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  just Stampede, Traxxas,
  to now include other
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  and cars. I still don't sell
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  enthusiastically promote
  those RC products I think
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     Take a look through
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  - Tony

  StampedeProject.com

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Lithium Power 101
Understanding Lithium Based Batteries & Chargers
As published in R/C Car Magazine November 2007
by Tony Arnold - StampedeProject.com

 Once upon a time, NiCad (nickel-cadmium) batteries ruled, then NiMh (nickel metal-hydride), and now power is making yet another shift to LiPo and lithium-ion batteries with exponentially increase current and runtimes per ounce, but to use this power we need to understand lithium based power.

Although there are tons of different brands, the RC industry has only two uniquely different lithium based battery packs available. Gel based LiPo (Lipoly or Lithium-polymer) packs such as those made by MaxAmps are the most common. The other type are lithium-ion based packs exclusively from A123.  Both have their advantages and disadvantages and to help us explain the lithium hubbub, I enlisted the help of my friends at MaxAmps.com and A123 Racing to help put together some key points as well as some suggested entry to pro level lithium based kits to test and make things easy.

Safety
Lithium based batteries definitely had their issues when they first came out and many people are still freaked out over the potential fire thing. Sure lithium based packs can catch fire in rare instances of negligence, damage, or defect and burn extremely hot, but those of use that have witnessed Nickel based batteries explode and then catch fire would argue that we are simply trading an equal but different amount of risk. 

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LiPo Safety
·Don’t Short, puncture/damage, over charge or over discharge.
·Store in a fire safe location
·Only charge with an appropriate LiPo specific charger
·Use of a optional balancer to balance cell prior to chargingis recommended
·Use a LiPo cutoff circuit to prevent over discharging
·Armor packs as your driving style requires
·LiPos should be used only by the more educated user.
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Including a couple normal battery safety items you should already have such as heavy gloves, a fire extinguisher, and a bucket of sand, a couple general precautions apply.  The biggies are; don’t short, puncture/damage, over-charge, or over-discharge the packs. This pretty much the same “don’t” list we all ignore for our Nickel base packs, however ignore these rules with Lithium packs and things get ugly.  If your negligent, you might have started what is called “Thermal Runaway” which is just what it sounds like, the pack gets hotter and hotter to the point of ignition. Old tech and less stable Lithium-ion based packs encased in metal would build pressure and literally explode during thermal runaway and gained lithium power a very bad rap.  A123 changed the lithium chemistry in their metal encased cells so thermal runaway cannot happen. The new LiPo packs are also much safer and if damaged the foil packs will simply start bulging like a sausage, end up popping like a balloon and could potentially self ignite, so grab your gloves and get that expanding pack into a fireproof container outside immediately and leave it there for at least 24 hours. Just like a venting NiMh cell, once a cell has expanded consider the entire pack toast and it should be marked for disposal.  

 
Safe Use
Scary stuff aside, lithium safety is pretty easy, a good LiPo charger assures proper charge, a cutoff circuit integral to your ESC or added eliminates over-discharging, and assuring secure mounting of the packs to the chassis and adding armor where needed, prevents pack damage in all but the worst wrecks.  Store Lithium based packs safely out of reach such as in a locked metal cash box, because it has a sweet smell that can attract pets and small children and can be deadly if eaten. I highly recommend using an optional “balancer” separately prior to charging or in line during the charging process adds an extra margin of safety and assure the LiPo cells in the pack are each charged equally and no overcharging occurs.If you really want to be hyper-firesafe, charge and store your batteries inside clay flowerpots with the pot inverted over the base to form your own fireproof container. 

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Super Safe Lipo

1. Store in safe & fireproof location 
2.  
3. Inspect the pack for damage or bulging prior to and after use and charging
4. Use a Balancer either before or during charging to assure uniform charging
   
5. Double check charging rates, capacities, and peak voltage, BEFORE starting charging.
6. Place the pack in a fireproof location during charging where it can be monitored.
7. Enjoy the runtime and power of Lipo
8. Repeat process

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Current Reality
The reality is that with better materials designs, cutoff circuits, chargers, & smarter users, those now extremely rare fires are typically caused by over-discharging without a cutoff circuit or pack damage.  Some companies are going the extra mile to solve these two issues by encasing LiPo packs in plastic to prevent punctures or with A123 they have encased their fire free and worry free dischargeable cells in metal cases and have a 15-minute charger to assure durability, cell balancing, and perfect charging. 

A123’s battery & chimp operate-able plug in and forget it charger system is probably safer to use than NiMh systems, however LiPo packs are not quite that easy.  LiPo is NOT for the beginners in the group. If you already know proper battery handling and how to select peak voltage, capacity, and charge rates on your charger, you are ready for LiPo,  however if you only know how to us a charger's auto mode, then either pick the A123 setup or hit the books on charging basics.  The only other critical safety issue is Lipo disposal that due to space considerations will be covered in a later article.

