In search of the correct battery for my petrol ignition

[Clive Moore]

Hi all,

I'm building a 1/4 scale Piper Cub with a petrol engine.

The engine needs a minimum 2200 mah NICD Battery of 4.8 or 6.0 v to power the ignition system.

 I can find lots of LiPo's and NiMh packs but not NiCd

The instructions clearly state "under no circumstances use NiMh or LiPo batteries, you must use NiCd"

Any one got any ideas where I can find a battery of this type and capacity? (I've tried the usual suspects)

Regards

Clive

PS I've no idea what the technical reason is for stipulating the use of only  NiCd's ?

[Tim Mackey]

PS The only Non Lipo cells I use these days ( for onboard glo units ) are the excellent Puffin hi-flo NiMh cells which are 4000 m/ahr cells and capable of up to 70Amp discharge. I can't see these being unsuitable personally, however a quick call to the ever helpful, honest, knowledgeable John Emms at Puffin will elicit a definate answer for sure !

[Ultymate]

Clive can I ask what make of engine you're reffering to I've flown petrol for several years now and never encountered this issue before at the moment I run mainly ZDZs and 3Ws and have used both nihms and nicads I usually stick with 4.8V as thats less weight and more duration for a given mah rating I have once used a lipo but had voltage reg problems so reverted to nihms but as for nihms versus nicads I just dont see the issue a battery is a battery 4.8v is 4.8v

[Tim Mackey]

Hi BC -  I was hoping you might see this post...'cos I know you is the petrol / sparky stuff man

[Ultymate]

Hi Timbo, I really don't see the logic of this insistence on nicads at all ! and a 1/4 scale Cub is not particularly large so I can't see the engine being that big. The third scale Balsa USA one we've got in the club flies on a 45cc jobby so 2200 mah seems rather large to me. That said I believe the new Zenoah 20cc is heavy on the current drain of the ignition. We'll await news of what the engine is and proceed from there

P.S. You mean I'm a petrol Head

[Tim Mackey]

Nah...I know you like the dark side too

Well I am with you on this one.....but you know how it is

A manufacturer recommends something, I / We say  "rubbish,  use this instead "

Model plane crashes and burns - guess who gets the stick!

[Clive Moore]

The engine in question is the new (ish) Jen 26cc petrol (details on the Just Engines web site).

Since putting the post up I have spoken to them re the logic of NiCd only and the answer is that they have had people complaining about poor running etc (not on this engine but petrol with electronic ignition in general).

The common denominator seems to be (according to Just Engines) that where problems have occurred people were running NiMh and some LiPo's. The problems went away when they were advised to use NiCd's only hence the statement in the instructions for the Jen 26.

I have also spoken to Overlander about this and they understand that LiPo's would be a problem but cant understand why NiMh cant be used.

One point of interest, I connected up the ignition using a 700mah 4.8 (fully charged) futaba RX battery and couldn't get it to produce any spark on the plug, when I used a 2200mah 4.8 NiMh I RX battery  got lots of spark? 

This is my first foray into petrol (fed up with all the unpleasant mess over my aircraft and electric just doesn't make the right noise for me) so any help and advice will be gratefully received. 

[Tim Mackey]

Well thats a pretty poor technical explanation by JEN of why hydride is not suited yet cadnium is

The 700 mahr battery was probably just incapable of delivering the current required.

The problem with LiPos is not the actual Lipos but the regulator required to use them at 4.8V and high current

With the disclaimer mentioned earlier I would just go for a decent capacity NiMh pack ( 4 cell ) say...3000m/a

[Ultymate]

That explanation stands up like a chocalate fireguard in front of the old logfire, sorry I just don't buy that I'd run with Timbo's suggestion above

[Doug Ireland]

The ignition unit that came with my MVVS 45cc engine will not run below 6v. I use 6x 1.2v NiMH soldered together and can get a whole days flying out of it. Not too heavy either.

[Tim Mackey]

Some systems are designed for the higher cell count, but I think Clive said his was happy on either 4 or 5, so I only advised the 4 cell pack because it will use less current.

