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.

Monday, July 14, 2008

Multiplex Easy Star Initial Experience

One of the things that I've decided to do lately is start flying planes again after a 23 years departure from flying planes.

I've Always enjoyed RC cars and the last few years have been fantastic with regard to electric-powered helicopters, now it's planes (again)!

So, where do I start? A few weeks back I purchased a MS Composite Swift II (lot's of fun and small). What a great way to get back into planes. Then a week later I purchased the Great Planes Slinger. I didn't even built it with the included (brushed Speed 400) motor, instead I chose to go straight for the gusto - 2900kV brushless, CC 35A ESC and A123 3S pack. Wow, what a load of fun!

So, my restless thumbs had me itching for something that I could practice a lot of orientations with - that also can fly VERY slowly when I'm attempting to orient myself with nose-in approaches and other things that I still have a slow response time with (i.e. left is right / right is left).

Last Saturday I picked up the Multiplex Easy Star and I think I'm in love (again). She was easy to build and I was careful when I selected my motor so that the 28mm can would fit most aircraft that I'm training on. It had fit perfectly in my Slinger (flying wing) and also in the Easy Star. I made some wiring changes and that was that. Everything else was a standard build except for the following:
  • swapped out the 400 (brushed) motor for me handy dandy 2900kV motor
  • CC 35 ESC
  • A123 2S1P pack - when I feel I've reached that point, I'll also use the 3S1P batteries - but for now, no need for all that speed!
  • added very strong magnets to the canopy and cockpit - didn't use the included canopy clips - everyone seems to have the same poor experience with the canopy clips (after one or two hard landings they don't line up well anymore)
  • added reinforced tape to elevator and rubber as hinge reinforcements (dabbed 3m 77 adhesive to the areas to be taped to offer even more 'holding power'
  • added the reinforced tape to the belly of the plane since that where she's going to 'skid' when she lands
  • added some crazy strong hatch magnets! I was told to cut the correct size holes (so they would be flush with the surrounding areas) and CA them into place. Well they're so strong that two of them tore right through all of the CA. I reinserted them, re-applied CA and overlaid them with the reinforced tape. After a weekend of flying and reconfiguring the cockpit, not a whimper of movement!
  • added a "foam barrier" between the battery and the electronics (esc/rx). I hit nose-first pretty hard with my Swift II over the weekend and crushed a AR6100e rx (about $50). I won't be doing that again.
  • added a foam pad in the nose of the cockpit to keep the battery from sliding (even with velcro) - a decent nose tap on a slightly rough landing caused the battery to wedge itself as far forward as possible causing a bulge (almost a break in the foam). I will also be fiber-tape reinforcing the nose since it has shown a weakness here. As I get better the plane's (assumed) weaknesses will magically diminish ;)
With the build completed late Saturday night, I took off Sunday morning to some fantastic sloping hills in San Diego (4S Ranch area). With my trusty DX6i tx in hand and several A123 packs, I was all set.

Amazing - with a total flight time of about 25 minutes, I consumed 1290mAh of a 2S1P 2300mAh A123 pack.
  • note: for $90 I get a DeWalt pack shipped to my door. That's (5) 2S1P batteries. Hmmm, I can fly all day
Here are some pics to better reflect my positive experiences with this fantastic training aircraft.

Great - now let's get started!

I'm absolutely thrilled to get this moving along as I've got a TON of things to share and document for both myself and to share with the world in general.

Thanks for stopping by!