TL;DR - Fully charged NiCd and NiMH packs need to be recharged regularly because they lose their charge at a rate of 20-30% per month. Lithium-based batteries lose their charge as well - but - at a lower rate of more like 5% per month. However, you shouldn't store them fully charged or they'll go bad (the exception being LiFePO4). Lithium batteries are generally based on three broadly defined chemistries, which in decreasing order of voltage are LiPo (3.7V),
Li-Ion (3.6V), and LiFePO4 (3.3). LiPos, the ones with the highest energy density, should NEVER be stored fully charged. Ever. Even just overnight. Instead, you should run the 'storage
voltage' mode on your charger expressly to take them to a a lower voltage before putting them away (I use metal ammo can). For further info, continue reading.
On Modeling's Different Chemistries
Rechargeable batteries form the backbone of the sport of Remote Control modeling. These batteries are divided into two distinct classes. There are the ones for powering the control-electronics, and the ones for powering the model's propulsion motor. Both classes can use either chemistry, but due to differences in how much they weigh, those for propulsion are overwhelmingly of just one type, LiPo. More later.
For powering control-electronics there are two basic chemical choices. These two chemistries make up 99.9% of the available packs. They're the ones based on either nickel - or - lithium. Rechargeable nickel-based batteries have been around since the 1950s - they even went to the moon! Lithium is the new kid on the block, but more than 30 years old it's no kid! Both are very developed technologies. Lithium has the advantage of packing more energy into the same amount of space - it has a higher energy density. However, this makes it less stable and prone to catch fire. Lot of this in the news, right? There's a reason. Anyway, let's first discuss the older of the two technologies, nickel-based. The primary point is; they're very stable and easy to use so there's still a market for this technology despite the relative weight disadvantage compared to lithium-based cells.
Nickel-based: of the nickel-based chemistry, there are two distinct battery cells. These are NiCd (Nickel-Cadmium) and NiMH (Nickel-Metal-Hydride). NiCd appeared first and date back into the 1950s when they were a wonder to a world more accustomed to dry carbon pile type batteries that weren't rechargeable. Imagine having to buy a new battery every single time you wanted to use your model - yikes!
Anyway, for our purposes, both NiCd and NiMH effectively produce 1.2V/cell and are typically combined in series into 4-cell packs (short for battery packs). The resulting voltage for one of these is calculated like this. Since they are in series, you add up the voltage and thus, a 4-cell pack is 1.2Vx4cells=4.8V and 4.8V became popular with modelers due to do with the preponderance of the electronics industry operating on 5V nominal and those components being used for RC stuff. Basically, this means a 4.8V DC pack - whether made up of NiCd of NiMH cells - is plenty close enough to work with 5V components. And this is good because the modeling world is a 'very' small slice of the electronics world and thus, making something 'special' for what are considered toy models isn't a big market. Making do with industry standard components is how things get done in modeling.
So a four cell pack look like it's made up of AA-Alkalines because the NiCd cells are of a similar size and shape. Then enterprising modelers discovered they could use 5-cells (thus making 6.0V) 'and' their electronics didn't burn up due to the higher voltage 'plus' the servos produced more power and operated faster. And virtually overnight, the word spread and 5-cell packs became the rage. But some servos burned up on 6V and this set off a race to develop servos specifically for higher voltage.
At about the same time nickel-based chemistry gave to higher energy density lithium-based chemistry. Now instead of 1.2V/cell we were looking at cells making from 3.2-3.7V/cell depending on the mix of chemicals. A further advantage of higher voltage cells is packs contain fewer cells, e.g. two 3.6V LiIon cells makes 7.2V versus needing six 1.2V NiMH cells to make 7.2V. Anyway, just as NiCd gave way to NiMH due to its having higher energy density, NiMH is giving way to lithium chemistries. All things being equal higher energy density is always desired. Note; another reason for NiCd giving way to NiMh is the latter uses cobalt instead of cadmium in the electrode, which is much more environmentally friendly.
