[w62820616]

Or they will drain each other ? If it does, is there any way to save that issue ?

 

[tangent]

It's okay if they are alkaline batteries, but not okay if they are rechargeables. It has to do with the difference in internal impedance. The effective impedance of alkalines is high enough that batteries in parallel will effectively "share" the load, while rechargeables will fight each other unless they are very well matched, and that state won't last forever even if you manage to match them at the start.

 

[nikongod]

Why do you want to do this? There may be a better solution or reason its totally unnecessary in general. 
 
Even with alkaline batteries I would lean very far away from paralleling batteries. Using a new and used battery together can cause serious problems.

 

[w62820616]

I have room for 2 9v batteries and I don't want put them in series ( my gain is only around 5) so 9V is enough. I was thinking maybe i can put them in parallel so that I get double the run time.

 

[tranhieu]

pardon my ignorance but how could you get double running time by having 2 batteries run in parallel?

 

[akgfan]

If you had room for six AA batteries, it would be better. 6x1,5V=9V and more mAh than 9V battery. It would last longer.

 

[jcx]

dual mono

 

[tangent]

Quote:

pardon my ignorance but how could you get double running time by having 2 batteries run in parallel?

You're effectively doubling the volume of the cells in the battery, so doubling the capacity of the battery.

 

[tranhieu]

Quote:

You're effectively doubling the volume of the cells in the battery, so doubling the capacity of the battery.

By doubling cell volume are you referring to the power provided by 2 parallel batteries (since output current is doubled while voltage stays unchanged in this config)?

 

[tangent]

That's another way to look at it, yes, but by "volume" I literally meant "volume", as in width-times-height-times-depth: there is twice as much battery-stuff, so a given fixed load will take 2x as long to chemically reduce the battery's voltage to a given level.