Can a lithium battery pack with a 3s 20A BMS be charged from a 12V 3A power supply?

No, it is not wise to use a 12V 3A power supply to charge a LiPo battery pack with a 3s 20A BMS.

1. LiPo batteries should be charged using a constant current source, not a constant voltage source like a power supply. Using the power supply in this manner can cause problems such as overcurrent which causes the power supply to overload and shut down, then restart and repeat the process. In order to properly charge this battery pack, you should use a 3s lithium battery charger specifically designed for this battery configuration, rather than a general-purpose power supply.
2. If you still wish to use a power supply, you may consider placing a resistor in series to limit the current, although this method may only achieve a maximum of 80% charge. However, it is worth noting that using the power supply in this way may not be the most efficient or safest method, as the voltage and current can cause problems. Monitoring the voltage of your batteries and turning off the power when they reach peak open voltage can help mitigate the risks, but this method is still risky and isn’t recommended for beginners.
3. The ideal charging solution for your 3s LiPo battery pack is a specialized charger designed for this battery configuration.

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