Is there a formula or rule of thumb for determining the lithium content by weight of batteries?
Short: 8 grams / 100 Wh or ~= 0.75 grams per 18650 cell in a typical laptop battery.
Longer: Guidelines below, but YLMV -*
I have traveled on occasion "at about the maximum limit" when all the batteries I carried were added up. But, in around 50 legs of international travel, nobody has ever asked or checked.
[I had more trouble carrying boxes of 48 new NimH AA cells, as they are XRay dense and they attract extra attention from security.] Rules stipulate that batteries must be carried as carry on luggage and this does seem like a good idea.]
I have carried out this investigation for my own purposes some years ago, but Gargoyled to see what others say. I arrived at the same site as above BUT at a page where they answer your question (or purport to)
On this page Safetravel.dot.gov (that's their name) who you'd hope would know (but possibly don't) say:
Equivalent Lithium Content (ELC). ELC is a measure by which lithium ion batteries are classified. 8 grams of equivalent lithium content are equal to about 100 watt-hours. 25 grams of equivalent lithium content are equal to about 300 watt-hours.
You can arrive at the number of watt-hours your battery provides if you know how many milliamp hours and volts your battery provides:
mAh/1000 x V = wh
Most lithium ion batteries marketed to consumers are below 100 watt-hours (8 grams ELC). If you are unsure of the watt-hour rating of your lithium ion battery, contact the manufacturer.
ie 100 Wh/8g = 12.5 Wh/g or 0.08g/Wh
A single 18650 call has a Ah capacity of 2 to say 2.5 Ah. Some claim higher but they are less believable (eg Everfire sell "3000 mAh" cells) and you also would rather they were wrong for this calculation.
On the above basis
a 3.6V nominal 2 Ah 18650 cell = 7.2 Wh =~ 0.6 gram
a 3.6V nominal 2.5 Ah 18650 cell = 9 Wh =~ 0.7 gram
To be easy and safe, say 0.75g/18650 cell
Laptop batteries tend to come in multiples of 3 or 4 cells.
Netbooks may have sub 18650 cells. Larger laptops tend to use 18650 cells.
Likely offerings are 18650 x
3, 4, 6, 8, 9, 12. giving
2.25, 3, 4.5, 6, 9 grams of Lithium per battery.
Happy the man (or woman) with a 12 cell laptop battery !
Even the 12 cell is so close to the limit that some refining of the calculations may show it falls below the threshold. It seems like this is aimed at laptop battery size or smaller, versus a larger than laptop division.
*-Your Lithium May Vary
MUCH more on web on this.
More later maybe ...
The following links are pertinent and may be useful.
Rushing out ...
I'll consider annotating them later:
Table good but does NOT relate to actual gross metal used
http://en.wikipedia.org/wiki/Lithium-ion_battery
Applies 8g/100Wh simplistically.
http://www.canonblogger.com/wp-content/tutorials/batteries.html
Discussion. May be useful
http://www.schneier.com/blog/archives/2007/12/new_lithium_bat_1.html
Discussion. May be useful
http://www.bobatkins.com/photography/li_battery_restrictions.html
Safe Travel again. Read !
http://safetravel.dot.gov/whats_new_batteries.html
Useful - citing safetravel site
http://www.tsa.gov/travelers/airtravel/assistant/batteries.shtm
Best Answer
Few things to take into account when replacing the batteries, are the cells conneted in parallel (higher current) or in series (higher voltage)? Does your device have a regulator and reverse voltage protection? Shorting an LiSOCl will just make it warmer, shorting a LiPo cell might cause an explosion.
You said it is a wireless device, 2 1/2AA LiSOCl cells in parallel can provide a 200mA pulse, check if your intended replacement cell can provide it.
Self discharge: LiSOCl cells have low self discharge and very long shelf life. Rechargeable cells have much higher self discharge and if the cell drops below a certain threshold, it cannot be used anymore.