Fly Electric!

'A123' Lithium Cells





A123 Systems introduced a new cell in 2006. It is known mainly as 'A123' but also 'M1'. The chemistry is Lithium Iron Phosphate which is sometimes abbreviated Li-Fe or Li-FePo4. These cells have most of the good features of both Lithium Polymer and Nicads. More specifically:

* Light enough to be in same league as lipos
* High enough voltage to be almost as good as lipos
* Flat discharge curve better than nicads and lipos
* Non-critical voltage extremes so less likely to be damaged inadvertantly
* Not sensitive to heat like lipos
* Inherently much safer and don't have to be monitored like lipos
* Cab be charged quicker than nicads! (15-20 minutes)
* Cheaper than lipos (if bought as DeWalt packs)
* Physically much stronger than lipos to withstand more crashes
* Easy to make DIY charger

Disadvantages:
* Only two sizes available
* Larger/bulkier than lipos
* Likely to need new chargers and balancers
* Have to consider ESC voltage and BEC implications
* Being marketed with their charge voltage (3.6v) rather than nominal voltage; very confusing for people (eg: both Li-Ion and A123 cells are marketed as "3.6v" but they are not the same measure)


CHARGING IMPLICATIONS

A123's are designed to be charged to 3.6v per cell. Lipo chargers charge to 4.2 so they must not be used (directly) with these cells. Don't be misled by the fact that your lipo charger is designed for '3.6' or '3.7v' lithiums (learn the difference between nominal and charge voltages). Your options for charging (from 12v) are as follows:

1) Upgrade or buy a new charger that can do A123's
2) Make or buy a Dapter - this allows you to use a nicad or lipo charger with A123 packs (it regulates the voltage correctly)
3) Make a cheap and very effective charger yourself...

The easiest way to make your own charger is to buy a 'universal car laptop adaptor'. These cost less than 8 from DealExtreme or ebay (delivered) but can only charge FOUR or more A123's. They require one or more mods; see here. To make a charger for 3 or less cells you will need a 'step-down' design (the laptop thingy is 'step-up').

In the two graphs that follow I illustrate how the laptop charger works and compare this to a 'standard' charge rate. The laptop charger (left) fills a four cell pack in about 15-20 minutes. This is well within the 5min charge speed which the manufacturer says is allowed, and on par with the 10A charger they sell. You won't find many commercial chargers with this capability at any price. Due to the characteristics of the laptop adapter, on a completely flat pack it starts briefly at about 16A but spends most of its time in the 7-9A region.

The right-hand graph shows a constant current, constant voltage (CC CV) charge using a regulated power supply at 2A. It reveals some interesting characteristics. A lipo reaches peak voltage usually no more than half-way through a charge (after which time current begins to drop). The A123 cell reaches full voltage a lot later (about 80% through the charge). As a result, the A123 pack takes the full charge current for longer so charges quicker than Lipos even when charged at the same 'C' rate. The A123's are pretty full after 60 minutes at 1C and the charge rate was down to less than 80mA at 68 minutes. A lipo at the same charge rate usually takes about 90 minutes.


'Laptop'
fast charge

Conventional
slow charge


CONTROLLER IMPLICATIONS

A123 cells have a fairly abrupt 'knee' at the end of their discharge (example). This means (i) it is very obvious when they are flat (same as nicads) but (ii) the drop to voltages that are too low can be quick. A123's are within spec at 2v and allowed (but not recommended) to drop to 1v. So a low voltage cut-off (LVC) is less essential but still important. Until you are able to buy ESCs with appropriate LVC's, look for those that have multiple settings. For instance, with the PC connector, the Castle Creations ESCs can be set to any LVC voltage. Some other brands offer 2.6v lipo settings or useful nicad ones. For instance some HiModel/eMax/Fdragon ESCs provide a nicad setting with a LVC voltage of 60-65% of initial voltage (this equates to about 2.1-2.3v per cell - perfect for A123's).

Most ESCs are rated for 3 lipo voltages. A123's have a lower voltage than lipos so 3 cells will be well within spec on 'lipo' ESC's but give you lower motor revs. 4 A123's are equivalent to about 3.5 lipos so yield higher revs, and more is always better! 4 A123's can be packaged as a nice rectangle and the laptop charger mentioned above is ideal. So 4 cells are my preferred number for A123's. The problem that you may experience is that 4 A123's have a voltage of 14.4v off the charger. 3 lipos are only 12.6v. Most ESC's are only rated for 3 lipos so you run the risk of damaging something with 4 A123's. So, buy ESC's that allow the use of BEC up to 4 lipos to be sure.

