These guys go crazy and connect up their car batteries to small lithium ion (Li-Po) lithium-ion-polymer battery
You can actually hear it explode and fizz, i think the same battery may even let go a few times before giving up.
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When Sony introduced the first lithium-ion battery in 1991, they knew of the potential safety risks. A recall of the previously released rechargeable metallic lithium battery was a bleak reminder of the discipline one must exercise when dealing with this high energy-dense battery system.
What wikipedia makes of li-poly
from wikipedia page on batteries.."A compelling advantage of Li-Poly is that manufacturers can shape the battery almost however they please, which can be important to mobile phone manufacturers constantly working on smaller, thinner, and lighter phones. Another advantage of lithium polymer cells over nickel cadmium and nickel metal hydride cells is that the rate of self discharge is much lower.
Li-Poly batteries are also gaining favor in the world of radio controlled aircraft, where the advantages of both lower weight and greatly increased run times can be sufficient justification for the price. However, lithium polymer-specific chargers are required to avoid fire and explosion. Electronic motor speed controls calibrated to prevent excessive discharge and subsequent battery damage are required as well. They are also gaining ground in PDAs and laptop computers, such as Apple's MacBook and small digital music devices such as iPods and other mp3 players, as well as portable gaming devices like the Sony PSP or Nintendo's GameBoy Advance SP, where small form factors and energy density outweigh cost considerations."
Another way over-recharged LiPo Battery
This is a bit more spectacular than the previous one! flamage!