Connecting Cells in Parallel: Effects of Series Resistor

[IntuitioN]

what happens if you connect two cells with different emf in parallel along with a resistor in series


eg:


l---100Ohm--l
l-----6V-----l
l-----9V-----l

 

[Meir Achuz]

Stop! Don't do it. The loop containing the 9V and the 6V battery is a "short circuit"
(as in "Don't blow your fuse.") . The imbalance is 3V which would give a large current
through each battery: I=3/r, where r is the (usually very small) internal resistance of the batteries. The 1 kOhm resistor would have a current of I=3/1000=.003 Amp.
But the batteries would quickly burn out.

 

[chroot]

It generally won't blow anything up, but it won't be good for either battery. The 9V battery will be trying to "charge" the 6V, but the 6V vattery cannot ever charge to 9V. It can potentially draw a large current from the 9V battery for a period of time, but the equivalent series resistance of both batteries will limit the current to levels that wouldn't be considered dangerous. Once the system reaches equilibrium, the 6V battery will serve no purpose, neither sourcing nor sinking any current.

- Warren

 

[Meir Achuz]

Have you ever shorted a 6 V car battery?

 

[chroot]

I have seen a 360 V electric car battery pack get shorted, and that's definitely messy. I assumed he was talking about standard 6 V and 9 V dry-cell batteries, though, which have high enough ESR to prevent a dangerous amount of current.

- Warren