72v + users, do you use a pre-charge resistor?

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Timma2500   1 kW

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72v + users, do you use a pre-charge resistor?

Post by Timma2500 » Jun 11 2012 11:04am

I've just stepped up from 12s lipo to 20s for another bike and i'm finding the initial plug-in of the battery produces a much larger spark than it did on the 12s setup.

Do you guys running say 72v and higher all use pre-charge resistor setups to conserve your plugs? Its really starting to hammer my Deans and bullets after only a few plug-ins and i know i'll need to do something about it sooner than later!

If ya's do use pre-charge setups, is there a simple way to make one?

Cheers,

Paul :D
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Re: 72v + users, do you use a pre-charge resistor?

Post by whereswally606 » Jun 11 2012 11:10am

yes I do, i just made a connector with 3 large seriesed resistors from an old power supply, does the job fine. only running 18s at the moment but should work in theory for higher voltages.
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Re: 72v + users, do you use a pre-charge resistor?

Post by pwbset » Jun 11 2012 11:53am

whereswally606 wrote:yes I do, i just made a connector with 3 large seriesed resistors from an old power supply, does the job fine. only running 18s at the moment but should work in theory for higher voltages.
could you post a photo or a little how-to for us more challenged members? resistor specs? just finished my 18s pack and am not digging the plugin zap either. thanks!

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Re: 72v + users, do you use a pre-charge resistor?

Post by Ypedal » Jun 11 2012 12:07pm

http://endless-sphere.com/forums/viewto ... =16&t=7994

it's not just the voltage, it's the controller's capacitors, heavily modded 18fet units with large caps make BIG sparks.. enough to spot weld andersons.. but my 100v crystalyte analogue controller is not bad at all, i can plug right in ( 3 years on same connectors and still intact )
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Re: 72v + users, do you use a pre-charge resistor?

Post by pwbset » Jun 11 2012 12:14pm

Derka. Thanks Ypedal. Monday morning... lack of caffeine affecting my brain. :D

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Re: 72v + users, do you use a pre-charge resistor?

Post by auraslip » Jun 11 2012 1:01pm

enough to spot weld andersons
Verily! All my bullets have huge burns on them from connecting my 18 fet at 22s. Luckily, the tip of the bullet doesn't actually make a connection so it can get burned up all it wants.
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Re: 72v + users, do you use a pre-charge resistor?

Post by d8veh » Jun 11 2012 2:59pm

I used one of these. Cost less than$5 to make ans saves a lot of messing about. Works perfectly even with counterfeit FETs. Thanks Jeremy.
I might be able to make a few more pcbs if anybody's interested.
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Re: 72v + users, do you use a pre-charge resistor?

Post by John in CR » Jun 11 2012 3:03pm

I just use two 50ohm resistors connected in series for 100ohms and the leads coming out of the same end, which makes using it easier. I make one connection first, pos or neg, and then connect the other through the resistor first, then plug. Different controllers have have different rates of draw down of the caps, so some I have to be quick with the plug or make the tip of the connection while the resistor is still touching.

Even if you get more elegant for the on-bike connection, you still need such a resistor handy for things like draining the caps and shorting the leads before opening a controller so you don't blow the board. I also like to be sure the caps are drained and short the controller leads for storage or shipping.

Warning- when you do short controller leads, be darn sure to drain those caps first. Due to battery IR the spark on connection that we've all experienced is nothing compared to the gunshot-like crack if you short the leads on a big controller with the caps charged. :shock:

John

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Re: 72v + users, do you use a pre-charge resistor?

Post by amberwolf » Jun 11 2012 5:10pm

Ditto. My tiny little 6FET Fusin controller on the kit I'm currently testing/reviewing has little male automotive bullets on it's power leads. I unplugged them from my CA's PP45 Andersons (yeah, I know) and hooked up the charger to the PP45s, leaving the controller leads dangling. It has no drain-down resistors, and will stay fully charged for quite some time (at least overnight). Then I leaned across the bike to grab something out of the front baskets, and my leg pushed the two bullets together.

CRACK! like a big branch snapping. All the dogs ran away except Hachi...and my heart was racing because even though I "knew" what had caused it, my body was still reacting to it.

This is at only 55-ish volts of the partly-discharged pack prior to disconnection...it would probably get much more interesting at higher voltages.
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Re: 72v + users, do you use a pre-charge resistor?

Post by icecube57 » Jun 11 2012 5:23pm

I use 5w ceramic 1k-1000ohm and the precharge time is 5seconds.

http://scriptasylum.com/rc_speed/nospark.html

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Re: 72v + users, do you use a pre-charge resistor?

