How to prevent Sparking when connecting batteries with an MGM controllers
 

Hi Guys,

Saw this on the MGM website and think that it will help some of us.

http://mgm-compro.com/pdf/en-antispark-090707.pdf

Will this still work with a Dean's Ultra and how many of us are already doing this?

I have mine set up that way. I use a deans plug for the neg(-) and resistor on the pos(+), plug that in first. I have a 5.5 for the main pos(+), and plug that in second.

You have a pic of that? Like they say, a pic is worth a thousand words.. :)

I use deans on mine, but I just have a seperate small connector (a 2pin JST plug) that has the resistor. Works really well, esp w/ my HV setup and the really large caps that are on the 9032.

A slight pain as you need an extra connector, but worth it. Just be sure to unplug the xtra plug once fully connected.

its on his muggy link in his sig. nice setup

First, you plug the main "neg" lead. Second, you plug the "by-pass" resistor lead, and third, you plug the main "pos" lead.
On the batterie side, the by-pass lead have a 20 ohms resistor in it. And i use Schulze 3mm bullet for it.
http://img220.imageshack.us/img220/6650/sparkmodfg8.jpg

This type of thing can be done with Deans just as easily (actually, probably easier):

http://scriptasylum.com/forumpics/nospark.jpg

Just hook the mini-deans up first, wait ~10 seconds, then hook up the main leads.

The resistor in the pic above is based on 1,000uF ESC total capacitance and an approximately 10 second charge time. The actual formula for picking the resistor value is:

R = S / 5 / C

R= Resistor in ohms
S= Time in seconds
C= Capacitance in farads

So, if your ESC has three 330uF caps (990uF total) and you want them charged in 5 seconds, the resistor to use would be 1010 ohms, or 1k.

The required resistor power rating can be found by the formula:

P = V^2 / R * 2

P= Power in watts
V= Battery voltage
R= Resistance in ohms

So, for a battery voltage of 22.2v (6s) and a 1k ohm resistor, use a 1W resistor.

MGM suggests resistor values of 20 to 50 ohm.

I'm sure they recommend those values based on the same capacitance and time-to-charge parameters. 20 ohms on 1000uF total capacitance would take 100ms to charge. It just depends on how long you don't mind waiting.

That is basically what I did with mine. However I did not want to use a Deans miniplug as it leaves the possibility of the little prong on the mini connector making contact w/ the main Deans plug. ==> Dead short.

If it was wired carefully to make that impossible then I would feel ok about it. I just don't want any lipo fires in my travel bag I'm carrying my RC stuff around in. Instead I just prefer this:
http://www.brushlessmotors.co.uk/ima...JST%20Plug.gif

You can use a smaller resistor, but then you have to get a higher watt rated resistor, and those get big and bulky. I used a 1/2W resistor (as its small and easy to find at ratshack) and an appropriate ohm value. I prolly should get a 1W resistor, but I just have it plugged in a few secs so its fine (its a 2200 IIRC.) Really you just need to take the initial "sting" out of it, and it always takes me a couple of seconds to plug in one, then grab the Deans and plug that in. Spark is tamed by then. Here's a handy calculator

BTW Brian, shouldn't it read [P=V^2/R ]?

True, a totally insulated and polarized mini-connector would be best. The diagram with the mini-Deans was just for demonstration purposes.

Like you said, you don't really need to keep the charging resistor circuit engaged the full amount of time. All you really need is 2/5th the total charge time (which is 86.5%) to take the "sting" out.

And yes, power is V^2 / R, but I always like to add in a little extra heat capacity for safety (factor of 2). But since the duty cycle is so low, you could probably easily get away with V^2 / R * 0.5. I tend to over-engineer. :oops:

haha, just keepin you on your toes Brian :)

For me, as sonne as the main "pos" lead is plug, i unplug the bypass resistor lead and insulated it.

P O W E R P O L E S :na:

Hi,

This is a very useful thread. I nominate this to be a sticky.