Meanwhile, the technically obsolete 5-cell NiMH remains a popular option for powering avionics (and especially the ignition of gasser engines) so we continue to offer these. Furthermore, for reasons having to do with increased reliability, we have them made with two leads instead of one. Basically, this lets us use two switches instead of one, which is important because switches are the single most most failure prone component in a model airplane. And having two means the odds of two switches failing at the same time are astronomical, so running two $10 switches instead of one is stupid cheap insurance! Note; all ProModeler packs use two leads once they're larger than 1000mAh because that's about where the split lies in terms of the size/risk/value for the models involved. Anyway, below is a photo of the popular 5-cell NiMH pack equipped with two discharge connectors.
- 5-cell (6.0V) NiMH battery pack is 2-3/4" x 2" x 9/16" and weighs 4.8oz
Tips on handling NiCd and NiMH packs
Note; both NiCd and NiMH packs can be stored fully charged - but - they need to be charged regularly because they self-discharge, e.g. lose their state of charge. And at a significant rate!
How quickly do they discharge? The rate for either may be as high as 10-20% in the first 24-hrs and from 10-30%/month thereafter (why is beyond the scope of this white paper). The point being, after a couple or three months they're effectively too discharged to use without recharging. Learn more here:
NiCd - https://en.wikipedia.org/wiki/...
Ni-MH - https://en.wikipedia.org/wiki/...
Lithium-based: of the lithium chemistries, there are four commonly available types, and in decreasing order of voltage they are LiPo, Li-Ion, and LiFePO4 (LiFe packs are the same 'stuff' but packaged differently). Learn more here:
LiPo - https://en.wikipedia.org/wiki/...
Li-Ion - https://en.wikipedia.org/wiki/...
LiFe and LiFePO4 - https://en.wikipedia.org/wiki/...
While the self-discharge rate for popular LiPo packs may be just 5% per month, e.g. significantly less than nickel-based cells, the very real downside of storing these bag-type cells fully charged is they're prone to 'puff' or swell. Thus, they 'must' be placed at their proper 'storage voltage' by your charger before being put away. Review the care and feeding white paper (link below) for recommendations on charging and storage of LiPos.
Care and feeding of LiPos - https://www.promodeler.com/ask...
With respect to LiIon and LiFePO4, both are built in cylindrical shells like NiCds (and ordinary Alkaline cells). Meanwhile, it's my experience packs marketed as LiFe have similar chemistry to LiFePO4 but are manufactured within a plastic bag instead (something a lot like a Ziplock bag).
Cells built within a 'bag or pouch' don't feel rigid like a cylindrical shell. Instead, they feel like something was poured inside and left to dry/harden but hasn't quite. The point being they're surprisingly easy to fold in half (like a taco), which destroys it. Basically, they're less robust than cells made within metal cylinders - but - because they can be made to fill more volume they have a definite place in the market. Of course, this is all just my opinion, so you'll have to form your own.
Anyway, it's also my experience pouch-type LiFe packs aren't (despite many, many who will attest otherwise) quite reliable enough for me. This is because I've seen too many packs with a dead cell (thus, rendering the whole thing useless for commercial purposes). And with the reference to 'commercial purposes', this just means replacing a cell involves too much cost (so we just toss them, which adds to the cost of the ones that remain in the batch).
Bottom line? We want no part of LiFe packs and no longer handle the product due to how many seem to just go bad sitting on the shelf in a short amount of time and the number of complaints they've generated (three different vendors, same results, and I don't know why). Thus, at this time I stay away from using LiFe packs that look like LiPo because they're built in a pouch (or offering them). Of course this may change. Meanwhile, if I want LiFePO4 chemistry (and I do) then I go for the cylindrical cell-structure.
Why? For me (as a businessman) if LiFe packs go bad it's a minor economic nuisance. However, when installed in a model, because there's what 'I' perceive as a significant risk of crashing for reasons unknown to me, then I as a businessman want no part of it regardless of how much money there is to be made in selling them - or - how many folks will step forward to relate how they've used LiFe bag-type packs forever without issue. Basically, 'everybody' has a different tolerance for risk and this speaks to mine.
For those who love LiFe packs of construction similar to LiPos, there are plenty of vendors willing to offer them (and quite cheaply, too). As for me, this alone speaks volumes if you care to listen to the message behind the pricing scheme.