Post by sn0wchyld » Jun 11 2012 9:08pm

yea i use 2 1W 150ohm resistors in series for my 24s pack to a 12fet lyen.
I leave the controller connected though while charging, so the anti spark rarely gets used.

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Re: 72v + users, do you use a pre-charge resistor?

Post by recumbent » Jun 13 2012 2:04am

sn0wchyld wrote: ...snip...I leave the controller connected while charging, so the anti spark rarely gets used.
Yes, I also leave my controler connected while charging, this way you can monitor your voltage through your Cycle Analyst if needed. Just connect your charge leads on the same line as your batteries + controler.
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Re: 72v + users, do you use a pre-charge resistor?

Post by miuan » Jun 13 2012 2:11am

I'm too lazy to precharge.. after 2 years of commuting it turned out you can get away with it just fine using 4mm bullets. Sure their ends get dark but the inside of plug is still okay and it's not overly warm at 74V/60A. No burned caps either. Guess it'd end up much worse at 88V but at 74V the spikes are probably well within 100V controller specs. I even operate 75V fets at this voltage without precharge and they still work. Lucky me.

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Re: 72v + users, do you use a pre-charge resistor?

Post by Timma2500 » Jun 13 2012 9:46am

Thanks guys! I'll get onto it this week and put together a positive lead resistor setup.

Do ya's think one of these will do the trick with a 20s setup?:
http://www.jaycar.com.au/productView.as ... ATID=968#1
amberwolf wrote:CRACK! like a big branch snapping. All the dogs ran away except Hachi...and my heart was racing because even though I "knew" what had caused it, my body was still reacting to it.
Its funny you say that Amberwolf, the 2nd time i connected up and got the almighty CRACK, our male staffy was beside me and my brother's even older dog was in the doorway. The staffy heard it, promptly crapped himself and ran for the hills! The older dog being deaf, just watched him run past and had no idea what was going on :lol:

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Re: 72v + users, do you use a pre-charge resistor?

Post by cohberg » Jun 13 2012 10:37am

Timma2500 wrote:Do ya's think one of these will do the trick with a 20s setup?:
You may have been referring the entire package of 8 resistors but, just as a reminder, make sure to run all 8 in parallel so that the instantaneous draw (up to 200W) at the beginning doesn't burn it out. A single 1/2 watt resistor will get pretty hot all by itself.

Edit: if you do run the 8 in parallel you would need a 8x 1/2W 2.4kΩ resistor pack for an equivalent 300Ω 4W resistor
Last edited by cohberg on Jun 13 2012 11:16am, edited 1 time in total.
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Re: 72v + users, do you use a pre-charge resistor?

Post by Ypedal » Jun 13 2012 10:42am

cohberg wrote:
Timma2500 wrote:Do ya's think one of these will do the trick with a 20s setup?:
You may have been referring the entire package of 8 resistors but, just as a reminder, make sure to run all 8 in parallel so that the instantaneous draw (up to 200W) at the beginning doesn't burn it out. A single 1/2 watt resistor will get pretty hot all by itself.
you sure bout that :?:

My loose understanding of this is that a smaller resistor will simply cause a slower rise in voltage until the caps are ready to go, then you can hook up the main power leads without spark.. too large a resistor will result in a small spark but no problem in perticular.
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Re: 72v + users, do you use a pre-charge resistor?

Post by mdd0127 » Jun 13 2012 10:49am

I had way too small of a resistor in my precharge circuit when I first put my bike together and it didn't work at all. It was either so small that I never waited long enough or it burned up inside and became an open circuit. I never tested it to see why it failed. I rigged up a 1K-1 watt to replace it and it worked really well but I never permanently installed it. I just use a dual stage power plug with a sacrificial tip. I don't mind the spark so installing the precharge resistor hasn't been a priority.
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Re: 72v + users, do you use a pre-charge resistor?

Post by Alan B » Jun 13 2012 11:04am

The value of the resistor determines how long the precharge time is.

The mass of the resistor determines how warm it gets.

The capacitance and voltage determine the energy involved. 1/2 * c * v^2. So voltage is squared, and makes a big difference.

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Re: 72v + users, do you use a pre-charge resistor?

