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The ABCs of batteries
Nobody asked me these questions, but I feel the need to answer them anyway. I am certainly no expert, and have learned a considerable portion of what I know about batteries in just the last few weeks, and in no small part due the information in this forum, shared by the experts here. Since I have some electronics background and couldn't easily find some of the pertinent information related to modding ecigs, I hope to enlarge our knowledge base a bit, and cure insomnia at the same time.
How safe are lithium ion cells?
The potential for injury and considerable property damage is definitely a concern. One of our members just experienced an explosive fire in his back seat from a lithium. There's videos of exploding lithium batteries all over You Tube, and seemingly endless recalls of laptop batteries. But, before you go throwing your ecig in the ocean (wouldn't help, anyway, lithium ignites in water), I advise a longer perspective. Despite the media frenzy, I have only heard of one injury. An early cell phone battery burned the face of a man. Commercial ecigs are relatively safe if used normally. They incorporate a micro chip that prevents both over-discharge rates and under-voltage conditions of the battery. Their chargers (usually) prevent overcharging and subsequent thermal runaway. However, electronics can fail so the most frequent battery castastrophes occur while charging. DO NOT stick your charger on a window sill, on carpeting, or near any other flammable materials (especially curtains), just because it is near the most convenient outlet. Find a spot on a flame resistant counter (kitchen, bath) and don't leave the house or leave it charging overnight.
Also, don't charge until the green light comes on and then give it few more hours, like I have heard repeated so often. That is a myth leftover from the days of NiCds and NiMHs that have a 'memory effect'. Lithium ions have no such memory effect. None. Nada. They don't need to be 'topped off' like NiMHs, nor frequently discharge cycled like NiCds. They do like a moderate discharge to 3 volts or so every 100 charges to recalibrate the electronic surveillence circuit that monitors its charge status.
AS AN ASIDE: I was one of the first victims of lithium ion. In May of 2001, one week before the dot com crash, I bought 1000 shares of Lithium Ion Corp stock at $5. They are now worth $35. Unfortunately, I mean the entire 1000 shares are worth $35. That is a serious burn. But I digress.
What about modders?
Aaarrrgggh, matey, we're pirates, not girly-men. We only have one leg and one eye, anyway. But, if you happen to be a nerdy pirate, then it would be wise to always use 'protected' lithium ion cells.
First, all rechargeables in a can have a vent. When a cell overheats and the pressure starts to build, the vent opens and dissipates the excess pressure to avoid an explosive event. Unfortunately, the vent can fail. And, in the case of a Li-Ion cell, thermal runaway often overcomes the vent's ability anyway. Because of this particular concern in Li-Ions, a thermal fuse, or more commonly, a positive temperature coefficient (PTC) current limiting switch, is incorporated near the vent. This usually prevents the cell from reaching thermal runaway temperature (about 166C). Since LiPos aren't in a can, they do not explode, they simply have a flaming vent occurance, known in the trade as a 'Holy Crap... I don't want to be a pirate anymore' moment.
But on with the 'protected cell' lecture. As a modder, you have the choice of a protected or unprotected cell. They sell both types at most suppliers. This is the final line of defense for us idiots. It is essential to choose the protected type in any single cell situation. This protection consists of a tiny pcb with a micro, a voltage sensor, and an electronic switch. It's frenched into the cell, just under the anode plate. Its job is to prevent thermal runaway and over-discharging of the cell. It monitors current drain on the battery and shuts the circuit down in runaway conditions. It also shuts the circuit down to avoid an undervoltage occurance. Most importantly, it prevents overcharging. These functions do not interfere with normal use, they simply add a mandatory insurance policy. (Okay, okay, maybe the overcharge mode can tangle with the charger's cutoff circuit and the charger light can toggle colors, but that isn't much of a penalty since you can just consider the change from red as the 'charged signal' and don't have to wait until it is fully green.)
Why do they sell unprotected cells, if they are so dangerous?
Some people use the cells in packs for higher voltage systems and purchase a separate protection board that covers the entire pack. Other situations assign similar protection duties to an external micro board, like in commercial ecigs. But modders have removed that external protection when they replace the autoswitch with a manual switch.
So then, why lithium ion if it is not the safest?
For ecigs, we need a small, light weight, long lasting, large output battery. Lead acid batteries are obviously too large and heavy. Non-rechargeable (primary) batteries are all ruled out for their short lifespan/cost ratio. Rechargeable (secondary) alkaline, nickel-cadmium (NiCd), and nickel-metal hydride (NiMH) batteries don't have enough output. Think of the capacity of a battery (mAh) as the size of a bottle of water. Its output (discharge rate) is the size of the mouth of that bottle. While it is true that you can short circuit any battery (or tear the mouth off a bottle) and get the entire contents out almost at once, you will ruin the battery (or bottle) in the process. Another trick you might be contemplating is to strap a bunch of small mouthed bottles together to produce a large output, but then you run into the size and/or weight problems again.
Li-Ion comes out on top for energy density (size, weight), capacity, and output capability. And due to laptop, cell phone, and R/C popularity, are the most available cells.
What about lithium polymer?
You had to get fancy, huh? Well, lithium polymer (Li-Poly, Li-Po) is a close brother to traditional lithium ion (Li-Ion, LiCoO2) and uses the same cobalt oxide type cathode. The difference between the two is the polymer electrolyte of the Li-Po. It does have the best output (largest mouth) with huge discharge rates of up to 30C. The term 'C' refers to the nominal charge rate of a battery. In reality, it is just the capacity of the battery (in mAh) expressed as amperage. Thus, a 500mAh battery charged (or discharged) at 1.5C simply means a charging rate of 750mA (1.5 x 500mA).
Will they work for ecigs?
Sure, quite well. And Li-Pos don't need to be encased in a metal can, so their energy density is even better than Li-Ions. But they are usually only available as a flat pack, since their main customers are cell phone manufacturers and R/C enthusiasts in higher voltage packages. Modders are prone to shove their electronics into a tiny flashlight case or juice box. But as a pocket recharger, hmmm....
Is that it?
No. Lithium Manganese (LiMg204, spinel) and lithium iron phosphate (LiFePO4, LFP) have recently been developed and hold some advantages over traditional lithium cobalt (mainly safety). But of course, then there are their disadvantages which mainly are that they aren't widely available yet, and they have different voltages and other characteristics which make them an extra hassle to use. But DX is starting to sell LiFePO4 cells in convenient sizes, so it may be time to experiment.
To spare us more clever dialog, how about a chart?
OK.
What about shelf life and self discharge rates?
Almost unimportant to modders. We use 'em and abuse 'em. It is interesting, though, that li-ions only last about two years, whether used or just sitting there. You would be best served buying your li-ions from a distributor with high turnover. Any store selling nixie tubes wouldn't be my first choice.
What about the notoriously poor battery life?
In commercial ecigs, the battery size is approximately AAAA (8mm x 40mm) with about 180 mAh capacity. Those five second vapes approach 5C, so the commercials are sacrificing battery life for greater than rated output. But we are modders, so to hell with that. When a battery mod is AA size (10mm x 44mm) or greater, the battery capacity jumps to at least 750mAh, and the output falls to only a little greater than 1C, bringing it back into spec (usually 2C) and gain a much longer battery life (unlike the commercial cigs false claim of 300-500 charging cycles).
Shut up and just give us a chart!
Of what? Okay, maybe this one. It has saved me a bit of time on occasion, when comtemplating which battery size to design around.
Be safe, mates.
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Originally Posted by mogur
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