For the small 1S batteries you can get this style of charger for $16 that will charge in parallel: https://www.amazon.com/Happymodel-Battery-Charger-Mobula7-Qu...

But for bigger batteries I recommend buying a charger that can handle 4 or more separate batteries at a time like the SkyRC Q200 or the Turnigy TQ4, or just do like me, charge each battery individually.