Post by cohberg » Jun 13 2012 11:09am

Ypedal wrote:My loose understanding of this is that a smaller resistor will simply cause a slower rise in voltage until the caps are ready to go, then you can hook up the main power leads without spark.. too large a resistor will result in a small spark but no problem in perticular
I'm not sure about the current limiting abilities of lower wattage resistors. I'm pretty sure that the wattage rating is just for max heat distribution. Obviously the little resistor isn't going to flow a lot of amps so that helps with the anti-spark.

That being said. If you spec your resistor correctly it should give you no spark even if you had a infinitely large wattage rated resistor.

Image

If you look at the simulation on the scriptasylum page you can see that there is a large draw at the very beginning. Afterwards it just tapers off. So for that first second, the resistor is being run at power levels multipliers above its rating.

I'm just speaking from personal experience where single 1/2W resistors would burn through heat shrink and brown within a couple uses in contrast to a correctly spec'ed 3 second precharge through a 10W resistor that had no spark.

What I did flub on was the resistor math. You'd need to buy the 2.4kΩ 8 pack to get 300Ω of equivalent resistance.
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Re: 72v + users, do you use a pre-charge resistor?

Post by Alan B » Jun 13 2012 11:27am

Initial resistor power is e^2 / r. This can be quite large and burn a small resistor. I use 100 ohm 10 watt wirewound which starts out at 50 watts dissipation but drops off quickly. Rule of thumb for wirewound resistor is it can handle a brief overload for 20 seconds or so.

Total energy in the resistor is equal to capacitor energy. So 1/2 * C * V^2 is energy for capacitors, and also for the resistor.

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Re: 72v + users, do you use a pre-charge resistor?

Post by icecube57 » Jun 13 2012 1:29pm

Thank you for pointing that out . I used the same script and again i get 1k ohm at 5w . People ask why i use the big bulky ceramic resistors... Im like Thats what it calls for. That first second at that thigh wattage could damage regular resistors...

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Re: 72v + users, do you use a pre-charge resistor?

Post by Alan B » Jun 13 2012 2:01pm

My 24 FET controller has six 100 uF caps at 100V. So presumably they are all in parallel and therefore 600uF. At 100V then 1/2 * C * V^2 would be 3 watt-seconds total energy. So a 5 watt wirewound would probably be ok since the wire is pretty tough and can handle the brief overload, but a carbon film or something like that with a low mass resistive element might exceed a safe temperature very quickly.

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Re: 72v + users, do you use a pre-charge resistor?

Post by GCinDC » Jun 13 2012 2:44pm

this is how i do it (or did, back when i was using andersons):
Image
plug in the pair, wait two seconds, the plug in the solo red. the resistor is under the shrink next to the black connector. bind it up tight, or better yet, lay it against something solid before taping it up so it never wiggles, cause if it does it will break and you'll be back to sparking...
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Re: 72v + users, do you use a pre-charge resistor?

Post by oatnet » Jun 13 2012 5:28pm

I use (2) power switches, and the second switch has a pre-charge resistor around it. When I close the first switch, power goes through the pre-charge resistor. When the v stops climbing, I close the second switch, and full power is available.

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Re: 72v + users, do you use a pre-charge resistor?

Post by sn0wchyld » Jun 13 2012 8:19pm

Timma2500 wrote:Thanks guys! I'll get onto it this week and put together a positive lead resistor setup.

Do ya's think one of these will do the trick with a 20s setup?:
http://www.jaycar.com.au/productView.as ... ATID=968#1
amberwolf wrote:CRACK! like a big branch snapping. All the dogs ran away except Hachi...and my heart was racing because even though I "knew" what had caused it, my body was still reacting to it.
Its funny you say that Amberwolf, the 2nd time i connected up and got the almighty CRACK, our male staffy was beside me and my brother's even older dog was in the doorway. The staffy heard it, promptly crapped himself and ran for the hills! The older dog being deaf, just watched him run past and had no idea what was going on :lol:

Paul :D
1/2 watts probably a bit small... i'd use the 1 watt stuff they have. ive got 2 of them and they dont get warm at all on 24s.

GCinDC wrote:this is how i do it (or did, back when i was using andersons):
Image
plug in the pair, wait two seconds, the plug in the solo red. the resistor is under the shrink next to the black connector. bind it up tight, or better yet, lay it against something solid before taping it up so it never wiggles, cause if it does it will break and you'll be back to sparking...
another thought occored looking at this... why bother with the resistor at all? if you're going to have 3 pins permanently connected then you may as well just let one spark, and then connect up your other 'clean' one. that way it doesn't matter how F'ed up the 1st one gets, and saves the trouble of having resistors wired up too (not that that's a big burden). Also means no waiting around for the caps to charge.

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