The proper tool would be something that would maintain a set voltage. Battery chargers don't necessarily do that. I haven't done any coding on my car yet because I don't have a proper charger. Here is a
Schumacher INC-700A (they run around $375):
This is a recommended charger on Bimmerforums in their Diagnostics section. Not knowing how many amps our cars draw during a session I would err on the too much capacity side. I certainly would not do anybody's car without one and I wouldn't do big stuff on my car without one.
There might be a way to set up some ham radio equipment voltage regulators that would regulate voltage at the required amps. But I don't have any further info on that.
I really want to get rid of my yellow lights and was excited to see Shipkiller's post, but I don't think I'll do anything for a while since there are other things I want to do that take priority over coding.