I have a battery pack of 2 18650 cells in series, so it is possible for me to charge it without connecting anything to the central 3.7v potential point as many answers mention? The battery in question is https://uk.rs-online.com/web/p/lithium-rechargeable-batteries/1449410/, which is sealed and has only 2 wires coming out. This means that I cannot access the central part. I understand that a 7.4v charging circuit is most appropriate, but is it possible for me to implement 2 TP4056 boards in series somehow? I wouldn't be able to do a setup like
given that the grounds would be shorted, and thus so would the second cell. So, does this mean I have to resort to a specialised 7.4v circuit, or could I manage with a 3.7V circuit (that I have 3 spares of with me currently, which is why i want to use it).
Furthermore, the datasheet on Rapid Electronics for that battery pack lists that it has over-charge and over-discharge protection - does this mean I can hook it up to a 8.4V (safe current limit of, say 500mA once near full) bench power supply? This would be even more convenient to charge, but I do not want to damage the battery if this is dangerous.