There’s life beyond the Li-Po as these cell types continue to prove
Remember also that the method of assembly and inter-cell connection quality can affect the performance greatly. Sometimes, as with most things in life, you get what you pay for, indeed those really cheap packs from a certain auction site that look so attractive may prove to have little in the way of longevity.
LITHIUM FLAVOURS
Generally speaking, most, if not all, lithium based cells have carbon anodes, and it is the different materials used for the cathode that create the different varieties and specifications. Let’s start by looking at the one that some of you may already know, the Li-ion. These differ from Li-Po cells both chemically and in the way they actually work, however lets just look at the important differences for typical applications. Li-ion is the normal choice for batteries found in mobile phones, laptops, and other portable devices. One obvious difference is in the packaging. Whereas Li-Po cells are encased in soft and rather delicate thin aluminium or plastic, Li-ion cells are more rugged and generally cylindrical. The Li-ion cells we will encounter are more akin to a dry cell, or even the more familiar nickel cell, and are encased in a metal can. They do come in a variety of sizes and capacities and there is also a type known as prismatic where the cell shape is not actually round at all, but moulded into suitable shapes to fit in the aforementioned mobile phones and music players.
Saphions seem rarely used, these packs are available from OverlanderCHARGING AND STORAGE
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Another variant on the Li-ion cell is the Saphion brand from Valence technology. This phosphate based cell is claimed to be one of the safest types of lithium battery available to modellers, and is available in a variety of sizes and configurations. It can be discharged at fairly high rates of 15C or more and benefits from a longer life span. Once again, however, they’re heavier and slightly less powerful than lithium polymer cells. Moreover, they’re not widely available in the UK, although Overlander and a few others have them in stock.
UP AND COMING
One particular type of lithium cell which is certainly gaining popularity is the LiFePO4. This unglamourous sounding lithium phosphate based technology consists of several varieties but differs from either Li-Po or Li-ion in several ways.
To get around this a regulator or a standalone BEC (Battery Eliminator Circuit) can be used which drops the voltage down to the more usable 5 or 6V required. This extra component however, increases cost, complication, and weight. The nominal voltage of a five cell nickel battery is 6V, and LiFePO4 have a nominal voltage of 3.3V, so two in series produce just 6.6V, which most radio gear is happy to accept, thereby eliminating the need for external regulators. In case you worry that this is still a little high, remember that a five cell nickel-based battery, fresh off charge, could well be at 7.5V or higher for a short time, whilst a fully charged LiFePO4 2s pack is only 7.2V.
A123 packs have been put to good use in certain types of models, Those you see here are available from Puffin Models
THE A123
By far the most suitable LiFePO4 cell for our application is the one known by several names including A123, M1, APR, or even (incorrectly) DeWalt. In fact the correct name for these 2.3Ah cells is ANR26650M1. They are manufactured exclusively by the A123 Systems Corporation of America, and are used in the manufacture of amongst other things, DeWalt power tools, from whence that particular identity was born. Many modellers have been known to buy DeWalt portable power tool batteries to get at the A123 cells inside. Thankfully, some retailers are now distributing them to the modeller. The designation M1 is derived from the last two letters of the product catalogue number and seems to be the most popular name used, so well stick with that for the rest of this article. A123 Systems produce another lighter, smaller, LiFePO4 cell of less capacity at 1100mAh and these could be ideal for use as airborne radio supply packs, as mentioned earlier.
POSITIVES…
The discharge curve of most other types of cell shows a gradual decline in both current and voltage as the cell approaches the end of its discharge. The M1, however, will hold its voltage and current capability almost right to the end, and exhibit a quite marked drop off right at the finish. Although this means there’s little warning of imminent power drop, the consistent high output available with A123 packs means you do not have to carry excess capacity.
Li-ions are usually good for only around 3C, above which they fall-off a lot. To get, say, 30 amps without suffering a large voltage drop, you’d need to carry around 10,000mAh of capacity. A pack of 2300mAh M1s will put that current out without even breaking sweat, and right until the pack is empty, too!
…AND NEGATIVES
First off they cost more than the equivalent capacity Li-Po. However, the expected life cycle, even when completely discharged at 10C (23A) is claimed to be at least 1000 cycles, so although they may well have a higher purchase cost, their total life cost is actually much lower than a Li-Po. They have a lower nominal voltage, so to get the equivalent voltage of, say, a 6s Li-Po pack, you’ll need a 7s LiFe. Meanwhile, they’re not as widely available as lithium polymer, although Puffin models can supply either single cells, or ready made packs, in differing configurations.
They are heavier and somewhat bulkier than Li-Po and at present are only available in two capacities, so this does rather limit their application to models of a larger size. Finally, remember that these cells do require a special LiFePO4 charge algorithm, which is now appearing on many multipurpose chargers, and of course, if you desire large cell count batteries and re-charge times of 15 minutes, your charger and PSU will need to be up to the job! Disposal should follow the same procedure outlined last month for lithium polymer cells.
Now mainly used to power transmitters and receivers, NiCds and NiMHs are still widely used although NiCds have all but disappeared from retailers.DEATH OF THE NICKEL CADMIUM
This type of care will help prevent the formation of damaging crystals on the cell plates. In extreme cases, one or more cells in a pack could deteriorate to such an extent that they enter a reversed voltage situation which will cause permanent and irreversible damage to the battery pack. Rapid charging of batteries that are not empty is bad practice.
Nickel based cells also suffer from high self discharge (except for a newer special type of cell which we will cover later) so if left for any length of time after charging, they will need a further recharge just prior to use. Although all batteries perform poorly in cold conditions, NiCd packs do fare slightly better than NiMH in this respect.
Although more environmentally friendly than their cadmium cousins, NiMH cells have some drawbacks. They can exhibit up to 50% higher self-discharge rates, and although they can stand very high usage currents, life span will be reduced considerably. Moreover, they cant tolerate regular deep discharge quite as well and have a slightly higher minimum cell level of 1V. Correct fast charging (again around .5 to 1C) is the preferred method but requires a different charger than that used for a NiCd, as detection of the smaller delta peak is much harder. Although better than NiCds, these cells can also suffer from memory effect and require occasional cycling to maintain them in best condition. Mind you, like NiCds, they don’t work well in the cold.
The NiCd/NiMH type can still find a use where weight is required, this is Dave Chinnery’s 123″ span Fairey Monoplane that flies on 21 cells.UNDISCOVERED SECRET
Low self-discharge NiMHs are perhaps one of the most exciting and relatively undiscovered secrets in the battery world. Introduced in around 2005, these have all the positive characteristics of regular NiMH cells but with one very important difference, their self-discharge rate is incredibly low. This means that they can be charged up and left for a very long time and still be ready for action whenever required. Several manufacturers claim that as much as 85% capacity remains after one year – even at normal room temperatures. These exciting new batteries are ideal for transmitters and are manufactured and distributed by several well known companies under different brand names. Some of the more common ones being the Sanyo Eneloop and the Vapex Instant. They can be charged and treated just like regular NiMH packs and I’m currently using four of these cells in a Spektrum DX6i transmitter with excellent results.
PB (LEAD ACID)
Not used in flight packs (well not anymore at least!) the humble lead acid or SLA (sealed lead acid) battery still has a place in the model flying inventory, from flight-boxes and starters, to powering battery chargers at the field.
Well, that just about rounds things off for now. I sincerely hope these two articles have helped you become a bit better informed where your batteries are concerned. And remember, look after them and they’ll look after you.
