Friday, October 6, 2023

 12V Mobile Power Unit


This project was conceived as a companion 12V power source for my RV.  

When working remotely from power mains, access to 12V power is useful, such as working on a trailer, or vehicle like an RV. RV owners know that things fail often, but that is a story for another post. Sometimes a  failure can strand the RV and even create unsafe driving conditions. The worst of these are substems that are stuck deployed such as steps, slides, and awnings. 

Common  uses for a mobile 12VDC Power Unit:
  • Testing trailer lights
  • Power tools that are 12V compatible such as a soldering iron. Having a mobile soldering iron when working on a vehicle or trailer makes electrical work much easier. 
  • USB charger: a 12V to 5V adapter for powering USB-charged devices. 


Panel view of 12V meter and discharge monitor

Battery Stowed in box

Design

Features of this mobile power unit:
  • Converts a readily available tool battery technology to 12V. In my case Ryobi batteries.
  • Provide for various ways of connecting to multiple types of  loads
  • Use standard power connections
  • Visible voltages for all electronics

12V Power Supply

Socket for the battery

This Ryobi adapter was used to connect to the battery. It comes with an integrated fuse and a main power switch.

Battery adapters for other vendors' batteries are also available on Amazon. They are often found under a "Power Wheel" search term.

Discharge protection

If you discharge a lithium tool battery below a specified level you will damage the battery. To prevent excessive discharge a monitor is used that switches off the load at a specified level.
Discharge monitor: Voltage Disconnect Module
This module should be set up to disconnect at a value specified by the battery manufacturer. 
Ryobi batteries discharge level should be set at 14V

14V- 12v converter


Tool batteries typically output more than 12V. Ryobi batteries output 14-21VDC so a down converter to 12V is necessary.
As the battery discharges the voltage will drop. A converter that will operate across the range of the battery's output voltage is needed. In the Ryobi case, this is 14-21V. 
A suitable current capacity should be chosen. This one can handle 10A. I would have preferred 20 amps because many 12V loads are that heavy. I could not find a converter that supported a 20 amp load across this wide of an input voltage range.
This converter advertised; built-in over-temperature, over-current, over-voltage, and output short-circuit protection. I have not yet tested those features.

Fuses

My implementation uses the fuse supplied with the adapter. Additional protection at the output could be added. A fuse could be provided as an adapter. 

Fabrication

Ammo Box


The supply is housed in a translucent plastic ammo box. You can get these on Amazon and at Walmart.
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Holes for mounting screws, voltmeter window, and output jacks are cut into the wall of the box  

Frame

I wanted the entire assy to be removable as a single piece. To that end, a 1/4" acrylic plate was used to mount all the parts. The plate was tapped for screws as needed for mounting the parts. Screws on the outside of the box mount the frame to the inside wall. Mounted to the frame are:
Shown is a converter module (upper middle of the frame)  that I am not using

Battery Adapter 

The adapter was spaced from the frame by a 1/4" acrylic subplate [see photo above]. The adapter is disassembled and two mounting holes are drilled. Then 1/2 of the adapter and the spacer plate are screwed to the frame after which the adapter is reassembled in place.

Converter

The 12V converter is mounted on the side wall opposite the frame.


Adapters

A series of adapters were fabricated to facilitate connecting the power unit to various loads. 

Example configurations:
  • A load can be connected directly or through an amp meter.
  • A DC motor can be connected through a reversing switch so it can be operated forward and reverse.
The adapters were fabricated using SAE cable jumpers so as to keep connections standard.

WARNING: When SAE connectors are connected end to end without an adapter the polarities are reversed. This can damage the load!

To prevent this occurrence use reversing plugs where necessary.  To determine if the polarity is correct, before connecting a load connect the Polarity Tester where the load would be connected. If the LED in the polarity tester lights the connection if correct if not add a polarity reverser plug.

Adapters for connecting to: 





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