Splitting LiPo Packs

[Karel Jankowitz]

I am busy coverting my Seagull Seafury to Gas 26 cc RCGF.  This engine weighs about the same as the original JBA 1.20 two stoke engine I use to have in it. However for the ignition I need a small light weight battery, normally a 4.8 Rx pack is used but this weighs in at 98g.  I tought if I split one cell of a 1600mAh 3S pack, this weighs about 40g, I will save about 60g on the battery and about 400g on the fuel, thus the total weight of the model will be less.  The 26cc is able to turn a bigger prop (16x10 instead of the 15x6) I will get better performance.  My flying field is amost 7000' above sealevel so I need all the power I can get.  The model weight with the glow engine was about 4.5Kg and hopefully I can get it to about 4Kg with the 26cc Gas engine.

 

The questions are:

Are there any dangers to splitting a 3s 1600mAh pack to get a cell?

 

Will the capactiy be still 1600mAh?

 

Will the one cell be sufficent for the electronic ignition?  It seems to run ok on the bench.

 

I would appreciate and advise previous experience by other modelers on any of the above.

[Tim Mackey]

There are inherent dangers when mechanically "interfering" with any LiPo battery.

With that in mind, if you wish to proceed..... then the following applies.

1 ) Capacity remains the same ( each cell in any series pack is the stated pack capacity )

2 ) I would say 1 cell will not be sufficient for the ignition system.

           What are the specs of the ignition system requirements?

 

Incidentally.... I cant see why you are so bothered about the few grams saving, on what is basically a fairly large model

[Jonathan Lewzey]

you could buy a single cell such as the kokam 2000 from robotbirds. Would save the pack and any complications that could arise from splitting the pack. Personally, I'd go for Nimhs.

[Brian Parker]

 

I don’t know the specification of your ignition system other than it is a CDI unit. 4.8volts is probably the minimum required for satisfactory operation.
 The CDI generated spark voltage is higher than with inductive systems but the duration is relatively short, thus the reduced voltage using a single Lipo cell will almost certainly have an effect on spark intensity. This will introduce poor starting and reduce the maximum power and RPM.

 Use a 2S lipo with a regulator or stick to the recommended 4.8v Ni-MH.

Or as many badged CDI units are rated at 6volts, 5 Ni-MH cells should (usual caveat) be OK, the increased power will more than compensate for the extra weight  but a fully charged unregulated 2S lipo might be pushing it a bit.

 Use quality high capacity cells.

[Boots]

If you are asking how to split a lipo, then I can assume you dont know enough to do this, I recommend not.

Check ignition system voltage and current !!!!

 

 then use either a 2 cell lipo and a  4.8v / 6v bec or 4 / 5 cell nimh light weight cells

[Karel Jankowitz]

Thanx for the responses. 

 

Splitting the pack is not the problem, I was just afraid that the capacity may be reduced, or that the actual cell may be compramized in some way.  It is really difficult to get singel cell and small capacity LiPo here. 

 

The fully charged cell is about 4.8 volts and the engine ignition works well until the voltage drops to about 3.2 volts when it starts to miss now and then at full throttle.

 

A few gramms in one place does not make a big differance but if you save it on several places the overall reduction is quite siginficant and vice versa.  The altitude really has a huge effect on the power of the engines and also reduces the wing effectivity.  The fact that in summer we get tempratures in between 35 and 40 deg celsius on a normal day does not help.  The only thing normally stopping flying is strong or gusty winds coming from strong thermals and thunderstroms. (Namibia)

 

The average size model here is about 50cc or larger and in the glow a .90 is considered small.  The smaller ones tend not to do well at this altitude and the wind do push them arround.  The bigger models don't have this problem.  You guys in the UK have the Wx that keeps you from flying.  Here we have strong winds and too hot days.  Trying to organise a float-fly day can some time be very difficult as the resevoirs just dry up especially during Winter, it is really hard to fly off mud.

 

Any good advice on cooling baffles in the cowling.  I was thinking to place some balsa wood (fuel proofed and painted matt black) at about 30 deg angle leading the air towards the cylinder head.  This will have to be fixed to the cowling.  A small ammount of air will be allowed to pass over the crankcase to the back for the carby, I think this air must be slowed down somehow to prevent ram-effect changing the mixture.  (The carb is a rearmount, but has ended up in the engine mounting box behind the firewall.

[Bruce Richards]

If you are getting a single cell Lipo to give out about  4.8V fully charged then I think you are over charging and you are damaging the Lipo and running the risk of fire. Lipo should never go above 4.2V ever. If by "about 4.8V" you mean 4.2 then that fine.

 

I agree with the other posters and I would use a good capacity 4-5 cell nickel pack.

Edited By Bruce Richards - Moderator on 05/11/2009 07:39:04

[Chris Card]

Have you taken into account the weight saving available from using a smaller tank, as your petrol engine should have a noticeably lower fuel consumption rate than the glow engine?

[Mowerman]

Assuming that the voltage of a single cell is OK and you manage to split the pack into single cells, the the big problem is how to connect the cells as the tags om the cells are aluminium and very difficult to solder ( do'nt ask me how I know this ).

[Tim Mackey]

4.8V !! You are introducing another ignition system into the equation altogether!

[Brian Parker]

 

[Karel Jankowitz]

My appologies, I measured the fully charged cell at 4.08 volts so it is still below the 4.2 volts max.  However I have asked some of the other modelers at our club and a few are using a single cell LiPo for the ignition on their RCGF enignes.  This is only on the smaller single cylinder engines (below 50cc) and they don't have any problems.  The have reported that a typical flight of about 15-20 min will need about 75-110 mA when recharging.  They did have not any problems with starting and running provided the carb and timing was well adjusted in the first place.

 

The original fuel tank of the SeaFury is a massive tank of about 600 ml (not capacity on indicated on tank).  When I flew it with a 1.20 JBA glow engine I could easly fly for 20 min at half to 2/3 throttle (no aeros) and still have half a tank left after landing.  For the petrol engine I intend to install a 12 or 16 ounce tank wrapped in thick foam to reduce foaming and to fit thightly in the original tank frame holes.

 

The cell I am planning to use is comes from an older type pack without balancing plug, that is why I stopped using it in the first place.  I have opended the shrink sleeving and found that the thin aluminium tabs has a thick blob of solder on. Unfortunately one of the cell's tabs broke off during the uncovering.  This cell I will discard.