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Intelligently utilizing lithium-based rechargeable batteries

PostPosted: Dec 15, 2012 8:42 pm
by mooreshire
(I wrote this for a recent issue of the Cascade Caver newsletter.)

Intelligently utilizing lithium-based rechargeable batteries underground

A lithium battery fire can be dangerous enough above ground, and any lithium cell is capable of failing in a way that results in a truly dramatic chemical fire. In an enclosed space a lithium battery fire could be an unquestionably deadly event. Scalding explosions aside, the massive quantity of noxious smoke produced by a burning lithium cell would rapidly render even a relatively spacious cave passage immensely toxic. While the most effective precaution would obviously be to avoid lithium batteries altogether while caving, it is possible to significantly minimize this risk by properly matching your hardware and battery selections. Some cavers regularly use disposable lithium primary cells purchased at the supermarket while fearing to use a properly constructed lithium-ion battery pack - I hope here to shed some light on the complicated reality of lithium batteries.

There are several different chemistries implemented today in lithium cell technology, and their safety properties have improved with industry innovations driven by the portable electronics and electric car markets. Disposable primary cells, which I will not be going into in this essay, can come with numerous chemistries - some of which have remarkable properties such as being able to function at very low temperatures or being less prone to catastrophic failure when abused. While disposable primary cells are built to emulate 1.5v alkaline batteries and rarely exceed 1.8v, rechargeable lithium batteries can range from 4.2v to 3.1v but only a few of the many technologies are readily available to the consumer market.

Without further dividing and confusing the situation by describing all of the available electrode material combinations and electrolyte options, we can generalize that two of the three readily available lithium rechargeable battery types are built with an eye for safety. Hobbyists regularly utilize Lithium-Polymer (LiPo) batteries in their remote controlled devices and, while they can offer the greatest capacity for battery size and weight, they are among the most volatile and should be avoided in this case in favor of more stable chemistries. Cellular phones and laptop computers utilize Lithium-Ion (Li-ion) batteries which have slightly lower capacities and voltages but are significantly safer and often built utilizing desirable precautionary components. Fancy new Lithium Iron Phosphate (LiFePO4) cells are designed with the hopes that they will resist exploding even if your electric car is in a high speed crash, with an additional tradeoff of voltage and capacity but a surprisingly high current output capability.

The inherent stability of a battery’s chemistry is only the beginning of the precautionary selection process. Inbuilt protection circuitry and a cell’s or pack’s exterior construction make a significant difference in the safety and ruggedness of a battery. Individual cells such as the familiar button-topped cells house the components in an only moderately crush resistant cylindrical housing. Hobby battery packs for remote controlled vehicles or robot building usually utilize several flat foil-contained rectangular “prismatic cells” held together with plastic shrink-wrap, and offer little to no physical protection. Cylindrical cells can also be connected and then shrink wrapped into higher voltage and/or higher capacity battery packs, and due to their wiring are slightly less protected from physical trauma than their individual cells.

Small purpose built Printed Circuit Boards (PCBs) or programmed microcontroller chips capable of performing electrical protection duties are not always included but can be easily integrated into any variety of battery. A simple label claiming that a battery cell or pack is “protected” is insufficient information for a proper safety assessment however as there are several such precautionary tasks that a PCB can implement and they must be properly matched to their cells and application. Most battery fires occur during charging, and the most common precaution for a PCB to provide is a check against overcharging or overvolting. Short circuit protection should also be sought after as it will protect an otherwise healthy cell from damage caused by malfunctioning equipment or improper handling. Low-voltage cutoff protection features are also regularly implemented to prevent the cell from being drained too low and ruined, unable to be recharged.

Not always present, overcurrent protection is also quite important as it keeps the battery from being asked to provide more current than it can safely, which can lead to a fire. While this protection feature is perhaps less vital for lower draw applications, for uses where high or unregulated current will be required careful selection of high quality batteries is necessary. For flat battery cells/packs with no rigid housing or packs of wrapped cylindrical cells with no inbuilt protection, the PCB is usually contained inside along the top or side of the shrink wrapping which may prove vulnerable to damage. Some battery packs feature balance charging, which allows a compatible charger to individually monitor the health of each cell in the pack but requires a separate balance-plug to be wired and used while charging.

Cylindrical cells have their protection circuitry contained inside the housing, and are either a few manageable millimeters oversized as a result or have a slightly lower capacity than an unprotected cell of the same manufacture. Lithium rechargeable cylindrical cells are built and sold in the same size as alkaline/NiMH batteries, but a simple misidentification can result in serious damage to most devices likely intended for only a third of the voltage of a single lithium cell. As such care must be taken when using lithium batteries that are sized to match a standard Alkaline cell.

