Pre-Charge Resistor help

I know this has been covered all over the place but I can't seem to find a clear and concise answer to this.

I am running a tidalforce 750x on the B battery (which is a self built headway 12s1p pack 43.8v).
I connect to the bikes controller via andersons and right now get a spark when i connect.

So i want to add a Pre-Charge resistor. I used the no-spark calculator on: http://www.scriptasylum.com/rc_speed/_nospark.html

and got a value of 200ohm 1.87watts for a 1 second charge so i figured I'd go will a 200ohm 5watt resistor like this:
http://www.allelectronics.com/make-a-store/item/200-5/200-OHM-5W-RESISTOR-SAND/-/1.html

let me know if i am on the right track with this.

And the no-spark calculator says, the resistor should stay plugged in, but fetchers diagram has a switch in series to disconnect resistor after charging circuit. DO i need a switch and if so what you out there recommend?

thanks for any help you can provide.

fyi i peak below 50amps if that is necessary for switch choice.

I first tried a 470ohm 10w resistor and the precharge wasn't enough for my BMS kicking off and big sparks when main power connected. So then 2 in parallel and everything worked fine but took a really long time to charge... with 3 in parallel the start was nice but 3 10w resisters is a pretty good chunk of resistors! I found some 120ohm 5w and now I just use 1 of those.

As for switch or not ... that's up to you! I personally use 2 switches; first is a lower amp for precharge and the second is a high amp for main power. I flip the precharge switch off after I flip the main on. I would like to find a "good" double throw switch with a small form factor but those I found for 50A or so had the high amp contact bassackards (1st contact in a rotation is high amp and stop 2 the low amp). I'm still looking for "small" double throw switch that can handle 50A or more on a primary pole. If anybody knows a good source let me know!

For awhile I just had 3 connectors that I used to plug in the battery; Black & Yellow (precharge) followed by Red for full power. It didn't seem to make a difference if I left the yellow connection connected. My head logic tells me path of least resistance and the resister is effectively no longer a resitor when paralleled with straight wire... though my EE brainwashing was MANY years ago so YMMV.

The only real reason to leave it plugged in is so you don't have the loose wires floating around. Using a permanent connection and a switch eliminates this problem.

There is a constant urrent flowing thru the precharge resistor, when you leave it connected.
It draws only ~200mA@44V but that is 9W power loss. It will overheat the 5W resistor.
The good news is, you'd only need a small switch for 200mA.
-Olaf

I'm good with some sort of small constant current if left connected. But I'm not sure where 44v would come from other than a very short initial moment of time? I started off along those lines, thinking I needed high watt resistor but that was before I tried the first 470ohm 10W and discovered that it didn't provide enough precharge for my controller. Light bulb... oh, that must mean that the controller has resistance too. Chuckle, I'm just learning again … maybe that was a wrong conclusion?

Hum... ponder... ug, the cobwebs of time are thick! Let's see... I do know the wad of 3 paralleled resistors 10w each (~155ohm) didn't get warm when left connected. Could this have been due to power multiplier factor of parallel resistors and the inaccurate finger touch heat measurement? Not likely as I also know that I notice no heat when touching my 120ohm 5W resistor during precharge use ... maybe I'll leave that switch on for a ride to see what happens now that I'm curious? If it gets warm, maybe it tells me the effective controller resistance drops near enough to 0ohms during operation that the precharge resistance becomes a factor worth considering again.

With 53V my 120ohm 5W resistor drops quickly to 9V across it and then down to 0.2mV (lowest resolution of my meter) when the main power switch is activated also. So, with the precharge only switch activated and standing still (0 throttle) the 5W resistor dissipates under 1W right (0.7W)? And if left on with the master switch wouldn’t the resistor power requirements be considerably less, such as to the point of insignificant? I assumed the 120ohm resistor in parallel with the bare wire to be about a .001ohm factor in the circuit. I ended up thinking .001ohms in series with controller resistance 1-500ohm was effectively 0 impact to; ebike operation, battery drain or preload power dissipation needs. Only the initial connection of contacts and charge of caps had the momentary potential to hit and maybe exceed the 5W rating.

Yeah, my bad!
Forget about overheating, as long as you close the main contactor soon after precharging. Don't leave the battery connected to anything after your ride. Otherwise the precharge resistor wil suck the juice out off it.

In other words: it's safer to have a switched precharge circuit.
Olaf
<edit> How could I write such a stupid answer?

Once the controller caps are charged the current flowing through the pre-charge resistor is almost zero and once the main switch is on the current would be almost un-measurably small.

Wouldn't it make sense to have the precharge switch as a momentary press button ? So you just hold it in for a second or 2 then flick the main power switch ?