Voltage Cutoff Circuits
A voltage cutoff circuit or detector is required for LiPo packs.  Under normal use a LiPo cutoff circuit simply plugs in between the ESC and battery and reads the battery voltage and tells the ESC to issue a sputtering warning or just shut down once the pack dips below 3-3.2V per cell.  Stop activity when it alerts you and unplug everything from the pack and the pack is ready for the inspect, balance, and charge process. Whether you are using an ESC’s internal cutoff voltage feature or an external unit, make sure you are matching your settings or external detector to the packs you are running – changing from a 7.4V to a 11.1V pack will require some changes or even a different external voltage detector.

 Newer ESCs such as the Novak Rooster feature built in a voltage cutoff feature for LiPos.

Ups
The main advantages of Lithium based batteries are that they run cool, deliver lighter weight per Mh and offer very high discharge rates in the 150+Amp range that make brushless systems perform like nothing else all. Going from NiMh to Lithium power is like going brushless all over again.  For example, a 3300Mh MaxAmps LiPo pack will run about as long as your GP3300 NiMh pack, however the LiPo pack will be less than half the weight and size, will provide consistent power delivery from beginning to end of the cycle, deliver better acceleration, punch, and higher top speeds thanks to higher current output, and be room temperature warm at the end of the run. Lithium packs also support a higher number of charge/discharge cycles than NiMh, are relatively memory-less, and have an almost insignificant self-discharge rate. The eventual disposal of lithium based packs is also environmentally friendly – what’s not to love. 

The 20% uplift per Mh in price is about the same as what we paid when going from NiCad to NiMh and as before, the performance justifies the price increase.  For example, a MaxAmps 6000Mh pack is $100, an A123 pack is $50 for about half that capacity, and an old school 3000Mh NiMh race spec pack runs about $40.  Just like the switch to NiMh, LiPo requires a LiPo capable charger like one of the Hyperion models discussed here or with A123, they have their own Hypersonic charger.

Downs

The downsides are the touchier safety don’ts and that you finally need to buy that better charger you have been putting off for the last 3 years.  You may also need to spring for a few external LiPo cutoff or low voltage detectors for those older ESCs than don’t have built in LiPo cutoff features. 

 
Jargon Explained

In either type of pack you will see the configuration stated as something like 2S1P 6000HV.  This means that there are two cells in series and there is one set wired in parallel.  For example, a regular 6-cell NiMh pack would be a 6S1P pack.  To determine the cell count multiple the two numbers together, so a 3s2p (6 cell pack) means a battery pack containing three cells in series, and two sets of those series cells are wired in parallel. Just a reminder: series wiring adds voltage (V) and parallel wiring adds capacity (Mh).  As we are used to Mh, many LiPo cells show the Mh rating in HV this simply indicates a high current MH rating.

An extra plug or “tap” exits Lithium packs and is used for balancing/monitoring the individual cells during charging.  Not all taps are the same from charger to charger and brand to brand, so make sure the packs you are ordering have a tap compatible to your charger – if you need to, you can change or adapt them later.  Also specify what type of connector, for these high current packs, low loss Deans connectors are recommended.

Hyperion taps are among the many different pack balancing taps offered

Which Type of Lithium packs?

A123 Racing touts their packs as the safest Lithium based batteries available with “inherently safe Nano Phosphate chemistry that will not explode or catch fire”. These packs are based on sets of larger non-standard sized 3.3V 2300Mh cells which can be tricky to fit into standard battery trays.  Their 4600Mh 4 cell pack will not fit in any tray I have, however the little 2 cell packs work great in everything.  The A123 packs deliver somewhat standard outputs with 6.6V and 9.9V packs with higher voltage outputs reached by bridging packs in series. Current is much higher so simply gearing up a bit will boost top end speeds easily to 7.2V levels. One huge upside is that the A123 packs not only survived my severe mistreatment but can also be discharged with no regard to cutoff voltage.  That’s right, no need for a cutoff circuit just beat ‘em like NiMh packs.  A123 packs can only be charged with A123 compatible charger such as the Hyperion model, however their own proprietary A123 15-minute charger is the safest bet.

 A123 2S1P packs fit great in almost any tray

LiPo packs are a lighter than the A123 packs, available in a broader range of sizes, capacities, and current outputs than the A123 offerings, but will require a voltage cutoff circuit either added or integral to your ESC to prevent over-discharging and require more of an eye on safety. Like most LiPo packs, MaxAmps’s packs are built by stacking and wiring flat rectangular foil encased cells for different outputs ranging up to multi-hour running 12,000Mh packs with up to 600 Amp outputs.  LiPo pack capacities for most manufacturers are typically seen in standard voltage configurations such as 7.4V, 11.1V, 14.8V, however MaxAmps’ even offers custom packs. MaxAmps’ packs are compatible with any LiPo compatible chargers, feature heavy duty multilayered layered shrink-wrap for protection, and are available with your choice of plug, balancing tap, and even warranty options.  7.2V packs in the 5000-6000Mh range fit perfect in most standard battery trays for almost an hour of fun.

 [IMAGE 7 - CAPTION - What do you think the run time is on this 12,000Mh pack?]

