Batteries in parrell

Will adding diodes to the batteries keep it from shortening out?

you have to place a diode in series in each battery

Add a small resistance in series with each battery instead. EC batteries/voltage-sources are ideal and have no internal resistance.

This is like real life. Batteries in parallel is not usually a very good idea. The battery with the highest voltage (and real batteries will always differ, whatever their nominal voltage) will "try" to charge those of lower voltage. A diode in series with each battery will avoid this. The downside is that you'll "lose" the forward voltage of the diode (0.6V-ish for a typical Si diode, 0.2V-ish for a Schottky). Generally if you see the need for batteries in parallel, use a bigger battery. If you're setting up some complex series/parallel array, use appropriate battery control circuitry.

You can parallel batteries with no other components if they are rechargeable. Otherwise, big no-no, and putting a diode or a resistor between them won't solve your problem.

@sircube Well if you put a Diode in series to each battery, it will most certainly solve the problem of charging the batteries. You still lose energy over them, but it might be useful in some niche applications. Also putting two recharchable batteries in parallel is usually not a good idea, especially if you are using some with high discharge ratings. This can exceed the maximum charging current very quick and cause serious damage. Anyway as mentioned before by @tonyinselby, for arrays and rechargeable batteries you should eventually alswas use dedicated control circuitry.

I have edited the circuit to add two diodes, will this make the LED have two times more battery life? Thank you for the help!

So you're LED circuitry is consuming 84,621mW of power and the diodes use 15,956mW. So you're using one fifth of you're total power just to have two batteries. It will stay on longer but not twice as long.

Little less than a fith but still not really efficient. Just swap batteries when they are empty

@sircube that depends on what the problem is. If it's purely battery life, either use a bigger battery, use a different technology (LiPo instead of NiMH for instance, or alkaline instead of ZnC) or, if you're able to, redesign your circuit. Does your LED need to run at 20mA or will 5mA be bright enough? Do you need a 1k resistor between Vcc and pin 7 on your 555 astable or can you get the same clock frequency with a 10k and recalculated R and C elsewhere? Calculating battery requirements is an art in itself, as manufacturers of electric cars know well. A simulator like this can only go so far.

That makes sense. I am using A27 12v batteries that come in a pack of two. I will loose power using diodes so I probably just stick with one battery. Any practical reason to using batteries in parrell?