Battery and Ignition question

[Justin Martin]

Hey guys, being a big RC plane enthusiast, as well as avid racer, everything from motox, mountain bikes, and karts, I have come to question some rules with the Rotax battery selection.

The standard battery is heavy and doesn't last long if you want to stay competitive, thus meaning you buy more batteries so you always have a fully charged bat on hand. And then there is the rotax "lipo" which is lighter weight, last a little longer etc


They charge you out the @$$ for that battery! The same battery that in the RCworld is about $20-$50

That being said why does rotax limit/spec the battery so much? As far as I See, as long as the battery puts out the same voltage, there is no advantage unless you are chasing weight. However for a lighter guy like me, I could run a much larger capacity Lipo, for the same weight as the lead battery, but it would last all weekend!

Does anyone have any insight as to why they spec it? A simple multimeter at tech could check and make sure its an acceptable voltage, otherwise their is no advantage or disadvantage between batteries.

[David Flori]

Good question, and one I am currently trying to gather information on.

What are the working solutions people have come up with for LIPO, just for a practice battery?

How are people controlling the minimum voltage per cell when using RC / general purpose packs?

What Amp Hour rating are people using?
What would be an acceptable capacity for a heavy usage day, with enough overhead to not cause any issues?

What about the fact that 3 cell / 3S pack nominal voltage is under 12 volt?
Does this have any bearing on their usage?


Picked up a few of these to test with.
1-8S 2in1 Lipo Battery LED Voltage Meter Tester Alarm
http://www.ebay.com/itm/1-8S-2in1-Lipo-Battery-LED-Voltage-Meter-Tester-Alarm-/110882319324?pt=Radio_Control_Parts_Accessories&hash=item19d119ebdc#ht_2064wt_986

[Justin Martin]

David in the RC world we have many many options for regulating voltage, you can trade amp hours / and current for more voltage, or use a voltage regulator which steps down voltage, either of which are super simple.

Lipo batteries come in all sorts of shapes and sizes, always smaller and lighter than NiHm packs, which are lighter than lead. This means we could easily have, what I would estimate as 4 times the run time, or more, than the stock lead acid battery, for approximately the same weight.

Each cell has a max voltage of 4.2 volts and minimum of around 3-3.1 volts. I tend to not run mine below 3.6-3.5. One of the biggest benefits of Lipo is its ability to hold a voltage. For instance, a normal battery not only sags under load, but the more you suck out, the lower the output voltage. This is NOT the case with a Lipo. You initially start out with 4.2v per cell and it rapidly drops, but maintains at around 3.7v per cell until the pack is about 75-80% depleted, then it starts to drop again. A Lipo also has the ability to have a very high current discharge ability. Such as a 3.4amp battery able to sustain a 100amp discharge until depleted

Now although there are so many advantages of Lipo there are some drawbacks. You will hear horror stories, but the truth is (in the RC world) if taken care of properly you will never have a problem.

Lipo batteries must have extra care when charging. They need to be charged with Lipo specific/capable chargers, and need particular attention at what charging voltage, as well as capacity you tell the charger to charge at. If either of these two are exceeded or wrong for the pack it can catch fire. Lipo batteries also don't like to be fully depleted, down to or past 90-95% is a NO zone. They also should be balanced charged ideally every other charge. Balance charging simply means the charger is keeping track of each individual cell and making sure it is at the right voltage. A lack in balance charging can occasionally cause two cells to be different, for instance one at 3.9v and one at 4.2, this can also create a dangerous situation. Also, soft case lipos used in planes or helicopters do not like impacts and can catch fire of involved in a plane crash (usually very very violent.). However RC car LIPOs are protected in a hard case and are much more abuse friendly.

Now I know the Cons sound drastic, and not so user friendly. However it is extremely simple, and can be sunny proofed as proven in the Beginner level of RC planes. These planes come with batteries pre determined, and chargers pre programmed for the specific battery. There isnt much to screw up in that situation.

In the RC world we also use voltage meters, which are tiny micro chips that read cell voltage, if any one of the cells drops below the programmed voltage they can scream loud enough to drown out the loudest of RC plane engines
etc etc. This is pretty much what you provided s link to. It reads voltage of each cell and not only provides real time voltages, but sounds an alarm when it is too low or out of balance.

When I have time I will be creating ky own practice battery, on which I am sure I can make 4-5 times the capacitance with the same weight, or less than the stock lead rotax battery.

