Inrush current limiters as spark protection

[milehigh]

Long time browser.. finally signed up to throw out a question…!

First time I’m building a HV system. I’m connecting this ANR26650M1B, 20aH pack (no they won’t all be in parallel hehe, I’ll flip every other line soon for a 16S8P pack) to this 300A ESC. Spark protection is obviously a concern, and the instructions for the ESC states that if the included pre-charge resistor isn’t used, damage will occur.

But manually connecting that resistor beforehand every time I switch battery packs doesn’t feel fool-proof enough, so I’m looking for other solutions.

I’ve ordered the AS150 anti-spark connectors, but I'm gonna run some 2-300 amps out of this system so with that I would have to use two in parallel. And also my BMS system has a nice current sensing system with the possibility to control a cutoff relay that I would like to use… and if a relay is in the circuit I guess the spark problem will affect that too.

So… another solution I’m looking at is inrush current limiters. There are these thermistors from Ametherm that take 50A each. According to the manufacturer support engineer they can be put in parallel as long as they are thermally coupled (in order to heat up together and work as a single unit)

http://www.ametherm.com/inrush-current/ms35-inrush-current-limiters

I’ve done some searching around the forum but haven’t found anyone using inrush current limiter thermistors for this. Does anybody have any experience with them? Would putting them in the circuit eliminate sparks everywhere.. so that connectors, relays, kill switches, etc would be unaffected? Sounds like a nice solution, but would it work…

Ah.. and a related question, for charging, are sparks a concern when connecting the charger to the battery pack, or is it just the caps in the ESC that causes sparks?

Thanks!

Olov

 

[Punx0r]

Yes, it's the input capacitors charging that causes the high inrush current.

Thermistors might do the job, I have no idea. It might be over-complicating it, but I have in mind your "ignition" switch immediately connects the controller to the BMS via a resistor and also starts a simple 555 timer circuit that closes a relay or solid-state switch after a couple of seconds bypassing the pre-charge resistor.

 

[Gregory]

OK so I'm not an expert so take this as general musings.

You don't use relays to switch 200A, you need a contactor. And a big enough one would probably negate the need for other spark protection. Does not slowly charge the ESC caps though. May be a workaround with a pre-charge resistor on a second relay?

Putting 4 of those ICL in parallel and then in series with the main power? Their resistance will be cut to a quarter, so from 5 Ohms each to 1.25 Ohms total. What will that do to inrush current, I suspect nothing (i.e. not stop sparks), but I'm not sure. Pre-charge resistor values are two orders of magnitude higher.
Then during use at full power (200A) they will be dissipating up to 100W at peak.

If you have a fancy BMS, does it have a on/off switch or thermal breaker. Because the MOSFETs are ideal to use to switch the current on and off, eliminating sparks. dnmun has frequently posted about using a thermal breaker or the MOSFET gate supply on his BMS as a cut off. Does not slowly charge the ESC caps though.

There are caps in the charger that will suck up current. normally the recommendation is to plug in the charger to the mains first to charge the caps, rather than the battery first which would cause the battery to charge the caps.

edit: PS I've never been bothered by sparks on 16S lifepo4

 

[milehigh]

Thanks for some great answers.. and musings!

PS I've never been bothered by sparks on 16S lifepo4

This. I think maybe I'm overthinking the whole thing. I might start building the circuit and see how much of a spark problem I'm actually gonna have.

 

[Deafcat]

contactor is the way to go. simple, cheap, reliable.

 

[fechter]

There are some automatic precharge circuits out there that may be applicable:
https://endless-sphere.com/forums/viewtopic.php?f=3&t=40142

On page 5, there is one that is designed to operate a relay or contactor. It precharges the caps and when they are charged, it activates the relay coil.
I saw a commercially available version of this somewhere, but can't remember where. It was made for an electric car.

With no precharge, even a large contactor could possibly get welded contacts and the sudden inrush of current could damage the main capacitors.

 

[TheBeastie]

Was just looking around on the subject, thought this guys post is pretty cool, shows you the worst thing that can happen with no inrush protection, fire .
https://endless-sphere.com/forums/viewtopic.php?f=2&t=18572&start=25#p381474