Also, batteries connected in parallel without protection circuit "automatically" even the SoC - so, basically, after connection, current from higher SoC battery flow to the lower SoC one.

That is, why it's so important to connect batteries at ~same State of Charge - very little differences won't do harm (and are unavoidable), but connecting full battery to 50% SoC one will result in very high current flow, which will:

1) probably burn out connection after few seconds, due to excessive heat, generated by unrestricted flow

2) if not, "age" batteries quite much, due to current out of sane limits

3) in worst case, make batteries to vent with flames/explode.

In case of connecting (in parallel) batteries of (almost) the same SoC (of course, when they are same type, same chemistry, same capacity etc), battery with higher SoC also charge battery with lower SoC, but only for a small part of second, without harmful long-time overcurrent.

/Estel