. You really shouldn't nee more tha a 3-4 cell depending on Voltage regulator. Also we can bring down the estimated price even more. $40 -$140 packs usually have a high discharge rating 40-65+ c rating . In this use the pack wouldn't need more than 20-30 amps I assume, probably much much less. Which means around 5-10c would be fine.(A c rating is just the packs capacity times the "c" for instance a 5 amp or 5000mah pack with a 20 c rating can safely exert 100 amps until depleted)

Packs also have a charging c rating. It is defined the same way but much less than its discharge c rating. All packs are safely charged at 1c, which would transulate into slightly less or slightly more than an hour. (1 amp pack charged at 1 amp hour would be 1 hour if fully depleted.) However with Lipo technology constantly increasing, most packs now of days can easily charge between 2-5c. For instance a 5amp pack at 2 c would be charged at 10amps and take about 30 minutes or so. Packs maintain their life longer if charged at 1c as much as possible. For instance a cheap pack constantly charged at 5c might only last 80-100 charges before noticing performance drop. However charging a battery in 15minutes is not needed in a kart, and is only used in the RC world because we hate being on the ground waiting for batteries to charge. They also use their battery in about 5-10 minutes of flying.... The real benefit for karting is weight to capacity ratio.

I hope this helps! I tried to keep it as simple, yet as explanatory as possible!

[Justin Martin]

Just weighed one of my closest lipos. It weighs 10.6 oz for 2.8ah (2800mah)

The rotax lead battery YAUSA is 6 pounds, or 96z

This being said, for the same weight as the lead I can have 90oz/10oz = 9 x (2.8ah)


This means for the same weight I can have a 25.2 ah Lipo battery! There you go, an easy 4x the run time.

Now I know someone will bring up the weight of a voltage regulator, but I personally have 1.7oz Speed controls that not only can out out 45 amps, but they need a processor as they actually have timing that it calculates for the number of electric poles in the electric brushless motor, it also needs to receive signal from the receiver on the plane. So what I am saying is for a negligible weight we have controllers in RC regulating motor timing, recording data such as temps, amps, usage, etc and receiving signal inputs from the RC pilot.... So this simple voltage regulator would be a non issue!

[David Flori]

RC nerd here too.....


So lets talk voltage regs, this is a bit of a unknown for me, I have dealt with step up/down converters, but small ones generally do not allow sustained high amperage loads, of which I would think this starter motor would pull? And idea of amp draw, or how many watts the motor draws at a heavy cold crank?
How much overhead does a voltage reg require to operate?
What are we talking about for cost?

I am sure that a fully charged 3S lipo fits the bill, but as you said, and I know from experience, they drop rather quickly to a lower sustained voltage.

Now how does the ignition system deal with the lower voltage of a 3 cell lipo, compared to a normally operating well charged lead acid at ~12.5 13.5V


How do the retail/rotax approved replacement LIPO batteries deal with the voltage?
What is their nominal voltage? Are they simply running ~11.1V nominal, and this is just fine and was considered when designing the rotax ignition system? Or are they massaging this potential a little higher to accommodate for the designed for lead acid system? And if so how do they accommodate this adjustment of potential?
What does a fully charged retail LIPO voltage read at?

And how do we propose to easily charge these units?
As much as I despise the wall wart units, it would be much easier to drag one of those durable units in a pit box rather than one of my expensive and fragile programable chargers.
At home no problem, but just as a backup or to top off, it seems their should be a easy way to do this also.

Ideas?

And while I see your point quite clearly, would it maybe be more realistic to meet the needs of the starting and ignition system and common user needs rather than make a battery that competes with capacity at equal weight.

My instinct tells me a lipo should be fairly durable in a high vibration environment, but I am thinking it might be good since the unit can be much smaller, to them mount the battery in a vibration damped housing rather than rigidly mounting to the chassis as the stock unit does, poor execution IMO.

[Justin Martin]

Glad to know I'm conversing with another RC nerd! It makes these convos much easier!

I would say a 4s Lipo would be easiest with a step down voltage.or even easier, a resistor to step it down. It could be a bit high at the higher end of a topped of batt, but of you had the right resistor for 3.7-3.8v per cell and drop it to 12.5-12.6 it would maintain fairly easily. And the battery should naturally be able to withstand the amps fairly well on its own. This , while not ideal would be a simple way to make a DIY practice batt. As for keep a solid, and legal voltage we would need a more suitable voltage regulator.

