How big are the caps on a controller?

[Tiberius]

What value of capacitance is there typically on a controller?

I'm trying to do some calculations on the inrush current and sparking when the controller is connected to the battery. Or for that matter, a similar package of electronics is connected to a similar power source.

What's the worst case?

Nick

 

[liveforphysics]

What value of capacitance is there typically on a controller?

I'm trying to do some calculations on the inrush current and sparking when the controller is connected to the battery. Or for that matter, a similar package of electronics is connected to a similar power source.

What's the worst case?

Nick

If I remember correctly, 6x2200uF is the biggest I've seen on a non-kelly, and this was a setup for low voltage. The 100v stuff generally only uses 1,000uF caps. I personally fit 4,700uF 100v caps on my home-brew tinkering stuff.

If you're looking to calculate the inrush current though, the capacitance value isn't part of that calculation. Just the combined ESR. If you're looking to find the energy needed to charge the caps, or get an idea of how long to have a timer circuit wait over a given resistance or something, then the cap value will matter.

I'm loving your idea diodes, and I can't wait to see what sort of slick item you come up with next.

-Luke

 

[Tiberius]

I'm loving your idea diodes, and I can't wait to see what sort of slick item you come up with next.

-Luke

OK, Luke, I admit I'm doing a little more than calculations.

I'm working on an inrush limiter device. The breadboard on the bench is working with 1000 uF "load" but not too well with 10000 uF.

To be honest, part of the trouble I'm having is with the test equipment. Its a bit difficult to create turn on transients without upsetting the power supply and the electronic load. I may have to go to batteries and resistors.

Nick

 

[thatguitarguy]

Is this why there is a big spark when I hook up my battery?

I'm pretty new to e-biking and only have basic electronics under my belt. I've been concerned about the spark that is discharged when I connect my battery. It's pretty big and I'm concerned that it will eventually cause some damage somewhere. I don't understand why there is one big spark initially, but then I can hook up the battery with no spark within a few seconds, but if I wait for a minute or so there is another big spark. Is this from capacitance in the controller, or is it building up in the BMS, or somewhere else??

Thanks for your help.

 

[Tiberius]

Is this why there is a big spark when I hook up my battery?

That's exactly it. There is a big inrush of current to charge the capacitors. The caps will keep their charge for a few seconds so a second connection won't show the same problem.

Nick

 

[thatguitarguy]

Thanks Nick.

I recently got a WattsUp that I haven't hooked up yet, partly because of my concern about the spark damaging something. When I hook it up inline should I hook it up to the battery before hooking up to the controller, or should I hook it up to the controller and just leave it there when I remove the battery for charging?

Terry

 

[Tiberius]

Terry,

I don't think it matters; its pretty much the same either way. It depends on whether you want it to stay on the bike or travel with the battery.

Nick

 

[liveforphysics]

Thanks Nick.

I recently got a WattsUp that I haven't hooked up yet, partly because of my concern about the spark damaging something. When I hook it up inline should I hook it up to the battery before hooking up to the controller, or should I hook it up to the controller and just leave it there when I remove the battery for charging?

Terry

The wattsup is a heavy shunt on one side, and just a solid wire passing through the other side. You don't need to worry about hurting it.

 

[thatguitarguy]

I'm thinking that I'd like to set up the WattsUp to stay with the battery. I want to keep my power wires as short as possible, but still be able to keep an eye on how my battery is doing. I can buy a cheap DVM at Harborfreight for $2.99, and I can run smaller parallel leads to a place where I can see it while riding. Does this seem like a viable plan, or do you have better ideas?

Also, why is there a separate negative charging wire for my ping battery, but not a second positive wire?

 

[Tiberius]

Also, why is there a separate negative charging wire for my ping battery, but not a second positive wire?

Now that I can answer. Its a fairly normal arrangement for a BMS (Battery Management System). The BMS controls the charging process and needs to be able to disconnect the charger, or at least reduce its current, when the battery is full. It only needs control of either the positive or negative line and it turns out that with existing components its easier to control the negative line, so that has become the standard way to do it.

Nick

 

[thatguitarguy]

Does the BMS only control the charging cycle? I thought that it protected the battery from being drained too far or too fast.

Thanks for your patience with this nooby.

 

[Tiberius]

Hey, we were all noobs once.

It depends on the BMS. But ones with separate negative leads for charging are unlikely to be able to disconnect the load on discharge.

Nick

 

[thatguitarguy]

My battery cost more than everything else on my bike, so I want to take good care of it. It's a ping 36V 20AH, and so far the lowest it's been after a ride was 39V, so I don't think I'm abusing it. I always pedal when I ride and just depend on the motor for hills, and to keep my speed up just a little bit.

Will the voltage read pretty high until it's dead and then just drop off suddenly, or will it have any kind of linearity as it's capacity is used up?

 

[John in CR]

Is this why there is a big spark when I hook up my battery?

That's exactly it. There is a big inrush of current to charge the capacitors. The caps will keep their charge for a few seconds so a second connection won't show the same problem.

Nick

I think it depends on the controller and the order of how you do things. On my first bikes where I used to disconnect the batteries, if I turned the controller off first, then the caps stayed charged for at least a day or two, so no spark on reconnect.

thatguitarguy,
The spark starts to corrode things fairly quickly, a precharge resistor is used to prevent that spark. This resistor slows the current to charge the caps during the connection, so no spark occurs.

John

 

[tycreek]

Is this why there is a big spark when I hook up my battery?

I have sevaral extra 470ohm 10w resistors left over and would be happy to mail a few (US). I used 2 in parallel and seems to be working for me.

 

[thatguitarguy]

So if I want to use a pre-charge resistor how would I hook it up? What value? Is this something that could be permanently installed, or would it just be used to pre-charge the capacitors in the controller, and then quickly hook up the battery before they leak down?

I guess a way around this would to be to permanently install the battery and charge it in situ, but I like being able to remove the battery for a number of reasons.

Edit:

Thanks tycreek, I missed your post before I wrote this. Do you have yours permanently hooked up? Does it change the performance of the controller?

 

[thatguitarguy]

I hooked everything up with the WattsUp on the battery first, and the spark doesn't seem as aggressive. I hope that's the case, but I'd still like to know how to wire in the resistors to control the spark, and if this affects performance in any way.