A lithium-ion battery will still charge (slowly) at very low current. To avoid overcharge you must keep the voltage below 4.23V. Normally this is done by reducing charge current when it gets to 4.2V. I don't know what a 'shunt' battery charger is, but proper Li-ion charger IC's and modules are cheap and readily available.
Now that you have your preferred gadget take a seat, and let’s explore the world of lithium-ion battery charging. Rechargeable power sources like lithium-ion batteries are quite popular because of their lightweight and high energy density. Lithium ions in these batteries travel back and forth between two electrodes when charged and discharged.
Lithium-ion batteries operate differently. They charge under a constant current and switch to a continuous voltage later in the charging cycle. The charging process reduces the current as the battery reaches its full capacity to prevent overcharging.
Another easy way to charge a lithium battery is to use Linear Technology’s LT3650, a DC-DC converter with integrated lithium-ion battery charger intelligence. A designer adds a few passive components to implement a complete lithium battery charging solution.
Using a car charger made especially for your device, you can charge your lithium-ion battery in your car. But it’s crucial to ensure the vehicle charger delivers the right voltage and current for your battery. What are some ways to make my smartphone’s battery last longer?
The notion that lithium-ion batteries should constantly be fully recharged to 100% before use is another myth. Data shows that partial charges can be more beneficial. According to Battery University, lithium-ion batteries do not require a complete charge cycle, and partial discharges with frequent recharges are preferable.