Lithium Battery on S/S CDI Ignition

[GailinNM]

I have been using a CDI ignition systems from S/S Machine & Engineering LLC for a long time now.  I have used about a dozen of them and my active stable has 3 Gauge1 locomotives and 3 stationary engines in it.
http://www.cncengines.com/ic.html
 I have been using standard batteries consisting of 4 NiMh AA cells and the work perfectly all the time. But only if the batteries are charged.

Since I don't run them all time I have to charge them all up just before any event. NiMh batteries self discharge rather quickly so every thing has to be charged with a week before use and if the event is like the upcoming event with 4 days of running spare batteries are also required.

The ignition systems are rated down to 3.6 volts of operation. The common 18650 Li-ion battery is nominal at 3.7 volts and the discharge curves show that by the time it drops to 3.6 volts at the drain of a CDI they are about 90% depleted.  I wired a 18650 holder up to a R/C servo extension cable end so it would match the S/S standard connectors.

I have run my stationary engines for a total of about 4 hours on the single 18650 4000mAh cell and the voltage has just dropped to 4.0 volts from the 4.2 volts it started with. My calculations show that it should run one of my engines about 25 to 30 hours before recharge.

The 18650 is becoming a very common standard battery for high power flashlights and video cameras.  This has caused the price be fairly low.  As my vision is poor I use lots of flash lights and have 5 that use 18650 batteries. The 18650 batteries that are sold have capacities ranging from 1500 to 5000 mAh. As the price differential is low it does not make much sense to buy any thing other than 5000 mAh ones now.  They can be had on eBay for as low as US$6 for a pair of 5000 mAh ones with a charger, shipping included. A battery holder like I used sells for about US$5 for a package of 5.   

One caution.  The most common battery charger being supplied right now has the wall socket pins that are made of 1mm stock.  This is too thin to make good connection or hold in a wall socket.  Standard prongs are 1.5mm thick. To modify them to make reliable connection I twist the prongs about 5 degrees, one clockwise and the other CCW.  Don't try this with out either a fixture or at least two pair of pliers.  The plastic in the charger body will break before the prong twists if you just hold the charger and attack with pliers. I know.

My fixture is just 1/8 x 3/4 inch aluminium with a slot sawed in the end to hold a prong while twisting with an adjustable wrench.
Gail in NM

[tvoght]

This is really good information Gail.

Thanks!

--Tim

[Jasonb]

Having just bought 2 ignitions from S/S I may give this option a try. How does it compare sizewise to the AA cells?

[GailinNM]

Thanks Tim.
Jason: Attached photo will give you a comparison.  The height is almost the same as the height of the coil on CDI, about 0.030 higher.  As I build small engines this was something I was looking at also. As I typically bury the battery in the engine base I plan to make up a charging cable for the charger so I can charge the battery in place.

One other item that MAY be noteworthy. I have not checked the low voltage dropout on the S/S CDI nor have I checked with S/S.  Just using their specifications.  On the early units I have from them the power goes directly into the board.  On the later ones there is a diode in series with one of the power leads.  I suspect it was included to give reverse polarity protection as lwell as raise the upper voltage limit.  If the low voltage operation does not meet the specification I think the diode could be bypassed to eliminate the voltage drop.  I don't have time now but I will hook one of my engined up to a bench supply to check this when I get back from next weeks steamup. Need to bring a supply into the shop where the smell of gasoline is acceptable.  Electronic's room is a spare bedroom and such odors are not very acceptable there.
Gail in NM

[dieselpilot]

18650 describes the cell as 18mm diameter and 65mm long. AA is 14500. 14x50mm.

That's great. I've been using single cell lithium for ignition in my flying models for quite a few years. It sure makes life easy especially in small models.

[PStechPaul]

Just beware that many of the low-cost Chinese lithium cells do not really have the capacity as advertised. I have measured some for both charging and discharging to the voltage limits, and you may be lucky to get 2500 mAh from a 5000 mAh battery as you have pictured.
 
Also, I have heard that the "Ultra-Fire" brand sometimes lives up to its name, so be very careful to keep them away from flammable objects or anything that might be damaged by fire. I have some that I keep stored in a "Lipo-safe" bag made of Nomex. It may be advisable to charge them only about half-full for storage, so there is less stored energy. Probably most problems occur during charging and fast rates of discharge, but sometimes batteries have internal manufacturing defects like pinholes which cause small hot spots that can expand and "go critical".
 
The LiFePO4 version has less capacity and is more expensive, but the chemistry is more stable and there have been less reports of fires. You may want to read more about these batteries on the DIY Electric Car forum, where most people use LiFePO4 and some have had problems with other chemistries:
http://www.diyelectriccar.com/forums/showthread.php?t=83134&highlight=18650+lithium
http://www.diyelectriccar.com/forums/showthread.php?t=93890&highlight=18650+lithium
http://www.diyelectriccar.com/forums/showthread.php?t=85458&highlight=18650+lithium
http://www.diyelectriccar.com/forums/showthread.php?t=82626&highlight=18650+lithium

[gldavison]

I've been using A flip phone lithium battery with a CDI ignition from S&S Engineering on a Hoglet for the last couple years. Its 1000ma. It will last for a 2 day show. Never had to swap anyway.
I had 2 identical phones. Salvaged the battery box from one phone for the ignition system and use the other one to keep the batteries charged.

Gary

[Admiral_dk]

There are two very important parameters with the use of Li-Po, Li-ON and other kinds of Lithium based batteries, namely the charge and discharge limit - both fatal for the battery if you go past them.

You will have to read the technical documentation for the type you plan on using since they do not have the same limits. Some have a low limit of 3.00 volts and another type has a limit of 3.60 volts and going just a few millivolts below WILL destroy the battery. This is the reason why all serious equipment has a hard limit sensor measuring the battery / cell voltage (more than one cell requires a lot more electronic circuit to ensure load / charge balance in the pack), so it can shut down the equipment at 3.05 volts in the first case or 3.65 volts in case two. The charger likewise has to measure voltage, charge and temperature of the battery (the same applies to Ni-Ca and Nimh batteries) to avoid trouble.

If you're sure that you never forgets to shut the ignition off before the battery voltage gets too low and take it out of the charger in time - no problem, but if you're like me and forgetfull (just had my 20 year anniversary on my brain damage from April 1st 1994), then you will be glad if the equipment has the necessary limits build in 

[spuddevans]

If you're sure that you never forgets to shut the ignition off before the battery voltage gets too low and take it out of the charger in time - no problem, but if you're like me and forgetfull (just had my 20 year anniversary on my brain damage from April 1st 1994), then you will be glad if the equipment has the necessary limits build in 

For the cautious soul, you can also buy so-called "Protected" batteries, they are usually a fraction longer due to them having a tiny circuit built in that shuts off if the charge goes above 4.2volts or the discharge goes below 3.6volts. I use this type in my torches to make sure I dont over-discharge.

Tim

[Camm-1]

I buy my ignitions from Hobbyking for airplanes and use 2s lipo(7.4V) ca 2000 mh and  with a cheap BEC that I can set to 5 or 6 V thats is the maximum for the ignition.
It last very long time
Of course you have to have a charger for lipo but because I´m flying rc helis I have it already.

[Admiral_dk]

Tim - thank you for that info, I never heard of those before, but they do make sense.