Clive, jut use the Nimh cells we suggested

( as stated...if the weight is OK then get some of the Puffin Hi-Flo 4000 jobbies from John Emms ) 

[Ultymate]

I think if memory serves me correct (which quite often it does'nt these days) the MVVS is the odball in this ballpark as it runs an elecrtronic choke setup as well as the now normal electronic ignition so at this moment in time quite different to the rest. In fact I think you can run it straight off a 2S lipo which would account for your 6 cell count Doug. I like to keep the cell count down or more correctly the weight, I just hate putting non-essential weight in an aeroplane unless it's there for balance issues, weight I see as the enemy of good flight characteristics, I was always educated to build in lightness

[Clive Moore]

Thanks to all.

My instruction book does indeed say that I can use 4.6v or 6.0v. So I have ordered a 4.6v 2700 mah NiMh from Overlander.

Now I just need to work out how to wire in a battery checker so I can see what the battery is doing (charge state) I guess I can use a Y lead off the battery plug with one arm going to the ignition and the other going to the battery checker.

[Ultymate]

Clive on most of my petrol models I use a normal charge socket and switch as usually used on the R/X and then most battery checkers just plug in to read batt state

[Tim Mackey]

Why not just fit a small "servo plug style" socket hard wired to the ignition battery, and fitted inside the fuselage,"poking out a little as it were" in such a way as to allow you to simply plug in an external battery checker such as this ( available from Al's at special price at the moment ( £6.50 )

The bonus with that arrangement is that you then simply fit the matching plug on to your charger and bingo....charge through the same port without ever needing to move the battery ever again

[Tim Mackey]

Ulty just beat me to it 

[Ultymate]

There are some failsafe switches and sockets on the market (usually inlude an LED but they do come at a premium price, I do have one of these on my largest model at the moment.

[Ultymate]

As Bond (Connery) would say Sccchhhhnap!

[Doug Ireland]

Ultimate,

You're quite right, the MVVS Ignition does drive an additional (optional) servo for the choke.

[Ultymate]

Yes I've not had "hands on " as yet but I have read up on the system, I think theres a PDF file type manual on their website

[Doug Ireland]

If you go to HH and look for the EVO 45cc donk,  there are two PDF files that give a better description of how to set up the auto choke thing than the one that came with my engine. MVVS 45 is identical even down to the part numbers to the EVO.

Once the ignition unit is set for the type of exhaust fitted to the engine and then adjust the choke servo accordingly, engine starting is a breeze!

[Chris Marshall]

My guess on battery requirements is to do with the ability of the battery to supply high pulse currents. electronic ignition units will generally take the power in big gulps, if the battery cannot supply these pulses the voltage will drop. NiCd cells are probably still the best for lowest internal resistance, and I think this is what is important here, rather than absolute capacity. Having said that the larger cells have lower internal resistance as a general trend. My theory is supported by the fact that the unit would not work on the 700mA receiver pack. Presumably this is AA cells which in general have a really high internal resistance. 4/5A cells are much better, even for lower absolute capacity, and sub C cells are even better. It would be better if the manufacturere gave a figure for battery pack impedance; or peak current requirements: it would be much easier to determine pack suitability then. This is essentially the same problem that has bitten people with Spektrum receivers long reboot time, when used with inadequate battery packs. It may also be that the ignition unit has too small a capacitor on it's input. This would smooth out some of the excess of current pulses.

[Chris Marshall]

Forgot to say that I see no reason why suitable NiMh cells should not work. More info from the engine manufacturer is needed.

[Simon Chaddock]

Hi

Chris Marshall is correct. It is ALL to do with the ability of the battery to supply a high pulse current, which is in direct relation to the cells internal resistance rather than their absolute capacity. NiCd are better in this respect than NiMH or Lipo. For a high pulse current the bigger the cell the better. Of course Lead/Acid cells (an even lower internal resistance) are better still!

Just a point but small petrol power tools use no batteries on their ignition circuit, why don't model flyers?

 

[Former Member]

[This posting has been removed]