Meanwhile, and in general I love LiFePO4 chemistry for control-electronics and ignition power - just not in the ones built in a pouch. This is because of all the lithium-based chemistries, they're the only ones I feel comfortable walking away from after fully charging - like I can with NiMH batteries. And what makes them better than NiMH is I can use it six months later - without first recharging the pack - because it's still got maybe 95% charge which is usually enough for a day's flying . . . after sitting on the shelf six months!
Note, LiFePO4 cylindrical cells are made by various companies but the ones I prefer are the A123-branded cells.
Anyway, while it's my experience you can charge A123 packs and walk away from them for a month or two before putting it in use (because they'll have only lost 5% of their charge and believe me, this contributes a lot to my preference toward using these for control-electronics despite other factors like slightly lower voltage/cell,) please note 'I' still tend to put them into storage mode, first. Why? Honestly, the why fills books and is, quite simply, far beyond the scope of this paper. Please just put it down to 'my' tendency to baby my battery packs because they're expensive.
And it's not just 'me' who feels this way because, as a friendly competitor wrote after kindly reviewing the contents of this white paper with an aim to helping me improve it and said . . . "It's my contention that pouch format packs REGARDLESS OF
CHEMISTRY are nowhere near as reliable as cylindrical cells. In a Giant
Scale fuel powered aircraft you are better off with a low impedance 5-cell NiMH or Nicad than a pouch-type 2-cell LiFe pack in terms of
reliability . . . and a real A123 pack, if not brutalized on a
charger, run flat, or cooked in assembly by hot welds or inept soldering,
will outlast and out perform any other technology pack available.
Factor in that longevity and reliability, and the price of a genuine
A123 pack is also the least expensive."
LiPo - handle with care!
Importantly, of the Lithium-based chemistries, the only ones I'll charge whilst within my model are packs made up of LiFePO4 cells. And I'll reiterate, these are only built within cylindrical metal cells similar to a AA Alkaline batteries you buy for a flashlight - but - are somewhat fatter and longer. Note; the typical 18650 cells is 18mm in diameter and 650mm long and the cells we use have the nomenclature of ANR26650M1-B.
Back to lithium chemistry within a bag. LiPos are the most popular
chemistry for propulsion because they weigh the least for the amount of
juice they contain and the space they take up. Read that again if you need to but all I'm saying is they give you the most ooomph while weighing the least and taking up the least amount of space in your model. Unfortunately, they're also far and away the more volatile of the bunch.
The basic rule of thumb with respect to LiPos is handle with care. This is worth remembering because if you short one out
you'll discover they pack a big punch in a small volume.
- A123 cell pack (6.6V - 2500mAh) is 2-5/8 x 2 x 1 inches and goes just 5.7 ounces.
What's important to take away from this is of all the lithium-based chemistries, other than the A123 you'll ALWAYS see me remove them from the model before charging. You should too. And this, principally, is due to fire risk. Google is your friend; look up the horror stories and then exercise due caution. You've been warned.
Shapes, sizes, configurations, composition, balance connectors, etc:
Shape-wise Lithium-Ion packs are also composed of cylindrical shells - just like the LiFePO4 and the nickel-based chemistries (NiCd and NiMH). While a 2S pack (2-cells in series) is most common, for larger models, it's not uncommon to see 2S2P packs (2-cells in series, then the two sets of these are wired in parallel).
These look like the photo below - but - remember, they're not 4-cells in series like a NiMH pack even though they look similar. And critically, they shouldn't be charged using a charger made for nickel-based chemistries. You've been warned! One giveaway these are different is the JST connector (the little white one). This taps into each cell so the charger can monitor the voltage of each cell as it charges the pack. This is a distinguishing feature of 'all' lithium-based chemistries. Why? it's because the chemistry is more volatile and has to be treated with greater respect. Respect whilst charging in practice means the charger needs to know the voltage of every cell in the pack.
You can do this quite easily if you have a wire going to every cell so you can measure its voltage. The JST connector is how these wires all come together for connecting them to the charger and for a 2-cell pack, it means 3-wires. The rule of thumb is one more wire than there are cells so a 6-cell pack has a JST connector with 7-pins for the 6+1 wires.
- LiIon 2S2P (7.2V) is 2-3/4 x 2-7/8 x 3/4 inches and goes 6.6 ounces.
