# Two power supplies in parallel - what would the resultant power, voltage & current be

1. Aug 16, 2012

### hannahnatasha

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 [Broken]

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!

Last edited by a moderator: May 6, 2017
2. Aug 16, 2012

### Simon Bridge

Re: Two power supplies in parallel - what would the resultant power, voltage & curren

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.

3. Aug 16, 2012

### I_am_learning

Re: Two power supplies in parallel - what would the resultant power, voltage & curren

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?

4. Aug 16, 2012

### skeptic2

Re: Two power supplies in parallel - what would the resultant power, voltage & curren

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.

5. Aug 16, 2012

### Studiot

Re: Two power supplies in parallel - what would the resultant power, voltage & curren

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

6. Aug 16, 2012

### Kholdstare

Re: Two power supplies in parallel - what would the resultant power, voltage & curren

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.

7. Aug 16, 2012

### Staff: Mentor

8. Aug 16, 2012

### altos67

Re: Two power supplies in parallel - what would the resultant power, voltage & curren

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.

9. Aug 17, 2012

### skeptic2

Re: Two power supplies in parallel - what would the resultant power, voltage & curren

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?