Calculating Power, Current & Voltage for Parallel RTGs

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Discussion Overview

The discussion revolves around the calculation of power, current, and voltage when connecting two Radioisotope Thermoelectric Generators (RTGs) in parallel to charge a battery. Participants explore the application of Kirchhoff's laws in this context and consider the implications of the setup on charging behavior and circuit design.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant questions the applicability of Kirchhoff's laws in the context of RTGs with their own voltage and current.
  • Another participant affirms that Kirchhoff's laws are applicable, suggesting a model where the battery acts as a load that transitions to a source as it charges.
  • A participant calculates the power output of each RTG, estimating it to be approximately 3.44 watts, and inquires about the cost of RTGs and the amp-hour rating of the battery.
  • Concerns are raised about the internal resistance of the battery affecting charging current, with a proposed formula for calculating charging current based on the difference between RTG voltage and battery voltage.
  • One participant advises against a simple parallel connection without proper control and sharing circuitry due to the nature of the sources.
  • Another participant suggests checking the battery's ampere-hour capacity and notes that the battery may behave like a capacitor during charging.
  • A recommendation is made to include diodes in series with each RTG to prevent one source from driving the other, which could lead to heat dissipation issues.
  • A participant expresses curiosity about the project involving the RTGs and asks for more details regarding its application and the isotope used.

Areas of Agreement / Disagreement

Participants express varying opinions on the best approach to connect the RTGs and charge the battery, with no consensus reached on the optimal method or the implications of internal resistance and circuit design.

Contextual Notes

Participants mention the internal resistance of the battery and the potential need for control circuitry, indicating that assumptions about the behavior of the components may not be fully resolved.

hannahnatasha
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I am working with RTGs right now. I want to hook two of them up in parallel to charge a battery. Basically, the setup I had planned is like this:
http://img339.imageshack.us/img339/9549/rtgparallel.png

I am trying to determine the power, current and voltage that would be given to the battery.
I have been trying to research and know about Kirkhoff's laws. However, do they work in this situation when the RTGs have their own voltage and current?

For reference, the RTGs both have a current of 0.248A and a voltage of 13.89V.

How do I calculate what is given to the battery?

Thanks!
 
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Yeah - kirkoffs laws work for this situation ... one loop has two PSUs in it and the battery is a load ... you'd normally model the real components as an ideal source with a series load. A totally dead battery starts out as a load and gradually becomes a source as it charges.
 


Learned about RTGs for the first time. Wow,they are amazing. In your case, they produce roughly 13.89*0.248 = 3.44 watts each, supposedly for decades.
Just curious, would you mind telling how much does those thing cost?
As for your question, If everything is straight forward, the current given to the battery would be twice.
What is the AH rating of your battery?
 


I assume the current is short circuit current which implies an internal resistance of 56 ohms. This means that as the battery becomes charged, it will continue to charge slower and slower. If the internal resistance is a constant the formula would look like this.

charging current = (Vrtg - Vbatt) / 56

For 2 RTGs the current would be approximately double.
 


Considering the nature of the sources you should not use your simple parallel connection, without proper control and sharing circuitry.
 


Check the charge capacity of your battery. Like how much Ampere-hour it can hold. The battery will probably behave like a capacitor being charged, with some internal resistance.
 


You may want to consider putting a diode in series with each RTG. If you connect two sources in parallel, it is likely that one will drive the other and a lot of heat would dissipate in
the wires.
 


hannahnatasha, If you wouldn't be breaking any confidentiality agreements, it would be interesting to hear more about your project. For what will the RTG be used e.g. space mission, remote sensing, etc.? What isotope are you using for fuel? How hot will the fuel get?
 

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