Zener diode conduction
You also have the BE junction of Q1 in series with the diode, so significant current will flow through this path only when the battery voltage exceeds about 6.8 V + 0.65 V = 7.45 V.

That's a tad high for a lead-acid battery; I would normally keep it to no more than 2.4 V per cell, or 7.2 V for a 3-cell battery.