Tuesday, July 15, 2008

A123 - By the Numbers

I'm a huge proponent of A123 (LiFePO4 - Lithium Iron Phosphate) batteries and have been converting everytihing nitro or LiPo to A123. With the elimination of bloating and LiPo-related safety issues, 15-minute charge times and high discharge rates (both sustained and peak) these batteries have been regarded as fantastic power sources.

With my charger in the field (see notes below for details) and some 12v 20AH sealed lead-acid batteries (I have 3 from old scooters) I can race 1/8th scale electric buggies, fly 600-sized electric helis or simply fly planes all day long. At 15 minutes per charge and using a dual charger, there's not much down time on a sunny SoCal day!

Before we go on any further, let's look at the cons vs. the previously mentioned pros. The things that I consider caveats with using A123 cells is the requirement for lower-than-lipo LVC settings (see note on this below) and the fact that the cells are heavier than LiPos. For instance, a standard 3S1P lipo weights in at around 160g-170g. An A123 3S1P that's wrapped with power cables and balance plug weights around 234g.

Keeping the weight in mind allows me to adjust my COG perfectly in helis and planes.

Now on to the ~good stuff~

Here's my collection of notes/links on A123 - you should have plenty of reading for the next 30-60 minutes!

Informative Links
Voltage comparison A123 vs. LiPo
  • Lipo cell = 4.2V peak, 3.7V nominal, 3.6-3.2V under load
  • A123 cell = 3.6V peak, 3.3V nominal, 3.0-2.5V under load

  • The M1/A123 cell has a diameter of 26mm (~1 in.) and length of 66.5mm
  • (2.62 in.) and weighs 72g (2.54 oz.).
  • Nominal voltage: 3.3V
  • Nominal capacity: 2.3Ah
  • Core cell weight: 70 grams
  • Internal impedance: (1kHz AC) 8 mΩ
  • Typical fast charge current: 10A to 3.6V CCCV
  • 70A continuous discharge
  • 120A, 10 sec pulse discharge:
  • Cycle life at 10C discharge, 100% DOD: over 1,000 cycles

Manual Charging:
  • Set variable voltage power supply to 3.6v-3.65v * #cells. Example: 5cell: 3.6v*5=18v or 3.65v*5=18.25. (range=18v-18.25v)
  • The current taken by the battery being charged will reduce once the battery is nearly full.
  • Once the current has dropped below the 0.1C rate then the battery is "full" (or near enough). In the case of 5S1P (2300mAh) a reading of 0.23A or less would indicate a charged pack. You can set a charge rate of 10A!

Standard Charging:There are more and more chargers that have the programming to be aware of A123 cell voltage. There is also the option to use an adapter (like the LiPoDapter, but simply called the Dapter - http://www.slkelectronics.com/lipodapter/index.htm ). In my case I use the Hyperion EOS 0610i 6S 16N DUO CHARGER with the built-in LBA10A balancer - this thing is incredible. I charge (2) 3S1P A123 packs @ 10A each with built-in balancing, status info and the ability to simply time the charge. I was previously charging manually but I decided to go the charging system route.

I was previously using the TP610C (ThunderPower) but I could only charge 1 A123 pack at a time and at a maximum of 8A (for some reason it never balanced my A123 2S1P packs correctly - the voltage in the cells were so far off from each other that the charger would stop charging). The TP610C is an 80W charger whereas my new Hyperion is a 180W charger!

Here's a great link to a review on current chargers: http://www.helifreak.com/showpost.php?p=691212&postcount=2

NOTE: If 3s is all you need to charge, do a search for "zip charge" on RCGroups.com to find out how to make a $10 charger that will charge a 3S A123 pack in less than 10 minutes from a 12 volt car battery...

Zip Charging: http://www.rcgroups.com/forums/showthread.php?t=747911

One more thing to remember if you go the 3s A123 route, you MUST set your low voltage cutoff (LVC) to 2-2.5 volts per cell. If you leave it in auto lipoly, you will have issues (early cut-off/low-voltage warning too soon).

Important note on ESCs, LVCs and A123s: A lot of ESCs won't let you set the LVC that low.Castle Creations' ESCs allow me to do this and it has worked fantastic for me. For heli and fixed-wing, I make sure to time my flights. With the steep voltage drop at the end of an A123 cell's charge you'll find yourself either doing an autorotation landing or trying to glide a plane in.

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