One other thing to note with lead acid batteries is that the voltage
will stabilize over about two hours once you let it rest. Resting means
no discharge or charge. If you wait a couple hours after disconnecting
the load you may see the battery voltage creep back up. The larger
battery may gain more than the smaller one. The opposite holds true
after charging. The voltage will drop over two hours once you
disconnect the charger.
Randy
ps. My experience is with large battery banks in PV systems.