Lithium battery on S/S CDI ignition

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Feb 17, 2008
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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.
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

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.

I was told the disadvantage of many LiOn cells were their relatively low C-rating (recommended max continuous discharge). I heard numbers like 1-2C. So by example, a 1500maH cell would ideally be suited to current load of 3A. (I'm not sure what current your ignition system draws, but anyway a larger/5000maH cell is great choice for this reason too.)

However, that 1-2C rating must be old news because I found these discharge curve specs suggesting much higher C ratings & negligible capacity degradation based on load. That's pretty impressive. NiMH curves were significantly more depressed by higher C. Not sure if these are the same specs as your brand but something to check into.

Yes, I believe NiMH is going the way of dinosaur.
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As you know lithium polymer batteries are great energy storage devices. The go months without loosing much charge and have very low voltage drop even at high discharge rates. We routinely discharge them at well over 20C and charge them at 2C.

You need to be careful charging them. Only a high quality, balancing charger should be used. Overcharging and over discharging both cause battery heating that leads to fires. We use controllers with a low voltage shutoff to prevent over discharging. In any case don't charge the batteries unobserved, and have sand or a lot of water (a pond) available just in case. I have a lot of experience with lithium polymer batteries and never have had a fire despite serious abuse. A friend wasn't as lucky.

Lohring Miller


Holy Crap Lohring! That's a TON of chargers! And that boat requires it's own trailer, can you post another pic of that thing! I have a little experience working with (errr destroying) Lipo and I have seen first hand the destruction it causes (think all those batteries on the table packaged in a brick catching fire at the same time). Normally I would say go Lipo, but in this case it doesn't seem to be worth the extra care and danger for this application. Although if you do go lipo, you can buy one of these low voltage alarms for cheap that cannot be ignored:
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The boat is the current electric boat speed record holder. Mike Bontoft built and drove it to a UIM record of 98+mph, nearly twice the old record. In 2008 the only high quality lithium polymer batteries and chargers we could find were designed for electric models. We ran 42 6 cell battery packs charged by 22 chargers. Generator capacity was limited so we could only charge at 1 C. I am 1/2 owner and provided the basic power plant design.

Lohring Miller




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