SPST switch help

Hello all,

I am encountering an odd problem. I am using a switch to power on/off my Arduino (Vin) inside a project box.
This is the switch: http://www.sci.com.tw/PRODUCTS/switch/(R13)%20ROCKER%20SWITCH/R13-135.htm It is a simple SPST rocker switch

The problem is quite weird, and I can't wrap my head around what is happening. Here are the conditions:

No Arduino in circuit:

  1. Normal operation. All is fine. Switch ON (closed circuit) = 12V from lithium-ion battery. Switch OFF (open circuit) = 0V.

Arduino in circuit:

  1. Switch from OFF to ON. No power to arduino. Volt-meter reads 1.1 - 1.8 Volts (this is the most confusing part to me...)
  2. Switch from OFF to ON. No power to arduino. Volt meter reads as above. Physically open and close circuit (removing gnd or 12V lead, re-attaching lead) with switch remaining in ON position. Arduino gets power! Full 12V...

I am quite confused. The symptom has occured with two different arduino uno's.

Any help or insight would be much appreciated!

The black wire of the battery should go to Gnd.
The red wire of the battery should go to a switch terminal, with the other switch terminal going to Vin or the centre terminal of the barrel jack.

How do you have it wired?

Runaway Pancake, thank you so much for the help. That fixed everything as you described.

I previously had it wired up battery - to switch post, battery + to other post, and in parallel to board gnd and Vin.

If I could, I would like to ask for further assistance.

This lithium ion 12V battery came with a barrel jack to solder up to the same switch to recharge the lithium ion. Would soldering the barrel jack positive to positive positive post on SPST, and gnd to gnd work just fine? My arduino, all IC's, and battery gnd, and barrel jack charging gnd would be on the same post. This would be okay when recharing the lithium ion?

Sounds like it.
If you would sketch that up, I'd feel... more confident about it.