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
Protection circuitry is a highly critical element in a safely usable Lithium-polymer or Lithium-ion cell. Raw (unprotected) cells are less commonly sold, i.e. many times, the manufacturer of the battery includes protection circuitry as part of the battery; however, for safety, do not assume this is the case and check the datasheet of the battery.
Purpose of Li-ion/LiPo protection circuitry
The protection circuitry serves to protect against short-circuit, over-discharge, and over-charge conditions. Loosely speaking, the circuitry relies on an over-current cutoff, an under-voltage cutoff and an over-voltage cutoff respectively for ensuring against those three conditions. Sometimes, protection circuitry also includes thermistor-based overtemperature protection.
Unprotected versus Protected batteries
You MIGHT conceivably get by with an unprotected battery, for simplicity, if you have certain components on your PCB, e.g., a fuse (prevents overcurrent/short-circuit), an undervoltage-supervisor IC (prevents overdischarge), and a smart battery-charging IC (prevents overcharge).
But my personal assessment is: Unless you are able to constantly monitor the battery, do NOT use an unprotected battery; protection circuitries are fairly simple to implement (e.g. using pre-packaged protection ICs), or alternatively, a protection-added battery is easy to purchase.
How to implement Protection circuitry (for a Two-cell pack)
If you choose to implement it yourself (versus buying a protected pack), then here are two protection ICs you can consider for your scenario (i.e. two-cell Li-ion or LiPo):
Best Answer
The black wires are connected to a thermistor on the end of each. The white goop is heatsink compound. They are used to monitor the temperature of the cells in order to provide a thermal mechanism for detecting full charge as well as part of a safety mechanism to prevent the cells from igniting.