View Full Version : Lipo Dilemma

10-10-2005, 01:29 PM
I recently repowered my RCM Funster (low wing version of the Telemaster 40) with an AXI 4120/14. The 1900 mah nicad pack I'm using with this motor doesn't seem to be cutting it. Duration is not good and the pack just doesn't seem to have the discharge capacity I need. I've been using the pack for awhile and I'm thinking it's on its way out. I know lipo's are the way to go in this plane but I see Lipo's as a large mine field of problems.

The motor with a 12X8 prop draws 49 amps on the bench so I need a pack that can provide at least 50 amps, and to get the proper voltage I'd need a 5 cell pack. Knowing what I need is the easy part; choosing, charging and maintaining the pack is where my problem comes in.

I've got an Astro 110Deluxe with the chip that converts it to a 109 so the charger is covered. My concerns fall into three areas: First is flight duration and time between flights, second is balancing and maintaing the pack, and third is the cost of the pack.

I'm looking for a ten or twelve minute flight using half to three quarter throttle with some bursts of full. And I'm looking for several flights during one trip to the field. Some of the charging routines I've seen call for charging each cell indiviually. That would effectively mean one flight and done! Unless I chose a very large capacity pack where I could get several flights out of one charge.

Balancing and maintaining the pack is another issue. It seems to me that every manufacturer is coming out with their own proprietary balancing system which means I'd have to use one brand of pack exclusively or I'd have to buy a balancer from each different company. I don't like either one of those options.

Then there's the cost factor. I just don't see myself laying out $450-$500 for a battery pack. One way to get the capacity I need is to parallel packs but that ups the cost considerably and keeping the pack balanced becomes even more of an issue (read that problem) with multiple packs in parallel.

What I need is a large capacity (4000mah +/-) 5S pack with a balancer I can use on other brands of packs that I don't have to sell my first born son to afford.

Battery technology is changing so rapidly; I was hoping to wait until spring to begin the search for "my" lipo pack because I figured things may be very different by then but with the apparent "failing health" of the nicad I'm using now I'm starting my quest a little early.

Any help or suggestions would be apprecaited.


Matt Kirsch
10-10-2005, 02:59 PM
Right now, you're asking for Mercedes features for Hyundai money... With time and competition in the marketplace, the prices will start to come down, but that's probably at least a year down the road.

The two integrated balancing systems I'm aware of are both charger-based. That is, the circuitry that monitors the cell voltage and balances the individual cells is external to the pack. The packs themselves are simple packs with taps to each cell; there is no special protection circuitry within the pack. Truth be told, one pack could be adapted to the competition's system with a simple connector swap. Third-party packs can also be adapted to either system through the addition of cell taps and the appropriate connector.

Both integrated balancing systems are signifigantly more expensive than "raw" LiPoly packs, though... You could simply buy raw packs now and adapt them later, or you can manually balance the packs.

Balancing doesn't have to happen EVERY time you charge. It's nice to have, but not absolutely required. There are literally tens of thousands of packs in use that don't get balanced every time they're charged, and I would dare say most of them aren't being balanced at all. That's not to say that it isn't necessary, or a bad thing to do. What I am trying to get across here is that cells getting out of balance enough to cause trouble is an extremely rare occurrence.

Matt Kirsch
10-10-2005, 03:01 PM

What I've been doing right along is building my larger LiPoly packs from smaller 2- and 3-cell packs, using series and parallel connectors. This way, I can easily break my packs apart and charge each pack individually once in a while, and assess its health.

Fred Marks
10-10-2005, 03:48 PM
Battery technology is changing so rapidly; I was hoping to wait until spring to begin the search for "my" lipo pack because I figured things may be very different by then but with the apparent "failing health" of the nicad I'm using now I'm starting my quest a little early.

Any help or suggestions would be apprecaited.


Open www.fmadirect.com (http://www.fmadirect.com) or


and read the Li Po Handbook. Then click on What's new in Li Po Technology on the home page. It will also be helpful if you read the excellent article Tom Hunt wrote in the Nov issue of Fly R/C. Tom did one of the most thorough jobs we have seen in measuring the flight profile of a typical electric airplane in the range of your airplane. With regard to cost: With the proper FMA Direct charge and discharge control devices and heavy dduty packs, your system should last a lifetime, even longer than a decent glow motor. Cost to operate a good brushless motor and Li Pos over a 1,000 flight life (arbitrary) including initial cost, is about 1/10 what the cost is for glow with fuel at $15/gallon.

10-10-2005, 05:10 PM
Another avenue? Take a look at the battery performance graphs on this site. Many of the new NiMH packs will supply 60A.

For more lipo lifetime info, read your PM.

Matt Kirsch
10-10-2005, 07:08 PM
Yes, there is that route as well. 1900mAh NiCds are pretty old tech. For the same size and weight, you can now get 3700mAh NiMH cells that are easily capable of the same discharge levels (50A bursts).

I've been pretty brand-neutral up to this point, but this is what I'd do: For about $200 at Tower Hobbies, you can get a 2S and a 3S pack of Kokam 3200 cells that are capable of 20C (60 Amp) bursts and weigh less than half as much as a NiMH pack of comparable capacity and voltage. It's the perfect solution:

The separate 2S and 3S packs are easy to check for balance, and a simple series connector is all you need to turn them into a 5S pack.

The cost really isn't all that much. You were figuring on $400, and this is about half that.

Weight reduction... 'nuff said.

60% more capacity... 'nuff said.

10-11-2005, 11:25 AM
Thanks for the info, guys. I've been flying electric for close to 15 years so I'm not new to this aspect of the RC hobby. Lipo's, however, are another story. Obvioulsy I don't know a lot about this new technology. Your help is appreciated.

10-11-2005, 12:32 PM
Another way to go is with a 5s2p 4200 Thunder Power Pro lite pack. Runs $280 and weighs 17 ozs. Add a balancer unit for $60 and you're all set.