I'm afraid that with that dinky little 100mA panel you will be sorely disappointed with the charging speed.
I've tried similar experiments with solar panels and you probably won't break your phone but it will probably refuse the charge altogether and give you something along the lines of a "charging was disabled to prevent damage to your device" ...
The 5V 100 mA panel probably WILL work correctly with some phones and more in parallel will work better.
Some phones will not accept too low a wattage.
You can add a simple clamp regulator to stop the voltage rising above 5V.
A 4.7V zener diode across the parallel cells (or several zeners in parallel for more wattage) will probably work. Place the ...
I would defenetly look at the multimeter readout. It looks like the battery is dead. When the voltage on 12V battery falls <9V it get's permanently damaged.
You can attempt to charge the battery using bench PSU, with low current at the start, however the capacity will be signifficantly lower forever.
I used to repair those batteries with charging them on ...
The takeaway message here is for MPT, all impedances must be matched at all times for all interfaces.
MPPT for most PV's is known to start at 82% of Voc and decline to 72% Voc at 10% Pin. (within 3% per this spec)
The impedance for a true CC is infinite and for a true CV is 0 Ohms.
Yet for a PV the Zo (min) = Voc/Isc = Vmp/Imp
Thus @ MPT Zpv= 18V/0.56A = 32....
In short: To signal the usb device that you are attempting to charge, that it can charge at a current higher than 500mA ,if if wants to, you need to short D+ and D- of the USB connection to that device.
I have read through the comments once again and it seems to me as though this is a misunderstanding of your expected operation of what you have put together.
Is it possible to approximate a battery's current (the "right now"
kind of current) capacity, given its absolute capacity and a voltage
Yes. The relationship between resting voltage and capacity of a Lipo cell is non-linear and varies depending on the chemistry. To get an accurate value you would have to measure the capacity of your ...
There is memory or hysteresis in voltage due to a double layer effect. There is also slight variations in polymer chemistry in particular Fe or iron that affect the relationship.
AFAIK 3.7 is 90% and 3.0 is 10% as a resting voltage.
ESR affects voltage drop or rise due current x ESR and this rises faster as SoC drops near and below 10%.
Peukert’s Law ...