Spark funs site describes the Li-? as "Polymer Lithium Ion Batteries "
so are they Li-Ion or Li-Polymer ???
I ask this because when i go to TI’s site there are 2 sections for Li-Ion and Li-Polymer charger IC’s. which one should i get for these SFE batteries.
i need to series 2 of the 2000mAh packs, so i am looking for a charger / protection IC for them.
They’re Li-Polymers
You need to be careful, though. You’ve probably heard the thng with the Dell batteries, right? Those batteries are also LiPolys. Just be careful when charging the batteries.
So on the subject of LiPoly safety, what type of use/abuse is it that makes them want to blow up?
Bare Li-poly? Overcharging, overheating, too rapid of discharging, puncturing. Most result in overheating, which gets the lithium up to flashpoint (151C? something low) and they go. They may puff up, which is stage one damage (internal gas creation), or may catch fire, stage two (lighting said gas). LI-whatevers do not like to be overcharged at all, and are tricky to get fully charged since they need a monitor that does voltage and current detection.
That’s the dangers. Now, a bit too fast discharging, or fast recharging simply can cut down the life expectancy REALLY FAST. Thankfully not explode, but if you keep using them, they start to lose current capacity, and therefore go into the “too rapid of discharge” region, and we see flaming cell phones. Most applications should have a discharge meter IC (to keep the unit from discharging too far), a charge IC, and a thermal monitor with circuit cutoff. I’m probably forgetting something in here, but those are the three. Like I said, almost all of the issues can be tracked back to something that will heat the batteries up too far and hit a flashpoint.
It seems like there are a few misconceptions of the dangers of LiPoly. Whenever i mention a LiPo application, i have been told that a certain percentage of the batteries just like to explode or buldge for no reason.
This worries me as i am developing a couple of systems with LiPo batteries, but I will assume, with the proper monitoring (like, keeping the battery from catching on fire, etc) i should be fine.
Yes, proper monitoring of the charge rate and amount, discharge rate (do NOT short!), and temperature will provide a nice long, safe life in your applications. People want to skip the protection devices because they’re a significant chunk of the cost of a Lipo cell, and they’re used to the “chuck and charge” capability of NiCD/NiMH batteries. RC guys often times skip these parts in their batteries since they push their units to the limits (and take a 90% loss of charge cycle in the process), but they’re coming around and realizing that turning their vehicles into blazing comets flying through the sky is not worth saving a few ounces in electronics weight.
The only thing you can’t protect sufficiently against is physical damage, but a plastic case is usually sufficient
Check the blog section of my website and let me know if you have any questions.
SOI_Sentinel:
RC guys often times skip these parts in their batteries since they push their units to the limits (and take a 90% loss of charge cycle in the process)
The commercially available LiPoly packs intended for use in R/C applications are supposed to have protection in them, but then again the manufacturers say you can get 20C continuous out of the cell. The thing that I’ve heard is that the older R/C pilots hook their LiPolys to a NiCd or NiMH charger, which is extrtemely dangerous, or they crash [shock damage] and charge the same pack later. Anyway, for the bare LiPolys you will need some sort of protection.
For stacking two in series, be sure that yu balance the charge in the two cells before connecting them. You’ll need to do this in the charger portion of the circuit.
Unless something’s changed in the last 6 months, commercial R/C lipoly’s may provide a tap set to access each cell individually to maximize charge and balance the cells, but they have NO other protection built in, not even a thermistor.
You guys have inspired me to short an old lithium baced cell I have hanging around from a cell phone.
I’ll be sure to film it.
How far should I stand away?
check out this video: http://www.cheapbatterypacks.com/liposackv.wmv
basically, I believe these are Li-Polymer batteries that were charged (for several hours) using NiCad or NiMH chargers. The results… speak for themselves.
The video is an ad for the ‘Polysack’; ie, something you put the batteries in to keep the potential explosion from burning down your house.
Definitely something to be very careful with, and not cut corners on…
Those were intentionally overcharged. The thing is, you CAN charge Lipos with NICD/NIMH chargers, but you need a volt meter, an ammeter, and the patience and attention of a saint. You have to sit there the WHOLE TIME watching first the volt meter then the ammmeter, AND know where to turn off the blasted contraption.
Now, the thing about R/C lipos. They charge them fast, so they tend to get hot. They charge them constant, so the lipos tend to get warm as they age (internal resistance goes up). Fast charging ages them faster, as does high current drains.
When I first started looking at R/C lipos, the current cell balancing multitap systems weren’t available. It was plug and pray, and knowledge that even with a well designed lipo charger, you were still talking about when the battery pack was going to fail, not if. You usually hoped it was during charging, where you could contain the damag,e not during in-plane discharge. Many a good model fireballed because of that. It’s still a concern on discharge if you don’t have a lipo level sensor onboard. Lipos have a very steep discharge rate at the end, so the last 10% can sneak up on you while airborne. The normal hope if that you get the warning, throttle back, and bring her home while you still are in the safe range. If the warning isn’t recieved, burn and crash. I think some speed controllers may now have a detector and automatic soft stop (50% power to allow for a controlled landing?). Still, danger lurks.
Modern multitap RC packs, when used with the correct hardware, now will diagnose and not charge bad cells and alert you to the imminent failure, so you don’t have a fireball.
All commerical LION/Lipos are supposed to have the safety circuitry built in. However, if the parameters aren’t entered correctly (say, too large a maH rating or maybe the wrong type of anode selected), you get a Sony battery.
I got the LIPO BACKPACK II boards in and assembled. http://ohararp.com/blog.html