The usual naming scheme for cylindrical lithium cells pertains to the measurements of their dimensions, with a AA battery size being a “14500” and the most common similar lithium battery slightly larger and named “18650”. 18650 cells, mainly due to the increasing use of that size and the manufacturer competition that has resulted, are rapidly proving to be the best consumer-grade cylindrical rechargeable lithium cells currently available. An 18650 Li-ion cell with no protection circuitry is unlikely to truly hold more than 3400mAh, and the inclusion of protection circuitry might reduce available capacity by between 400-600mAh. Most of the cylindrical lithium batteries on the market today boast unrealistic capacities while their actual capacity is much lower - online research can usually either confirm a claim or provide the correct information for the brand in question. Similarly, the overvolting claims are usually lower than true performance reveals but still well within safety parameters for the cell. Third party analysis is the only trustworthy source for critical information on precise protection circuit and capacity performance.

Appliances such as headlamps featuring cylindrical cell holders sized for lithium batteries like 18650s are becoming increasingly common, and a well designed device will safely house and provide additional physical protection for the batteries inside it. Physically protecting spare cylindrical batteries or battery packs while transporting them is very important too, although most of the battery holding cases sold are merely organizational and are constructed of light plastics with simple hinges and latches providing little to no strength against abuse and are not waterproof at all. Waterproof battery enclosures on the devices themselves as well as waterproof cases or bags for carrying spare batteries are essential precautions against failures due to water. An awareness of the maximum current draw of your equipment in relation to any overcurrent limits or lack thereof is essential as well - don’t go underground with a new absurdly bright light without knowing that it won’t demand enough current to cause your protected cell to cut out or unprotected cell to overheat.

Battery packs containing multiple cells wired together can also come housed within their own hard enclosures. Some are protected inside a plastic box with a cable jack and power switch on one end such as the common inexpensive 12v closed circuit television battery packs utilized by the newly released Blind Bat cave photo/video light. Some headlamps or other devices contain or require proprietary battery packs with specific protection attributes and custom connectors for attaching the battery to its device and the charger such as Petzl’s lithium power packs. Enclosed hard battery packs are safer than their wrapped cousins (even wrapped cylindrical cell packs) but are rarely waterproof unless built for an outdoor related purpose such as powering a bicycle light. A shrink-wrapped pack might be further protected underground by making sure it stays in a plastic bag with only its cable running out or even tightly wrapping the pack in kitchen cling wrap, then placing it inside a slightly padded and/or semi-rigid case or pouch - the use of waterproof connectors and plugs helps prevent a short circuit.

LiFePO4 batteries, with their reduced capacity in a given size, are unlikely to be more desirable than Li-ion batteries although this difference is much less noticeable with large packs such as those that might be powering video or communications equipment. A properly protected Li-ion cell can prove nearly as safe in a damaging accident as a LiFePO4 cell will and the added capacity will reduce the need for additional spares. LiFePo4 batteries are, however, usually capable of safely delivering significantly higher amperage than other lithium cells and perform better in truly high output scenarios such as sustained overly-bright spotlights, video lamps, power tools, or even portable defibrillators. Though harder to initiate than in other varieties, the catastrophic failure of a LiFePO4 battery can still result in the generous production of toxic smoke.

The first sign of a failing battery will be it getting noticeably hot after which a distorting or swelling of the cell itself will signify that damage has already be done and the battery should be retired, and next will come the fire itself. Lithium cells require special disposal or recycling and should never be opened or punctured or bent or dented or burned. As with any battery, the voltage coming off a cell when completely full after a fresh charge is higher than the listed voltage and equipment must be rated to work within this peak voltage range. Each Li-FePO4 cell is likely to start producing 3.3v, drop quickly to 3.0v, and shouldn’t be discharged below 2.8v - Li-ion cells can put out 4.2v right off the charger, drop quickly to 3.7v, and shouldn’t be discharged below 2.7v. Battery packs configured using series-wired cells in order to increase final voltage are subject to a compounding peak-voltage effect due to addition, while packs configured in parallel to add capacities will average the peak variation between cells and therefore have a narrower peak range.

Chargers are NOT cross-compatible between lithium battery types due to these differences in voltage and using a charger intended for one type on another is likely to result in severe damage to the battery or even a fire. Single purpose lithium battery chargers should only be used to charge the type and configuration batteries they were designed for. Most featureful programmable hobby chargers are capable of charging all three of the readily available chemistries but great care must be taken to not charge a battery with the charger in the wrong configuration (though many such chargers can detect a discrepancy and abort automatically). Many users choose to charge their lithium batteries in fireproof bags or boxes and most manufacturers warn against leaving lithium batteries charging unattended. Cylindrical cells with proper protection PCBs are of significantly less concern and can usually be safely charged on a simple matched regulated wall-charger with the assurance that between the charger’s monitoring circuitry and the battery’s PCB any malfunction will be averted.

