Connecting Cells in Parallel: Effects of Series Resistor

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Discussion Overview

The discussion revolves around the implications of connecting two batteries with different electromotive forces (emf) in parallel, specifically when a resistor is included in series. The focus includes theoretical considerations and potential consequences for the batteries involved.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant warns against connecting the batteries, stating that it creates a short circuit due to the voltage difference, which could lead to large currents and potential damage to the batteries.
  • Another participant suggests that while it may not cause immediate catastrophic failure, it could still be harmful to the batteries, as the higher voltage battery would attempt to charge the lower voltage battery, leading to high current draw until equilibrium is reached.
  • A participant questions the practical experience of shorting batteries, referencing a high voltage scenario to illustrate the potential dangers, while clarifying that they assumed the discussion was about standard dry-cell batteries with higher equivalent series resistance (ESR).

Areas of Agreement / Disagreement

Participants express differing views on the safety and consequences of connecting the batteries in parallel. There is no consensus on whether it is safe or advisable, with some arguing it could lead to damage while others suggest it may not be catastrophic.

Contextual Notes

Participants reference the internal resistance of batteries and the concept of equilibrium in current flow, but do not provide specific values or conditions under which the discussed scenarios would occur.

Who May Find This Useful

This discussion may be of interest to individuals exploring battery configurations, electrical engineering students, or hobbyists working with battery-powered circuits.

IntuitioN
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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
 
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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.
 
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
 
Have you ever shorted a 6 V car battery?
 
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
 

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