I have purchased a Voltage Alert Pen from Aliexpress.com recently and found the pen is required to supply 3.6+ Volt to light the included white LED with a button, not two AAA batteries.
You will know usually LED flashlight need 3 or more alkaline batteries.
Since of it, I had released another order of AAA size Li-Ion battery and charger.
Today I have received the AAA size Li-Ion Battery, 10440(Brand Ultrafire) and successfully turn in operative.
To fill in the space of the empty one battery compartment, I have made an ordinary discharged AAA battery a dummy battery with wrapping aluminum foil, like the photo.
I think the way of making a dummy battery will be pretty useful when we apply that kind of Li-Ion battery to existing application.
I will not explain furthermore the know-how because everybody could understand the situation easily.