Note; it's also worth noting the composition of LiPos is all over the map and while they're all called 'LiPo' the actual chemical make up is usually either LCO or LiCoO2 instead of just Li. Meanwhile along with LiFe (Lithium-Iron-Oxide), these packs are shaped like bricks because they're manufactured in flat flexible packs (technically, bag-cells are soft-sided and flat so then can be made an infinite number of dimensions). This, versus formed within rigid round metal shells of set diameters and lengths. Another thing is you'll run across the term prismatic and this means rectangular cells like a LiPo but but encased in a plastic or even metal. We don't really see these often in modeling but I mention it in case you run across the term and wonder.
Anyway, while LiPo-style vary in length, width, and thickness, our 2S2200 (pictured below) is representative of the typical bag-style construction in both size and shape. And propulsion batteries are overwhelmingly the same brick-like shape.
Note, a distinguishing feature of packs intended for control-electronics is the discharge connector. Like the NiMH shown earlier, the 2S2200 below also has two JR-style discharge connectors (in addition to the JST balance-connector found in lithium-based chemistries). JR-style refers to the small black connectors and they are rated by the manufacturer at 5A each, which isn't adequate for a propulsion setup where discharge may be on the order of 10X as much, or more. Note, these can dissipate several watts as heat as more current is forced through them so 5A isn't a wall, but just where they're rated. Anyway, for propulsion uses, because they have to handle more current you're going to see a significantly more robust connector (more later).
- 2S LiPo (7.4V) is 4" x 1-5/16" x 11/16" and weighs 4.3 ounces
Remember I said the LiPo must never be charged within a model due to fire risk? Fortunately, because many of my models are electric powered (propulsion) instead of using an internal combustion-engine (fuel - either nitro or gasoline-based), removing the control-electronics battery for charging if I use LiPo for propulsion is less of a pain in the hind end than it may seem for two reasons. Easy access and the use of BECs instead of batteries.
Addressing access first; savvy designers - quite aware of the fact the batteries need to be removed for charging due to fire risk - make it easy for the pack to be easily removed via access hatches - sometimes the entire canopy assembly comes off revealing the entire structure near the CG of the model.
Easy access is not, however, a prime design constraint for batteries intended for control-electronics because the mass of the pack is quite a bit less and are often used for achieving final balance. To the point in many models it actually becomes inconvenient to remove the receiver pack (hence my preference for LiFePO4 or NiMH packs which can be safely left in place).
Further note; with respect to propulsion, the only realistically viable chemistry for most models is LiPo because of it's significantly better Watts/pound. Quite simply, they weigh less than a comparable pack making the same amount of voltage and of the same capacity but composed of cells using other chemistries. It's not really even close!
Anyway, the most important thing to take away from this is; you never leave your LiPo or LiIon packs in your model for storage or charging. Let me repeat because it bears mentioning again, the only battery pack I'll leave within a model is one made up of NiCd, NiMH, or A123 cells. Otherwise, I'll never leave a battery pack in a model for storage or charging . . . NEVER. Word to the wise, eh?
Connectors for propulsion packs:
Below 50A the Deans Ultra plug is very popular. These have been cloned in the orient and the problem with this is some are good, some are less so because the driving force for this was price. The genuine item is a proprietary product of a USA manufacturer and I rather like them for smaller models because they're compact. The downside is many find they're more difficult to solder than other types of connectors.
I'm in this camp 'personally', plus, I find it's easier to deal with bullet connectors because you can very easily configure series packs, e.g. take two 4S packs and make them into an 8S pack. Meanwhile, there are two companies trying to promote a plastic housing for the bullet connectors. These make it easier to make the connection but of course, they're incompatible. Grrr!
The blue ones are a proprietary product of Horizon and the yellow ones are an A-Mass product. Both house ordinary bullets in various sizes ranging form 3mm on up. Since we don't want to pick a fight, we offer both on our website but personally, I don't want to be roped into anybody's proprietary anything and thus, 'I' tend to use plain bullets. Especially because they're far more compact than the things encased in a plastic shell. Anyway, as a modelers we've never had it so good because we're offered a cornucopia of choices! You might say we suffer under a burden of too many good solutions. Anyway, make your choice and move on.