Lipos are very safe with vibration. And I have a few old chargers I no longer use for RC that would be up to par for being tired around to the track. Yet when I think about it my RC tools are banged around just as much as my kart stuff

[Justin Martin]

So most easily, what is the peak current (during start up) and average current while running (ignition)

Then using ohms law we can use voltage=current X resistance

[David Flori]

So a voltage reg can step down I take it?

My understanding of a voltage reg is that it usually will take dirty current off the gen/alt magneto, and smooth the oscillations to a safe charging current, ~14-14.5VDC

So assuming this is correct, can they then take a source such as a 4S lipo, and down convert this to a healthy 13VDC with enough amperage to start?
My guess would be no, as the battery the VR is intended to charge should be picking up the task of heavy drawing starting current, and I would assume the VR would be merely utilized by the charging system?

Again very little experience with VR, so I may be way off base, or dont understand the full capability of what a device like this can be retasked to accomplish.

What kind of unit did you have in mind?
From any specific readily available source?
How large would one have to be to be capable of accomplishing what you outlined?



I am going to do more testing on a 3S and see how far I can take it.
That was the purpose of the ebay link, was to show where a easy managment system could be found to take care of the critical issues with using a lipo, and the issue of monitoring the voltage.

This way I can quickly see what the cells are running at as well as set up a alarm that will scream at me when they are getting close to low.this will also allow me to see when I need to preemptively top off the cells to keep them in a healthy range.

However I dont believe the rotax ignition system will allow 9VDC and operate optimally, I am thinking there are going to be a lot of misfires at 9VDC, so that could be a easy lipo cutoff

[David Flori]

You would not happen to have an amp clamp to test draw by chance?
My multimeters ≤ 10 amp

[Justin Martin]

Voltage regulators do many things, such as do their best at regulating at a predetermined voltage.

In the RC world we use voltage regulators to step down voltage from 7.4v (2s pack) to 6v for the receiver.

It is much easier to block voltage or use a resistor to step down voltage than it is to step up voltage.

Keep in mind that a common electric speed controller in RC helis, planes, cars etc control power of the motor by regulating voltage.

My biggest problem is not know the current draw of the starter. I'm not concerned with ignition as it lasts all day on the current battery.

[Justin Martin]

Well I think I have a simple solution. I can't use a resistor to regulate the voltage, if I am using the same battery to power the starter. (To many amps.)

However I just came up with a simple answer. I should be able to wire in a bypass switch used for the starter, I think the starter will handle a fully charged battery no problem. The bypass switch would provide a path of least resistance straight to the starter, however when the switch is open it forces the power to go through the voltage regulator, prior to timing system.

So all I simply need to know is current draw of the timing system, which I assume is 1-2 amps. If this is the case I need to know exactly the average amp draw the timing uses, and I can calculate which resistor would step down voltage the best.

[Justin Martin]

Update.

With help from some fellow RC guys I discovered Castle creations has a BEC which is suitable for 20 amps. Prior to this I only knew about the 10amp BEC.

I am planning on using two 2, 4s 5ah batteries wired in parallel. With a starting bypass switch that allows full voltage and amp draw for the starter, however when the switch is open it is forced through the regulator. This will maintain 12.5 volts at up to 20 amps, and I doubt ignition pulls anywhere close to that, more likely 1-2 amps.

I will also wire in an 18amp circuit breaker before the BEC (voltage regulator) just in case, that way I don't accidentally over amp it and destroy a $40 part. All of this would weigh less than the original battery, but provide a total of 10ah!

[Curtis Ruth]

You do not need to charge your standard Rotax battery throughout the day. With normal use you should be able to go through an entire day's racing activities with no issues in the battery department. We ran a race a few weeks back with 1 hour of practice and then a 1.5 hour race with our rotax with no problems in voltage or engine perormance.

The lithium battery is just a good option to reduce weight.
_________________
Innovative Karting/ Intrepid karts
Glendale, Arizona
WWW.INNOVATIVEKARTING.COM

[Justin Martin]

Well, I guess my reasoning is either an all weekend battery, or 75-80% weight savings.

For the heavier guys they could save a ton of weight. For us lighter guys I can gain 4x the run time. Which is really no big deal, BUT it would be cool for me to leave my house in Tucson and practice or race 3 days in a row in Phoenix at PKRA. Meanwhile having one less thing to worry about.

[Nick Weil]