Both types have their place.  If you want almost guaranteed safety, with the benefits of lithium based power, easy plug and play charging, and want to continue on your abusive path, go for the A123 packs & charger. If you want a high level of flexibility, extremely high capacity, even lighter weight, potentially higher current output, and obscene runtimes, then LiPo packs like those from MaxAmps will be your best bet.

What to Look for in a LiPo Charger

For the A123 packs their buttonless idiot-proof 12V powered Hypersonic charger a great option exclusively for charging A123 packs. Connect charger to car battery, plug the A123 pack into the charger, and a light to lets you know when it’s done.  This couldn’t be simpler.

 LiPo compatible chargers are now everywhere and will charge any manufacturers LiPo packs assuming you have the appropriate battery taps and will even pull double duty to charge your Nimh/NiCD packs.  Don’t get confused by Lipo charger ratings all you need to know is how many “S” your Lipo packs will be.  As an example a 5 Cell charger will support up to any 5S based pack regardless of the MH or “P” rating, but it may take a little longer to charge the higher Mh packs.  Most LiPo chargers will ask for the pack's voltage and capacity/rate.  Most LiPo packs are happiest with a 1C (1 X capacity / 1000) or less charge rate - 5000mh pack want a 5Amp or less charge rate. Too fast of a charge and you could be setting up for thermal runaway.

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Lipo Voltage Cheat Sheet
1S = 3.7V
2S = 7.4V
3S = 11.1V
4S = 14.8V
5S = 18.5V
6S = 22.2V
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Recommended Starter Systems

From A123 their Hypersonic dual 2300Mh 2S1P battery and charger starter kit for $220 delivers a system safer than almost any other battery system, packs that deliver 3300 type runtimes & Lithium punch, will fit in almost any trays, and includes their 15-minute charger.

 A123 2 pack and charger starter kit.

From MaxAmps.com choosing a 3300Mh and a 5000HV Lipo pack and nice little wallet sized metal encased Hyperion EOS5i (5A, 1-5S LiPo, 1-14 NiCD/NiMh) charger will cost about $250 but will provide a flexible performance charger, and LiPo packs with higher current, a battery tray fit, and extended runtimes. Add a balancer and you have a very nice sport system that will take you far and your LiPos very happy.

 The Hyperion EOS5i with a 6000Mh or 5000Mh pack is a good starter system.

Recomnded Pro Level System
The advanced charging and power nerds in the crowd will want to grab the fan cooled 180W 10A $170 Hyperion EOS1210i charger that can handle the 12S Lipo and 30 cell NiMh/NiCD charging needs of the racers and hard core bashers.  Geek out by networking two optional Hyperion LBA10 balancers to the EOS1210i to charge two packs of same cell type and capacity with every cell in both packs being perfectly charged and balanced, and even charge custom A123 build packs now being offered from… you got it MaxAmps.com. For all day fun, the $150 12,000Mh 7.4V MaxAmps pack will deliver hour+ runtimes – think about it, single pack track days.

12S and 30 Cell Charging, cell balancing...Not much this Hyperion 1201i charger with optional balancers won't due.

My Opinion
Having had personal experience with both A123 and MaxAmps, I have an opinion on each type of battery and each manufacturer.

The abuse tolerance and care free use of the A123 packs has been stunning. As a Beta tester for A123 almost a year, I have been down right abusive to their packs and they still work great.  My only complaint is that fitment can be challenging with the 3-4 cell packs, however creative mounting solutions for these packs have been rewarded with impressive performance.  A123 Racing is part of a sizable company that makes batteries for a variety of industries, so resources are plentiful for their RC division.  

[IMAGE 9- CAPTION - A123 Packs are worth the mounting hurdles]

My "Joe Consumer" experience with MaxAmps packs, products, and service has been positive. LiPo packs vary greatly in price and their LiPo packs are a very good value with high power, and good durability. For those more stupid stunts, I added some flat lexan armor for several of my MaxAmps Lipo packs.  Their flexible, powerful, and simple Hyperion chargers have performed perfectly. MaxAmps has fielded tons of my questions from packs to chargers to balancers, delivered good customer service, and has been a good single source for a variety of products and was a good starting place for me into the world of LiPo.

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[IMAGE 1 GROUP WITH IMAGE 8 - CAPTION - Special NTB (Nitro to Brushless) Conversions like this Brushless Project Revo'lution demand Hi Power & high Mh LiPo Packs.]

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Conclusion
Lithium power has had some bad PR, but many of the issues can be traced back to “user interface error” rather than a product fault.  By observing some common sense safety guidelines potential problems can be almost completely eliminated.Lithium power is here to stay and with ROAR finally adding an exhibition class for Lipo, the writing is on the wall for NiMh.  For the “gas’ers” new LiPo packs make all your electrics perform better and longer with less weight, for the “shockers” lithium power delivers a RedBull like kick to the life blood that runs your system.  If you are up for the next generation of power, educate yourself, use common sense, assure charge and discharge is within spec and keep the packs relatively protected in some way and you will be rewarded with high power and longer runtimes with less weight and the next evolution of power for RC Cars. 

SOURCES: MaxAmps www.MaxAmps.com,  A123 Racing www.A123Racing.com