Meanwhile, the best bullets I've found are made by A-Mass so they're what we offer. Better being defined as consistent which as an engineer I view as very important. Honestly there's not a lot of money in selling these thing because everybody and their brother offer them online. Anyway, we do it principally as a convenience for you. Last thing and I'll move on; others tout their proprietary bullets as best but once again, I fail to see the benefit of getting roped in to any proprietary solution and since it's your money, I advise you just vote with your wallet!
At ProModeler we offer battery packs composed of all these chemistries. Moreover, partly because I want to offer the best deal possible (but also because I have the habit of grabbing them off the shelf for my own personal use), I tend to buy the best cells available. This is why I like buying from the companies sullying the cells to the top brands in the business (they'll tell me because they're proud of who they sell to). Having cells as supplied to competitive products of the top brands gives us the same superior performance - but - we can price them down with the no-names using cheapo cells. Why? It's because (if we're being honest) we don't spend a dime on advertising and promotion (no fancy adverts in the magazines and no team pilots, etc.).
Since I can buy in bulk and sell them cheaply we sell out very quickly, which is important because the battery-business is similar to the green-grocer business in that the product gets old whilst sitting on the shelf - just like bananas go bad. The point being, just like you don't buy spotted bananas you don't want batteries that have been sitting around on the shelf a long time. So we buy small batches and for this reason are often out of stock and are awaiting more. Don't get frustrated when we don't have the pack you want and use it to your advantage by knowing they'll be fresh when you get them through us.
Packs for internal combustion engines:
While I fly a lot of electric-powered models, for my nitro and gasoline powered models, I've largely switched to lithium-based battery packs for the control-electronics because they weigh less for the same output than nickel-based chemistry. Specifically I use packs made up with A123 cells because I've judged them to be the best and because I'm often using the pack to help me balance the model. The point being, once it's lodged against a firewall and buried under a fuel tank - because I'm too lazy to remove it for charging - I want to use a pack I know is safe to leave in place.
I also like these for ignition modules where a 2S2500 will easily last a whole weekend without recharging. However, if there is a downside of the A123 battery packs (other than cost) it's that they 'only' produce 3.2V/cell versus the higher 3.6V/cell of a Li-Ion or the 3.7V/cell of a LiPo (all values nominal).
recall I mentioned modelers switched from 4-celll to 5-cell packs because servos produced higher torque and faster speeds on higher voltage? The corollary is the produce less at lower voltage, right? Since I design our servos specifically for HV (high voltage) this means I consulting the specs page to ensure I'm going to get the performance I need at the voltage I'm planning to use to feed the control-electronics system.
Pay attention to what I just said; you should consult the servo's specifications-page to ensure the servos you select will delver the torque and speed you want at the voltage at which you intend to operate them.
People ask me all the time about the relationship between voltage and current. A smart fellow in one of the forums critiquing this paper at my request intelligently observed I should point them to the usual internet primers that compare voltage, current, and resistance using a bucket a water with a hole in it. There are many and this one is pretty good because it even includes a brief video:
where the larger the hole equates to more current. I like another analogy, the one that compares how high you mount a pet door.
Voltage is like how high you mount a pet opening in your door. You can mount it close to the ground at 4.8V high. A bit higher up the door at 6.), midway at 6.6V, higher at 7.4V, or as high as your pet can jump at 8.4V. Yes, I know I am stretching the analogy but bear with me because I'm going somewhere. The specs also mention current and current is like the size of your cat or dog. The point being, if you mount the pet opening it has to be large enough for your pooch, right? This tongue-in-cheek graphic tries to make the point and no, the pooch wasn't hurt in the process!
Anyway, with respect to the specs, we try to make it easier to understand by sharing the details for the performance of each servo at 4.8V, 6.0V, 6.6V, 7.4V, and 8.4V - here's an example using our very popular 180oz-in standard size servo . . . the DS180DLHV, which sells for just $40. Note in addition to just the voltage range, and tying into what is mentioned above, please note we also share current consumption so you may chose the proper wire gauge for servo extensions and basically plan your installation ahead of time.
