Precharge resistors

[ebike11]

Hi all
I have a question about precharge resistors.
I am using a large 96V fardriver controller.
I purchased a 20W 2k ohm resistor and wired it up. I plugged it in before the main battery to controller connectors.
However Im still getting sparks on my Anderson connectors. I even waited a minute after plugging in the resistor connectors but it still happens.
I do have a second 20w 2k ohm resistor but it is a different brand/shape but they are both cement resistors.

My questions are

1. Is it ok to wire 2 different resistors together in parallel

2. If so, i assume the 2 resistors together will give an output of 10W 1k ohms? Will this charge the capacitors faster to reduce/eliminate sparks?

Thanks!!

[DogDipstick]

You use a common ground right? Meaning... you ground the controller before you try to precharge? The Andersons and the precharge both have a clear path to ground?

Two in parallel would be 1K at max disapate 40w.

[ebike11]

You use a common ground right? Meaning... you ground the controller before you try to precharge? The Andersons and the precharge both have a clear path to ground?

Two in parallel would be 1K at max disapate 40w.

Hi again!
Yes I wired it like you mentioned before.
The resistor and andersons are both grounded.

On the controller side i have 12awg tapped into both Neg and Pos. with an xt60 then another xt60 and wires tapping in the battery Neg and Pos with the resistor on the Pos. side of the controller.

I wired it like in the diagram

Do you think 40W 1K would be better or worse than 1 x 20W 2K ?

Thanks!

[Blacklite]

Unless there is a ridiculous amount of capacitance in the controller a 2k resistor should charge up the capacitance in a few seconds. Are you sure you have it wired correctly? When you say "the resistor is grounded" what do you mean?

The power rating on a resistor is just how much power the resistor can dissipate as heat before burning out. It has nothing to do with how much power is dropped or passing through the resistor, which is determined by the voltage across the resistor and the actual resistance of the resistor itself.

[ebike11]

Unless there is a ridiculous amount of capacitance in the controller a 2k resistor should charge up the capacitance in a few seconds. Are you sure you have it wired correctly? When you say "the resistor is grounded" what do you mean?

The power rating on a resistor is just how much power the resistor can dissipate as heat before burning out. It has nothing to do with how much power is dropped or passing through the resistor, which is determined by the voltage across the resistor and the actual resistance of the resistor itself.

Hi!
I attached a photo.
Can you check it out? Thanks!

[999zip999]

Grounded ? First plug in neg then pos pre charge resistor then main pos.
This is done with 5.5mm gold plated. bullets and 4mm on pre charge resistor. Andsons junk. Xt90s junk not for bikes.
You can always put lipstick on a pig

[ebike11]

Grounded ? First plug in neg then pos pre charge resistor then main pos.
This is done with 5.5mm gold plated. bullets and 4mm on pre charge resistor. Andsons junk. Xt90s junk not for bikes.
You can always put lipstick on a pig

So you cant plug in the resistor neg. and resistor pos. at the same time on a connector? Neg must be first then pos. THEN main leads together?

[DogDipstick]

Get your meter out and tell us the voltage that the precharge achieves on the terminals of the controller, before you connect the main leads.

Plug ONLY the precharge ground and positive in, and you should see a voltage on the controller POS and NEG terminal. Tell us what this voltage is.

You should see a voltage on the controller POS and NEG terminals. Before you plug in the main leads.

[fechter]

Your controller must be drawing some current before you turn it on so it's not able to fully pre-charge.

Try a smaller resistance value, like 100 ohms. It doesn't need to be 10W either, the current inrush is very short, so there shouldn't be much heating. If there is, something else is wrong. A 5w resistor should be plenty big.

[999zip999]

The resistor I use is from RadioShack is 10 ohm 10 watt for a 72v pack. No spark. Yes hook up ground then resistor then main pos.

[ebike11]

The resistor I use is from RadioShack is 10 ohm 10 watt for a 72v pack. No spark. Yes hook up ground then resistor then main pos.

Ah...maybe that was my mistake??? I have the resistor pos and neg on the same xt60 connector so they connect at the same time once the xt60s are plugged in.

[ebike11]

Your controller must be drawing some current before you turn it on so it's not able to fully pre-charge.

Try a smaller resistance value, like 100 ohms. It doesn't need to be 10W either, the current inrush is very short, so there shouldn't be much heating. If there is, something else is wrong. A 5w resistor should be plenty big.

