How to prevent Sparking when connecting batteries with an MGM controllers

[Finnster]

Originally Posted by BrianG

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



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.

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:


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 ]?

[BrianG]

Originally Posted by Finnster

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:


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.