Very nice mod.
I agree with dentillozie that it is probably voltage sag. 2 cells will sag less than 1, but not by much. You could have some resistance, but it doesn't look like it. Probably have an emitter with a high Vf. If you can measure the voltage accross the emitter, that would give you some good info. Here is a chart djozz made when he tested an xml2 on copper some time ago. Link to thread in picture.
