Parallel charging is widely used method of charging LiPo batteries for RC. The method was reviewed hundred of times, but to me, almost all the reviewers have some kind of misunderstanding and use “common sense” models like filling the volume with water. This leads to wrong conclusion that parallel charging is safe for any combinations of batteries.
Actually, this simple case of parallel charging can be easily modeled with simulators, like free Texas Instruments TINA software (available at ti.com).
This simple model includes: 1) unit step constant current generator, which providers 2A of current immediately after turning on (charging current from charger), 2) each of the batteries are modeled as a huge capacitor, and internal serial resistor of 0.1 Ohm.
In real life LiPo charging requires constant current charging at 90% of time, and then constant voltage final stage. Here we consider the most important constant current stage.
Case 1. Both batteries are the same (100F) and have 3.3V of initial voltage.
The result is pretty obvious: source current of 2A is equally divided (1A) between each of the batteries, voltage at each battery changes linearly with time (V=Q/C=I*t/C).
When batteries are of the same capacity and discharged equally the charging current is distributed equally between batteries, this is expected and safe.
Case 2. Both batteries are the same (100F) but one of the batteries discharged more (3.3V vs 3.8V of initial voltage):
Since battery 2 has more voltage, current from this battery is added to the source current until both batteries are charged to the same voltage. On the plot below, battery 1 starts charging with current of 3A, battery 2 is being discharged at current -1A (opposite direction). The voltage shows that battery 2 loses voltage at the initial stage and starts gaining when battery 1 is charged to the same level;
in contrast to Case 1 this scenario is not safe: when batteries are charged differently, charging current of the battery with lower charge can be multiples of desired current. In the above example we have 3A current instead of expected 1A.
Case 3. Batteries have different capacity (100F and 300F) but both batteries discharged equally (3.3V of initial voltage):
Here we have situation, when one of the batteries always have higher voltage (larger capacity requires more time to get the same voltage). At the initial stage the both start at the same current of 1A, but then battery 2 (larger) charges at higher current of ~1.5A while the other (smaller) has charging current of ~0.5A.
Equally discharged batteries of different capacity have different charging current. This is safe because larger battery can be charged at higher current. Charging current will be safely redistributed between batteries in accordance with their capacity.
Real life situation is a mix of Case 2 and Case 3. I would recommend parallel charging of batteries of about the same capacity and discharge level. Avoid situation, when one of the batteries is discharged significantly more than others (this may produce unexpectedly high currents and destroy the battery and cause fire).