Also does anyone remember a more complex way to prevent the precharge spark without having to make it a 2 step process with the resistor ? I remember reading it somewhere but can't remember the details

Does the precharge resistor go on the positive or negative lead?

veloman said:

Does the precharge resistor go on the positive or negative lead?

Click to expand...

Positive.

Where would one put the pre-charge resistor on one of Lyen's controllers? It does not have the smaller red wire like the older 846 models had. Thanks!

Doesn't matter about the smaller wire--that is just an enable/disable wire.

Precharge needs to go in series with the main power input wire to the controller (big fat red battery wire).

I've been thinking about this, and I believe that with the right switch a pre-charge resistor isn't necessary.

I have one installed on my bike, yes, but I don't here or see a spark when I engage my switch.

I got mine here: http://www.harborfreight.com/battery-cut-off-switch-92688.html

The guy that wrote the how to build a 55mph ebike ebook, greasypants, uses a more expensive version of these on his 110v bike without a precharge resistor.

I'm not including one in my next build.

auraslip said:

I believe that with the right switch a pre-charge resistor isn't necessary.

Click to expand...

Higher power systems can benefit from precharge: Larger caps will have greater inrush current. Switches and contactors can get burned points from the arc.

Higher power systems can benefit from precharge: Larger caps will have greater inrush current. Switches and contactors can get burned points from the arc.

Click to expand...

How many on and off cycles do you think it will take to burn out the switch that I posted above?

auraslip said:

How many on and off cycles do you think it will take to burn out the switch that I posted above?

Click to expand...

Depends on your system.

Higher voltages and larger caps will be more likely to cause arcing; gradual burning and pitting can create high resistance in the contacts. Then it could get hot and melt the housing.

Contactors frequently have features to reduce the potential damage: contact-plating, magnetic blowouts, evacuated enclosures.


@Hyena:

Richard's diagram above shows a contactor coil being energized by the precharge; theory being, the big relay won't close until the caps are up to voltage (IIRC).

Click to expand...

What sort of diode across the the coil relay? 1N1004? bit small ? 1N5404?

what exactly does it do?

NeilP said:

What sort of diode across the the coil relay? 1N1004? bit small ? 1N5404?

what exactly does it do?

Click to expand...

Go big... 1N5404

"Coil Spike Suppression Diode = is a protection device across the contactor coil. Energizing the contactor coil creates a magnetic field that forces the solenoid arm to engage the contacts. When the contactor is turned off, the magnetic field collapses causing a back “voltage spike” that can damage the controller. The diode safely suppresses this energy."



Edit: removed landmine. ES remains independent!

Out of curiosity, does the relay open *and* close when you turn the key switch on and off? Or do you need to use the kill switch?

Second question: DC-DC convertors. Do they also need a pre-charge resistor, or will any switch do? If any switch will do, then I'll just leave the battery connected to the controller and power up and down with the ignition wire.

auraslip said:

Out of curiosity, does the relay open *and* close when you turn the key switch on and off? Or do you need to use the kill switch?

Click to expand...

Sorry... lost this thread somehow.

In the above diagram, the kill switch must be used to open the relay. Both the keyswitch and the killswitch are needed to close the relay; first the keyswitch for precharge, then the killswitch to close the relay.

auraslip said:

Second question: DC-DC convertors. Do they also need a pre-charge resistor, or will any switch do? If any switch will do, then I'll just leave the battery connected to the controller and power up and down with the ignition wire.

Click to expand...

Probably not needed, unless they are very big converters - if they have a big spark on connection, a precharge might be a good idea.

Tyler - thanks for answering that questions. Now that I've learned a bit more about relays, I know how noob it was

Sorry to bring back a thread from the dead.

I need a bit of help with this.
I've got a 9fet that connects to a 20AH 10S lipo pack.

Bought a pair of 50ohm 10w resistors; is this appropriate? too much? too little?
As for the precharge switch, will a 3a rated switch do? or will it blow out in short time?

Thanks
-David

Depends how fast you want to precharge... your low-value resistors may spark as much as having none. 500k-1K is more typical, to limit inrush-current.

This page is handy: http://www.scriptasylum.com/rc_speed/nospark.html

The slower the charge, the lower rated pre-charge switch you will need. (Lower current.)

Tyler, a second is fine for me.

The problem i have is that i don't know if i add *all* the capacitor values up in the controller, or just some to get that value for the no-spark calculator.

Yeah a 3 amp switch should be fine... you can also do something like this


as far as cap values...add them all up.... it doesn't have to be too exact. I'd put the resistors you have in series. If they don't spark like that, it should be fine.