Oh by the way..my voltage is 117.6V fully charged

[ebike11]

Get your meter out and tell us the voltage that the precharge achieves on the terminals of the controller, before you connect the main leads.

Plug ONLY the precharge ground and positive in, and you should see a voltage on the controller POS and NEG terminal. Tell us what this voltage is.

You should see a voltage on the controller POS and NEG terminals. Before you plug in the main leads.

Hi again!
Yes i measured as you instructed.
Im getting 10~12V on the controller main leads when the leads are not connected to the battery

The controller is seeing some power because it beeps faintly and the Cycle Analyst screen with random flashing and display Low Volts etc because the main leads arent connected

[999zip999]

I use 4mm bullets for the incline resistor. And 5mm bullets the all single. As the xt series aren't and pos in one connector . Like I said earlier are junk Yes the xt90s are under rated for your needs. Or junk. JUNK.

[ebike11]

I use 4mm bullets for the incline resistor. And 5mm bullets the all single. As the xt series aren't and pos in one connector . Like I said earlier are junk Yes the xt90s are under rated for your needs. Or junk. JUNK.

I am not using xt90s for my main leads..but i think xt60 ot xt90 connectors are fine for the resistor wiring as ive seen people using small awg for that part. Main connectors and wiring are much bigger though

[mxlemming]

You can use pretty much anything for the précharge connector. I'm using xt30. The current requirement is Vbat/R so 117V/100 ohm=1.17A. tiny.

You need lower resistance. Like fechter said. Below 100 ohm. The xt90 anti spark connectors use 2.2 ohm. They rapidly die with bigger higher voltage controllers because of the power dissipated in them.

Use a few W resistor and it'll be fine. Some have pulse power ratings If you get enthusiastic, but trial and error is your friend here. If you twist the throttle without the main connector plugged in it'll burn whatever you use.

[999zip999]

Do you have do you have a handlebar off and on switch ? What exact controller do you have ?

[mxlemming]

Screenshot_20220807-003831.png (184.8 KiB) Viewed 67 times

This randomly selected 100 ohm to220 resistor for example.

It has a graph of energy vs time.

Your controller probably has about 2500uF of capacitors. Probably.
Time constant RC is 0.0025x100 is 250ms.

Therefore you can safely dissipate at least 17J based on that chart.

The energy in the caps is 0.5xCxV²
0.5x0.0025x117²=17 joules by sheer coincidence.

That resistor would handle the inrush.

The controller will then draw current through it until it's plugged in with the main plug. Maybe that's 100mA. Who knows. Measure it.

Power dissipation is I²R=0.1²x100=1W. Happy days it's rated for 2.25W in free air or 50W on a heatsink. In fact you could draw up to160mA with no heatsink and 0.7A with one.

You'd then have a voltage drop of 0.1Ax100R is 10V. This is fine. 10V won't spark much, if atall.

There's nothing more to this.

[ebike11]

Do you have do you have a handlebar off and on switch ? What exact controller do you have ?

I have a 96v fardriver

[ebike11]

You can use pretty much anything for the précharge connector. I'm using xt30. The current requirement is Vbat/R so 117V/100 ohm=1.17A. tiny.

You need lower resistance. Like fechter said. Below 100 ohm. The xt90 anti spark connectors use 2.2 ohm. They rapidly die with bigger higher voltage controllers because of the power dissipated in them.

Use a few W resistor and it'll be fine. Some have pulse power ratings If you get enthusiastic, but trial and error is your friend here. If you twist the throttle without the main connector plugged in it'll burn whatever you use.

Im using 20W 2k resistor but maybe its too big then.

[ebike11]

Your controller must be drawing some current before you turn it on so it's not able to fully pre-charge.

Try a smaller resistance value, like 100 ohms. It doesn't need to be 10W either, the current inrush is very short, so there shouldn't be much heating. If there is, something else is wrong. A 5w resistor should be plenty big.

Yes i could try a 100ohm resistor...at the moment im using 20W 2k.

[fechter]

You could try 100 ohms and see what voltage you get on the controller before making the main connection. Even 10 ohms might be OK. You want the voltage after precharging to be within a few volts of the pack voltage.

[whatever]

I used to use an led with the resistor, led would be on when current flowing ( precharging), when caps full the led goes out, you know its ready to hit the on button.

[999zip999]

Good trick