What's important in the above chart? All of it! Seriously, those of you quick at math will realize the first two columns, 4.8V and 6.0V are for guiding those of you interested in continuing to use nickel-based packs of 4 and 5 cells in series. Meanwhile, the nest two columns; 6.6V (nominal) is for 2-cell LiFe and LiFePO4 because they're a bit hotter than 3.2V/cell when they first come off the charger, while the 7.4V column is for Li-Ion and LiPo (7.2V and 7.4V/cell respectively). This is because 7.4V is plenty close enough for the purpose . . . but what about 8.4V?
We share performance specifications for 8.4V because of the popularity of using a BEC (Battery Elimination Circuit), which is just a fancy name for 'regulator'. This is a bit of hardware either incorporated into the ESC (Electronic Speed Control) or packaged as a stand-alone device. The ESC is the throttle for the electric motor and the BEC substitutes for a control-electronics battery.
Anyway, the BEC is used to steal juice from the propulsion battery pack and use it for the control-electronics. Basically, this device lets you parasitize power for control purposes by diverting it from propulsion. This works out OK because control-electronics use but a small (insignificant) fraction of the available juice in the propulsion battery.
Personally, I'm not in love with the whole concept but must confess to doing it for smaller models where the added weight of a separate control-electronics pack 'may' be an issue. Plus, a laziness-factor enters the picture for the many who just don't want to be responsible for charging two separate packs. Once again, personally, I view things in terms of risk vs. reward and for me, the question of how reliable the added components of the BEC circuit are vs. a battery with two cells and three solder joints. This enter my mind because I am an engineer by training and it's as natural as breathing to evaluate risk due to adding more and more components.
Anyway, with more valuable models I tend to view the battery pack as more reliable than the components of the regulator. Especially when added to those of the speed control or ESC because if you fry the ESC, then instead of a routine dead-stick landing you're going to lose control function as well (meaning you're going to walk to the crash to retrieve the pieces - and hope nobody was injured in the process).
To reiterate, with respect to larger models where the added mass of the battery pack isn't much of an issue, then for control-electronics I pretty much always use a dedicated pack - I prefer them made up of A123 cells.
For propulsion, by and large the winner is LiPo. These can be manufactured in a nearly infinite configuration of lengths, widths, and thicknesses. Varying the cell count adjusts voltage of the nominal 3.7V cell to whatever you want from a pack. Common propulsion pack configurations begin at 3S and go on up to 8S . . . within a single pack! Modelers tend to use multiples of these to create, for example, a 12S system by selecting two 6S packs and wiring them in series.
- This 4S LiPo pack generates 14.4V and three in series result in a 12S pack
Of the various chemistries available to modelers for their rechargeable battery packs, fully charged NiCd and NiMH packs need to be recharged regularly
because they lose the charge at a rate of 20-30% per month.
Lithium-based batteries offer three chemistries, which in decreasing
order of voltage are LiPo,
Li-Ion, and LiFePO4. These aren't stored fully charged and instead, are
placed at 'storage
voltage' by your charger before being put away. Importantly, the only
Li-based packs I'll charge whilst within my model
are those made up of A123 cells. For propulsion, the predominant chemistry is LiPo and you NEVER charge them whilst mounted within the model.
many smaller models are electric powered and use BECs, this is less of a
pain in the hind end
than it may seem precisely because for propulsion, the only viable chemistry is
LiPo. Since these batteries are removed each time for charging anyway, the need for removing a safer technology like LiFePO4 (often used to aid in balancing with larger models) isn't much of an issue because the BEC obviates the need for a separate control-electronics battery pack in the first place. And for larger models, which are usually (but not always) powered by engines instead of motors, the best lithium-based chemistry in my opinion for control-electronics is the LiFePO4 battery pack composed of cells otherwise know as A123. I use these for both control-electronics and for powering ignition modules.
Last thoughts; taken in its entirety, the above is a somewhat crude and incomplete product regarding the state of battery chemistry within the RC modeling world. But it is, I think, good enough to acquaint
you with the essence of the subject and guide toward online resources to help round out the picture. Furthermore, I remain grateful to those who have educated me and helped critiqued this information always with the aim of helping improve it, including but not limited to Rotorhead (my pal and electrical-mensch Steve Poretz), John61CT, SilentPilot, and Hangtimes (a competitor, Steve Anthony of NoBS Batteries). Anyway, any and all errors are solely my own.
Comments? Further questions? Feel free to give me a call at 407-302-3361