Batteries demystified - part.2

There's life beyond the Li-Po as these cell types continue to prove

In part.1 we looked at the popular and versatile lithium polymer cell and this time we're going to discuss most other types of battery.
Firstly, let's examine some of the many other types of cell available to us while explaining the rationale behind selecting particular types for certain roles. First off though, remember that several different manufacturers / pack assemblers use the same basic cell when making up their products, then wrap it all up in a nice shiny label with their own particular brand name. However, even these battery packs can and do vary from one brand to another. Some of the reasons for this are down to general quality and matching of the individual cells - the better assemblers take care to choose individual cells from a batch, closely matched in things like internal resistance etc.

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

Perhaps the reason Li-ion cells have not found the popularity of Li-Po amongst us modellers is because of their lower voltage, higher weight, and inability to stand very high discharge rates without dropping voltage. They have a nominal voltage of 3.6V, a recommended maximum charged level of 4.1V and a lower minimum level of just 2.5V. Like the Li-Po they also require a special charger with specific charging algorithms of CC/CV. Generally speaking most Li-Po chargers can be safely used to charge Li-ion as their terminal voltages are very close and they can happily take a charge rate of 1C. Some of the later generation cells can be charged at up to 2 or even 3C.

Storage and handling requirements are similar to Li-Po packs which, of course, we discussed in detail in Part.1. By way of a reminder, consider the following. Store a Li-ion pack at say 25°C 40% charged and expect a capacity loss of around 4% in one year. At the same temperature, but stored fully charged, the same cell will lose 20% a year! Proper cell balancing is also necessary and despite claims of greater safety, care should still be taken to avoid high temperatures, overcharging and other such mishandling. There's no doubt they're safer than Li-Po and it is this aspect which probably attracts the majority of users.

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.

Firstly, due to its material structure its capacity is actually lower on a like for like weight basis than any of the other lithium based cells. However, this aside, there are important benefits over the other types. For instance, there is a growing trend to using lithium batteries in flight packs due to their very low self discharge, ability to deliver high currents and hold voltage well under load, however the voltage of a single Li-Po cell is too low to power our radio gear, whilst two cells in series is generally too high.

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...

Perhaps surprisingly, generic lithium based cells actually have quite poor electric qualities, through intrinsically low electrical conductivity, and several techniques are deployed to improve this. What makes genuine A123 Systems cells stand out from the crowd is their patented Nanophosphate technology.  Put simply, this process involves coating, replacing and then converting the LFP material into nano particles - all clever high tech stuff resulting in a cell that can handle very high currents, rapid charging and excellent thermal stability.

The 2.3Ah M1 cells have huge current capabilities, officially up to 70A continuous, with 10 second pulse currents as high as 120A - that's a whopping 30C continuous with 50+C bursts!  Another big advantage over lithium polymer is their very fast charge times. These little powerhouses can be charged at up to 10A (4C) which potentially means a re-charge time of less than 15 minutes. A123 Systems even claim that they can be charged in as little as five minutes without damage! They also seem far more tolerant of abuse than Li-Po and do not suffer the same fate if allowed to fall below the recommended low voltage point of 2V. As A123 say themselves, A123 materials are designed to ensure that all the lithium is fully extracted from the cathode when fully charged. As a result, safety issues relating to overcharging are eliminated because there's no lithium left to plate on the anode in an overcharged state. This contrasts with conventional Li-ion cells, which only extract half their lithium when they reach their upper cut-off voltage. Conventional Li-ion cells are easy to overcharge and, once in this state, they can continue to extract lithium, putting the cell in a dangerous mode and making it prone to fires and explosions.

This safe construction also means many fliers are happily charging their packs in-situ. One flying buddy has even managed to let an old 8-cell pack drop to 0V, yet after recharging it is now performing almost as well as ever with no signs of ill health, although long term overall pack life has most likely been reduced.

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.

The LVC (low voltage cut-off) point of most speed controllers is usually only adjustable for either nickel or Li-Po cells of varying counts, so some experimentation is needed to allow the M1s to reach their correct low voltage level, thereby preventing the ESC from cutting power to the motor prematurely. M1 cells have almost nothing left at 2.5V, and pushing them right down to 2V or lower is when excessive heat and cell unbalance occurs.

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

I suppose no article on batteries is complete without a mention of the old stalwart nickel cadmium (NiCd) and nickel metal hydride (NiMH) cell. These are still the choice of most fliers for radio power and indeed certain electric model powertrains. Sharing common features in most respects, these types do however also need proper attention if they are to perform correctly, so lets start by looking at the NiCd cell.

NiCd cells are on death row due to their environmental implications. Disposal of cadmium - a toxic heavy metal substance - requires special recycling processes. Many modellers will bemoan the passing of these cells, as they found favour with those wanting high discharge currents and fast recharge times, coupled with a generally trouble-free durability. They have a nominal cell voltage of 1.2V and a good cell will peak at around 1.5V or even slightly higher when fresh off charge.

Most NiCd compatible chargers will use what is known as NDV (negative delta voltage) detection to correctly terminate the charge process when charging at reasonably high rates of .5 to 1C. The common wall wart style charger is set to deliver only a small trickle charge current of around .1C and will normally need to be left on overnight. NiCd cells do exhibit what is commonly referred to as a memory effect and although not balanced in the same way as lithium cells, they benefit from both an initial forming charge from new, followed by periodic cycling, whereby the pack is fully discharged down to a preset level (normally 0.85 to 1V per cell) and then recharged fully.

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.

NiMH.

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.

It should be noted that there is a considerable difference between a standard car battery and a proper leisure or marine battery. The latter are specifically designed to stand a deep discharge and recharge, which is far more relevant to our use. Lead Acid batteries should not be fast charged - around .1C is the best rate for maybe 24 hours or so. Several trickle type chargers are now available for very reasonable money, designed to be left attached constantly and keep the battery topped up to a satisfactory level at all times. A PB cell has a nominal voltage of 2V and should be charged to around 2.45V, meaning a charged 12V battery will show around 14.5V or slightly more when full. Cells should not be allowed to drop below 1.75V if possible, as permanent damage will be inflicted. Finally, disposal of nickel based and Lead Acid batteries should be via a proper battery disposal site.

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.

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