Using laptop batteries
I just snagged a few good older laptop batteries (Li-ion, 14.8v, 3600 and 3800 mah) along with a laptop to charge them.
What I want to know is if these can be successfully used to power bike lights, and if there's a good way to get the juice out safely I've done some reading on these type of batteries and know that they are a bit more volatile to use than Nimhs or NiCads (something about venting flames <G>).
I did a search of this forum, but didn't find much, which is strange because I know it must be here somewhere- maybe my search terms were wrong.
Anyway, advice will be appreciated.
Lithium cells have a flamable electrolyte. They also have very low internal resistance. That means they are capable of producing very high currents when shorted. This can lead to what is called "thermal run away" which loosely translates to "She's goona blow up" even if you disconnect everything. They also can go "boom" if overcharged.
I would hesitate to recommend the use of DIY lithium batteries unless you are willing to spend the time to research the possible dangers.
RCgroups has logged over 100 submissions on Li battery "vent with flame" incidents including 2 totally destroyed cars, and at least 1 fire damaged apartment. Most of the incidents occured when damaged batteries were left unattended while charging. A smaller fraction happened after an RC vehicle crash.
Li-poly isnt the same as li-ion. All the RC guys use li-poly.
Originally Posted by Walt Dizzy
If you use laptop batteries and leave the protection of the pack in place (ie drain and charge through the protection PCB) you dont have a thing to worry about, they've probably got the best protection there is.
I'm using a 4x18650 pack with purpose-made protection PCB, have been using it for over a year without a single issue.
I do however keep some safety precautions in mind, like charging outdoors, keeping them from overdischarging (bFlex voltage warning) etc.
Need optics for DIY bike lights?
No, but yes
"If you use laptop batteries and leave the protection of the pack in place (ie drain and charge through the protection PCB) you dont have a thing to worry about, they've probably got the best protection there is."
"Li-poly isnt the same as li-ion. All the RC guys use li-poly."
Since the original poster didn't specify what he was going to do with the batteries (like maybe take them apart and reconnect in a different configuration) it is good that you specified to leave the protection PCB in place. That may be the best protection there is, but it's still not safe to charge lithium batteries unattended (inside a home) or to charge a damaged battery. It is good that you pointed this out as the kind of precautions you take despite saying that with intact protection circuitry "...you don't have a thing to worry about"
It's also true that Li-poly is not the same as Li-ion, but there have been cases where Li-ion batteries have vented with flame. There have been enough problems with laptop computers catching fire from battery failure that Sony to name one was obliged to recall some of their Li-ion batteries:
I use Li-ion cells, and have a fair degree of confidence they won't burn my house down. But it can happen. I'd rather err on the side of being alarmist than to give someone a false sense of security.