Charging extra Fairphone 6 battery outside the phone, possible?

Hello everyone.

My Fairphone 6 is not currently charging. I have an extra OEM battery - I bought one after the issue appeared along with USB-C charging ports to try to fix the issue - it didn’t.

I already started the process for sending the phone for official repair, waiting for an answer.

It is not a good time for me for this however, I cannot wait for however long this takes without a good primary phone, I live in an island and expect any RMA process to take up at least a month. If I could delay this 1-2 months by manually charging the batteries would be ideal for me.

So to the question. Is there a safe way to charge the battery outside the phone? I am willing to sacrifice the connector on one of the batteries to create a jig if no other option exists.

If something exists that I can buy even better.

Thanks for any help.