Most of use like to use BEC when we can. The BEC electronics in most ESC's is of the 'linear' type. This means they have to pull whatever voltage is fed to them down to 5v. This generates heat, vast amounts when servo currents rise. When these regulators reach a critical temperature, they stop working to protect themselves. When this happens you crash! 4 A123's being a higher voltage and giving more motor power will therefore generate more heat in the BEC regulator chips than 3 lipos (more volts to drop and you will probably fly more 'energetically').

Most ESCs do not have heatsinks on the BEC regulators so adding a heat sink will an important way to help them dissipate more heat. To be completely effective, you will also need a stream of air from a small fan or by appropriate placement in the plane. I would buy ESC's that (i) say their BEC can be used with 4 lipos (so you know they are electrically in spec) and (ii) have as high a current rating as possible (eg: at least 2A and 3 or 4A preferred). The more servos you have and the more current they draw affects the current rating required and how well you cool the ESC. ESC's by the way usually have many evenly-spaced chips on one side (the FET's that drive the motor) and the BEC regulators are usually on the other side of the board, usually have 3 leads and can be fairly chunky in size. The number imprinted on it will usually have a '5' somewhere to indicate 5v output.


COMPLETE TABLE OF DIFFERENCES

Here is a more complete list of differences as they apply to general sport flying:
Characteristic A123 LiIon/LiPo Nicads/NiMHs
Voltages
Peak charge voltage 3.6v officially but can go up to 4.2 with little damage (but not regularly) 4.1 or 4.2v but significant damage likely from about 0.1v above that No limit but usually charges to over 1.5v
Nominal voltage 3.3v 3.6 or 3.7v 1.2v
Voltage under significant load 2.95v @30A (actual test) ~3.3v ~1.0v
Minimum discharge voltage 2v officially but almost empty at 2.5. Can go to 1.0 3v in theory but many have to go down to 2.75 to get capacity and some require above 3v No limit
Voltage sensitivity Not sensitive to straying outside recommended voltages (2-3.6) Extremely sensitive to straying outside voltages (usually damages pack) No significant voltage boundaries
Discharge curve Very flat Noticable reduction in power over discharge Fairly flat
Balancing Minor issue; required but less frequently and with less precision Major issue; complex with large packs Not normally required
Other characteristics
Capacity 2300 and 1100mAh only Wide range of sizes Wide range of sizes
Weight 70g for 2300 Similar for same capacity and C rating but newer cells can be lighter Much heavier
'C' rating 30C 20C common, 30C becoming more available Not usually a constraint
Charge rate 5mins claimed; 10A (15mins) on 'their' charger; float charge allowed 1C (1.5-2hrs); float charge not recommended Nicads 25mins, NiMHs 1C (1hour); float charge OK but Nicads can develop 'memory'
Memory No No Nicads Yes (bad)
NiMHs No
Shelf losses Low Low High (bad)
Cycles 1000 claimed Often only 10-50 but can be several hundred Not normally an issue with active cells but they often die if not used
Internal resistance Low Most are much higher (bad) but improving steadily Low
Packaging Hard aluminium cylinder Soft plastic bag Hard steel cylinder
Availability Initially as DeWalt DC9360 and DE9360 36v power tool battery pack; a few 'modelling' dealers emerging Widespread Widespread
Cost Expensive list price; better on ebay More expensive than A123's but dropping progressively Difficult to compare
Temperature on charge Excellent (very little rise even with fast 15min charge immediately after flying) Can be dangerous (extremely high risk of combustion if settings are wrong) Nicads OK (but warm after 25min charge)/ NiHMs poor (some get hot at 1C)
Temperature in use Excellent (luke-warm at 15C) Dangerous especially when not cooled or worked near their C ratings Nicads excellent / NiMHs good
Size Bulky Compact Poor (need 3 times as many for same voltage)
Environmental issues Said to be OK Less OK Nicads are bad, NiMHs OK
Long-term Storage Not known Deteriorate if not stored at mid-voltage (~3.8v) in cold conditions Nicads tend to die if not used regularly; NiMHs tend to fade away too
Other issues Don't turn the 'button' on the cell.
Not recommended but ends are solderable
Terminals are flimsy
One terminal may require aluminium solder (if tab breaks off)
Not recommended but ends are solderable


Other lithium pages:

Main lithium page
Charger circuits
Balancing issues and circuits

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