It is likely that many readers have at least seen a video of a lithium fire. Almost all of the popular and informative YouTube videos showing a failing lithium battery in all of its glory are featuring the volatile LiPo batteries, but the severity of the smoke produced during failure is indicative of the technology. Li-ion batteries are far less prone to failing, as Bear Grills demonstrated on his popular survival show by wrestling for some time with a hunting knife trying to coax a cell phone battery to start a campfire for him. The image of an electric car in a high speed collision is enough to invoke a sense of the additional safety element inherent of Li-FePO4 batteries. An additional dangerous component that isn’t demonstrated in any such video is the quantity of heat produced when a lithium battery fails, which is substantial enough to severely injure anyone in contact with the burning cell and do substantial damage to equipment.

In conclusion, the dangers of lithium batteries failing catastrophically in a caving scenario are indeed very real. Proper cell selection, pack construction, and handling can reduce these risks to within a reasonable margin as to fully justify their use, but care must be taken both below and above ground to protect your batteries from water and trauma. Significantly higher capacities and voltages and available current await cavers willing to engage in the necessary research, equipment pairing, and general caution. Rechargeable lithium batteries are the present and future of compact mobile energy storage but far from all of them are cave worthy.

Re: Intelligently utilizing lithium-based rechargeable batte

PostPosted: Dec 17, 2012 12:33 pm
by NSS8921
Well done, James! And thanks very much! What 18650 battery and charger do you recommend? Should Tenergy PCB batteries and charger be disposed of?

Re: Intelligently utilizing lithium-based rechargeable batte

PostPosted: Dec 17, 2012 2:13 pm
by Chads93GT
PILA IBC charger is great. wish it had a digitaldisplay readout like the lacrosse charger for the AA nad AAA's though

Re: Intelligently utilizing lithium-based rechargeable batte

PostPosted: Dec 17, 2012 3:42 pm
by Extremeophile
NSS8921 wrote:Should Tenergy PCB batteries and charger be disposed of?

I read through this rather quickly, but I didn't see any mention of Tenergy. What is the basis for this question? Virtually everyone that has shopped for 18650 Li-ion cells has seen Tenergy cells, which are often very economically priced. My cells happen to be LG because I was looking for something with slightly higher capacity, but it sounds like you have reason to question the safety of Tenergy cells. I do have a Tenergy charger that allows me to customize the charging current and measure charge capacity. I have been using this for 2 years without issue. Is there some reason to doubt the safety of this equipment? Please elaborate.

Re: Intelligently utilizing lithium-based rechargeable batte

PostPosted: Dec 18, 2012 1:38 am
by mooreshire
NSS8921 wrote:Well done, James! And thanks very much! What 18650 battery and charger do you recommend? Should Tenergy PCB batteries and charger be disposed of?

I use a programmable hobby charger, but any Li-Ion charger that automatically lowers the current as the cell fills and then stops charging automatically when it senses the cell is full should be fine. There are many companies making top-notch cells right now, perhaps too many for me to list. :) I happily use Orbtronic brand batteries, but have heard good things about AW, Redilast, Callies Kustoms, XTAR, EagleTac, and EnerPower+ to name just a few.

As for Tenergy, most of their batteries lack the important overcurrent protection. They claim to be "protected" but if you read the fine print you might see that they are only protected against overcharging and over-discharging (which is important but not full coverage). The best batteries will have overdischarge, overcharge, short-circuit, and overcurrent protection and they'll list numbers for each.

If your light requires three and a half amps of power in high-mode and you provide a battery that is only really prepared to offer two amps or less, then you could have a problem. Most Tenergy brand 18650 cells claim a maximum discharge rate of half of their capacity per hour, which might only be one and a third amps. Even if the light uses two or more batteries in parallel configuration, that's still asking for more current than the batteries are rated to provide. The nicest cells are rated for more than seven amps of current draw however (more than two times their capacity per hour)!

Re: Intelligently utilizing lithium-based rechargeable batte

PostPosted: Dec 18, 2012 7:25 am
by NSS8921
Derek - my concern regarding Tenergy batteries is based on my experience with Tenergy NiMH AA's. I bought in bulk several years ago and used a Tenergy charger. Having used rechargeable batteries for three decades, I have no doubt that these are the absolute worst! Cost-wise, they have been more expensive than disposables. I have three Tenergy 18650's and I expect the quality to be similar to the AA's. Given the potential dangers associated with Li-ion batteries, I may be wearing a couple of bombs on my head! And now I find from James that the 18650's are not fully protected.

Chad - noted regarding the Pila IBC charger. The same that Marduke recommended.

James - thanks again!