Lithium - Polymer Batteries
Three years on from the first Lithium article, the technology moves on to bigger things

Right from the first experience of the early Lithium Ion batteries for indoor flying there was no doubt that it was just a matter of time before the technology would dominate the world of electric flight, both indoors and out. So the question was not whether to make the switch, but just when - and which cells to choose?

Last Summer (2005) one of my 2400 packs was beginning to show signs of deterioration and I had to accept that continuing to use these batteries another season would really be pushing my luck, so I decided then that 2006 would be LiPo year. So, as the 2005/06Winter drew to a close, I reviewed the fleet. All these models are currently flying on sub-C cells:

To be honest, the smaller models don't get flown a lot and whilst it would be nice to fly them on LiPos, I don't feel that they would justify the investment in their own right. Equally, at the other end of the size and weight scale, the DC3 has logged only 25 flights in nearly 6 years but this is in part due to the fact that I only have one 18 cell battery. It also is a bit on the heavy side at 10lbs, so being able to shed some weight by changing to LiPos would be very attractive but again though, it doesn't fly enough to justify the investment in a dedicated power pack.

The workhorses of the fleet are the three 14 cell models (the Limbo Dancer on 16 has chalked up over 250 flights and may be pensioned off soon), so I started by looking for suitable batteries for these models. Straightaway, I hit a snag: NiCd cells have a nominal voltage of 1.2v, whereas the latest LiPos are nominally 3.7v. So, a 4 cell pack of LiPos comes out at 14.8v, 5 cells at 20.5v. A 14 cell NiCd pack comes rather awkwardly in between at 16.8v. So the first conundrum is should the 14 cell aeroplanes be switched to 4s or 5s LiPo packs?

Next is the question of cell capacity. Until recently, LiPo cells were limited in capacity and, more importantly, in discharge rate so, as well as stringing cells together in series to get the right voltage, it was necessary to strap them together in parallel to operate at the sort of current levels we are used to for electric flight. The last year though has seen the emergence of cells which are claimed to be capable of sustaining current draws of 20c or more which, for a 2400mah battery for example, means a current of 48amps. One such range of cells is made by the Korean company Enerland and these are used in battery packs made by Flightpower, Hyperion, Polyquest and no doubt several others. Towards the top of their range are the 3700mah cells which, on a like-for-like basis, offer the potential for a 50% increase in capacity over my 2400mah NiCds, and at less than half the weight!

To start the ball rolling, I ordered two 2s packs and one 3s pack so that I could join them together to make either 4s or 5s to find out which configuration best suited my 14 cell models. I started with the two 2s packs strapped together and recorded the following figures:

(I didn't measure the voltages in the last two cases - I am a bit reluctant to insert the Whattmeter into the leads to a brushless ESC, so just used a clamp ammeter to record current.)

I have to say that these results came as something of a surprise! Each battery was first run for 10 - 15 seconds to get rid of any artificial 'fresh off charge' effect, then the three tests recorded here were done one after the other. As you can see, the 4s Lipo equalled or out-performed the 14 cell NiCd in every case. No doubt a more youthful NiCd pack would have done better, but it was clear from these measurements that a 4s pack should give me practically the same power output that I have become accustomed to with my NiCds - and in an aeroplane which would be more than a pound lighter!

The next stage was to draw up another table recording the position of the NiCd battery in relation to the cg and calculating where the Lipo pack would have to go to maintain the balance point. There then followed a couple of days of re-working of battery stowage arrangements. This generally proved not as problematic as feared but on the Chorus Gull, with the LiPo pack as far forward as possible, it was still necessary to relocate the receiver battery forward too, in order to restore the cg to the rightful spot.

I have now flight tested all of these models and they all perform brilliantly on the 4s LiPo packs. The Capiche in particular has been transformed. It now has unlimited vertical capability and no longer has a tendency to flick out at the bottom of outside loops. This was designed as an i.c. model and it is now flying at (or maybe even slightly below) its design weight - and it shows! At this point I ordered two 4s packs of 3700mah LiPo cells.

As for duration, I have increased all my Tx timer settings by 50%, simply on the basis of the increased battery capacity. In fact, I am still finding that there is plenty left in the packs after landing because of course, with the decrease in weight, the average in-flight current consumption is also lower than it was when carrying the NiCd packs around.

Heartened by all of this, I now re-considered the 18 cell DC3. After all, it did manage a rather underpowered maiden flight on 16 cells, and with LiPos, it would be a lot lighter. A quick check showed that with a 5s pack pushed right up against the noseblock, it would just about balance, so I decided to give it a go on 5s rather than 6s as I had originally planned. An initial ground test showed that with the 5s pack the power would be down just below that which I had on the maiden flight. Although the model would now be lighter, that maiden flight was very hairy and I had no desire to repeat the experience, so I changed the gearing slightly to bring the current draw roughly back to where it had been with the 18 cell NiCds. RPM was still down a bit due to the voltage reduction of course, but was up to the psychologically significant 7000rpm.

The DC3 flies well on this set-up. Interestingly, the flap-elevator mix needs some adjustment - it is as if there is no longer enough weight to keep the model moving against the flaps without a bit more down elevator being applied. This impression was re-inforced on landing. Normally this is a buttock-clenching one-shot affair, but this time I seemed to have plenty of time to make sure I was lined up on the runway, regulate the descent rate and even pick my spot to set it down. This model has shed nearly 24oz in weight and again is now towards the lower end of the recommended weight for the i.c. version.

One drawback of LiPo batteries is that they are still rather slow to re-charge. For me though, this is more than offset by their ability to hold a charge to the extent that I can keep them charged up and ready to go. So enamoured am I with this prospect of being able to go flying when the weather turns out better than forecast that I am now looking at eliminating the receiver batteries from some of these models by using switched-mode regulators (sold under the name of U-BEC and S-BEC for example). All I have to do then is make sure that I am not caught with a flat transmitter pack! This may be the subject of a later article.

At the time of writing, the Lazy Bee and Crossfire have yet to experience LiPo power, but the P38 flies well on a 2s pack (the 3700mah cells have a form factor very similar to a 7 cell sub-C pack and, since the P38 battery stowage is very close to the cg, I didn't have to re-work the battery stowage at all ). I have updated the Key Data pages on the most of the above models so just click on the names in the tables for more information. Alternatively, if you are thinking of taking the LiPo plunge, please feel free to e-mail me for the latest news.