Read up on Kirchoff's Law. The common battery gets punished for opposite leg charging or discharging differences. I see people down here doing 80mph with an emergency spare tire fitted. It works until it fails or the differential declares "enough is enough!"
Off-the-wall configurations will entertain off-the-wall consequences. The OP wishes to retain this configuration so just replace the abused battery and keep rowing. Pretending this can be resolved can be compared to wondering why playing 51-card solataire is so hard...
