When do we need blocking diodes?
If the voltage of the two power sources (the panel and the battery) are different, then you'll need, at least, one diode to prevent the current from the battery through the panel.

If your batteries are NOT rechargeable, I'd also put another diode to prevent the opposite situation of the panel voltage being greater than the battery voltage, what would cause it from being "charged", which is not allowed in a non rechargeable battery.







simulate this circuit &ndash; Schematic created using CircuitLab