Regards,
Joe Ling

How I did it
 

Plug in the deans (-neg and resistor) then the 5.5mm (+pos)

And I can still charge with the deans (no adapters) as all my chargers have deans.

http://www.rc-monster.com/forum/atta...1&d=1203398616

bdebde, that looks nice and neat. Time to take out the soldering iron again once the controller comes back from MGM.

I second the sticky status. I had found this info on the MGM site, but I would have know it a lot sooner if it were a sticky. Good info for everyone running high powered brushless.

Cool. I'm going to have to stop by ratshack soon and figure out which resistor I need for my 4s and 5s (5000mah, roughly) packs. The theory end of it I'm not real clear on.

The resistor value is not dependant on pack capacity, but pack voltage and capacitance of your particular ESC.

The ESC capacitance will determine the resistor value. The battery voltage will determine the resistor power rating.

If you don't mind waiting between 8 and 10 seconds for the caps to charge, use a 2200 ohm 1w resistor. That will be fine for anything up to 40v battery and an ESC capacitance of 1000uF (which is pretty typical). If your battery voltage is lower, that's fine.

If there is enough interest, I can add a page to my "R/C calculations" site to assist in choosing the proper resistor and provide drawings on how to do the necessary wiring.

If you're up for it, that'd be great. I really like the deans layout, that is very simple and intuitive and then you plug the main power in with the bullet connectors. I think that's the method I'm going for.

I assume there's no detriment to having that resistor plugged in all the time with the deans connector?

As much as I generally understand electric theory, sometimes sitting down and calculating it goes over my head. I just don't sit and pay enough attention, really. :)

Done!

Look for the "No Spark" Calculator :smile:

Thanks for that. A little extra wiring for some peace of mind. I've always hated that big spark, it just seems wrong and dangerous to the lipo pack.

So can you leave the resistor plugged in while driving or will the resistor get burnt out? It would look neater than having it unplugged.

I have ran mine with the "no spark" lead still connected. I can't see any reason why you would need to disconnect it when running. All the currant will flow though the path of least resistance eg. deans plug, bullet connect etc.
Cue BrianG or one of the other guru's to chime in here with more technical reasons why.

Technically, whether you leave the small connector plugged in or not depends on your choice of connector type. If there are any exposed terminals where they have a chance to touch against anything, I'd just leave them connected. Other than that, it makes no difference.

I prefer to disconnect mine as I'm drawing a bit over 600mW on a 1/2W resistor. I left it in once for a while and it got a bit warm. Its easier/safer for me than trying to find a 1W resistor and redoing it. If you've got headroom in the resistor I don't see a big need to unplug it as long as it stays cool.

Once you connect the main connectors, there should be NO heat on the resistor AT ALL. However, if you leave the main connector apart, that will heat up the resistor and slowly drain the battery.

Hmm that's not what seemed to happen to mine... It's still a live circuit, you should get some current flow over the resistor, yes? Could have been from motor/wire heat from the run.... but I didn't really investigate it much. I am talking about leaving it plugged in while driving the car.

Maybe mike can offer this in a kit for the MGM's with wires, resistor, and connectors to make it easier for everyone to set this up

@Finnster: Don't know why you had heat on the resistor while running. Once you connect the main wires, they effectively short out the resistor so there is no voltage drop across it (which means no current flow and therefore no power dissipated). I would suspect the heat came from some other heat producing thing in close proximity, or maybe the main wires heated up and the small wire/resistor heated as well.

@austinmaxx: Should be easy to do, but there would have to be kits for the different connector types and different resistor values for different voltages and capacitance for that specific ESC. Once you calculate what resistance and power value you need, you should be able to get a resistor close to what you need at RadioShack, PartsExpress.com, etc. And this type of thing could be used in ANY ESC, especially when using 5s+ voltages or ESCs that handle really high currents (because those usually have higher capacity caps).