. And it requires the cell voltage to be higher than the battery voltage. So you would need to use at least two of your solar cells in series, or probably three.
But it's not clear that you need a load sharing circuit. It depends on the characteristics of your panels (open circuit voltage and short circuit current), and the current draw of your Pro Mini circuit. If the load current is less than the charge termination current, or even if it's higher but can be periodically brought lower using sleep mode, then you really don't need load sharing circuit. You would just power the load from the battery.
The disadvantage of doing that is that when charging terminates, the battery will take over supplying all the load current even though the panels are still capable of sourcing current. So your battery will cycle back and forth between 4.1V and 4.2V.
But as long as the battery is not fully charged, the panels will contribute as much current as they can at the battery voltage.
Anyway, we need to know more about your panels and your circuit's current requirements.
You could use a TP4056. A CN3791 would probably be more efficient, but I don't know how much more. Adafruit makes a solar charger that does include load sharing, but of course it's expensive.
You need 3 panels in series to start with. Then you should test how much you can get out of them with your average light conditions. In practice you need some sub milliwatt adjustable load to play with.
I asked Google about charging lithium-ion batters and got this response:
V= Lithium-ion (Li-ion) batteries are charged using a two-stage process: Constant Current (CC) and Constant Voltage (CV). The CC stage brings the battery voltage up to the target level, followed by the CV stage where the charging current gradually decreases as the battery voltage reaches its maximum. Proper charging is crucial for maximizing battery lifespan and performance.
Doe your charging scheme come close to this specification?