How do different voltages affect parallel-connected batteries?

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    Parallel Voltage
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Discussion Overview

The discussion revolves around the effects of connecting batteries with different voltages in parallel, specifically examining the voltage measurements observed in a setup involving a 24V battery (composed of two 12V batteries in series) and a 12V battery. Participants explore the implications of resistance in wiring and the behavior of real batteries under load.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant notes a voltage measurement of about 18V on the 12V battery side and 22.2V on the 24V side, questioning the reason for the voltage discrepancy.
  • Another suggests that the resistance in the connecting wires may be causing a voltage drop, impacting the measurements.
  • A different participant explains that connecting batteries with different voltages in parallel is problematic, as ideal voltage sources would not allow for this. They highlight that real batteries will exhibit voltage differences across their internal and wiring resistances, which could lead to overheating.
  • One participant emphasizes that the voltage output of a battery is affected by the current drawn, indicating that the voltage may not remain constant under load.
  • Another participant points out that the placement of voltage probes on the wire can influence the voltage readings due to wire resistance.
  • A warning is issued regarding the potential short-circuit created by this configuration, advising caution about leaving such a setup unattended.

Areas of Agreement / Disagreement

Participants express differing views on the implications of connecting batteries with different voltages in parallel, with no consensus reached on the best approach or the effects observed.

Contextual Notes

Participants mention the effects of wire resistance and the behavior of real batteries under load, but do not resolve the mathematical implications or assumptions regarding the setup.

mapa
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Hi,

I wired a 24V battery --which were actually two 12V in series-- parallel with a 12V battery.

The Voltage turned out to be about 18Volts when I measure the 12V Battery side. When I measured the 24V side, it turned out to be about 22.2Volts.

What is going on? Why does the voltage change when different sides are measured? How would I go about calculating voltage when the batteries are in parallel and have different voltages?
 
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Maybe the wire you use to connect the terminals have some resistance causing the voltage drop.
 
If batteries were ideal voltage sources, it would be undefined to connect batteries with different voltages in parallel, because the meaning of any components being in parallel is that they have the same voltage across them. With real batteries, the difference between the battery voltages is the voltage that will appear across the battery internal resistance and the wiring resistance, so be careful of the wires getting very hot. Another effect with real batteries is that the battery will fail to put out the advertised voltage, because the voltage produced by a battery is a function of the current. The voltage across a battery is approximately constant if the current is small or zero, but as the current increases the voltage drops.
 
You also have resistance in the wire, so voltage will depend on where on the wire you attach your voltage probes.

By the way, you did create a short-circuit. I hope you didn't leave batteries like that for long.
 

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