In your Part list is no active semiconductor.
I would solve that problem with a NE555 astabile Multivibrator and a "Power" Mos FET Transistor.
When using the C-MOS Variant of 555 ( 7555 ) the Multivibrator draw very low current.
For the LED's you can use Low Current ones, when they are bright enough.
An astabile Multivibrator with 2 Transistor's is also possible.
But I think you have to install an additional driver Stage ( FET or Darlington Transistor ).
What kind of accu you want to use? Lead, NiMh, LiIon, LiPo, or LiFePo4 ?
All of them have assets and drawbacks.
I think a good choice for the Operation Voltage is around 12V.
So you can built strings with 3 or 4 LED's in serial configuration. 7 Strings with 3 Diodes will give you 21 LED's