Inrush from 48V battery into 9mF of power supply output capacitance - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: May 03, 2020, 03:03:37 pm
content: HiWe are developing a 12kW , 48Vout Power supply with 9mF of output capacitance.It is for a customer load which also has a 72Ah lithium battery connected in parallel with the load. (there is no inrush circuit on the load).So we designed an inrush limiter to staunch inrush from the customer’s battery into the power supply’s output caps upon connection…however, the customer has requested we remove this, and that we simply make the connection, and tolerate the contact sparking as their battery inrush’s into the power supply output capacitors.How long do you think the connector will last even if its highly rated up for such treatment?

author: coppercone2
author_rank: Super Contributor
timestamp: May 03, 2020, 03:10:09 pm
content: did you measure the current with a rogawski coil?It sounds like you would need something that looks like swtichgear to handle thatBut, it sounds like its in the ball park of a big welder. Maybe look at a big mig welder, like the ones they have on trucks that go out to weld big stuff.But, with a welder, you don't have 9mF of capacitance. So you might want to look at a giant shop spot welder. Maybe find a cheap place that you know does not maintain their equipment right and measure the contact resistance on their spot welder (then you will actually know the state of 90% of your equipment once its in the field). I am sure the company that does not want inrush protection on 9mF is going to have agreatmaintenance track record.So you will have something like a 500A lenco there?https://www.arc-zone.com/cable-connector-lc-40hd-blueI find those connectors kind of suspect btw, since they accept wire with a screw. You probobly want to mod that connector for reliability, at least I want to, to braze electrical copper to it with a connection for a screw lug. If you do use that, I would recommend getting a 4/0 ferrule tool, and not doing the 'shove a little bit of copper shim in there so it works right'.Keep in mind LENCO is for normal welders. If you have a capacitor on the input I doubt it will hold up great for some reason. IIRC the spot welders I have seen use crimped lugs. And I only saw one ancient one taller then you with capacitors the size of coke cans.Will it be as bad as a normal spot welder? no. Will it be as good as a normal welder? no. Do I know how to measure connector integrity and make a MTBF for a unknown application? No.Arc start circuits on welders typically use something like a injection coil, so its not really capacitance either. I don't think anything really uses 9mF, that is alot. It makes me think of pulsed supplies for lasers and physics experiments more then a normally working power supply of this power.Usually inrush protection seems like something you would want to market for your own sake, because it protects customers equipment. It might like barely work, then you switch to a lower ESR capacitor and you might have a problem in the future with complaints, contracts, etc. And people getting scared plugging them in and getting injured because you know it will happen during a emergency.IMO someone asked you to build a piece of shit. Also keep in mind, if its mobile, the scooter in amsterdam that was jamming street lights and causing electrical problems with its arcing. If something messes up and makes a pulse shaperish thing, you are talking pS times.And it will be way less tolerant of bad installation IMO. Normally it would get hot and maybe get caught during a inspection, show up on Ti, etc... but this one will do something 'bad'.But, it might be very normal for EV cars. I know nothing about those.It kind of sounds like something someone would want to do in a car?

author: bdunham7
author_rank: Super Contributor
timestamp: May 03, 2020, 04:50:04 pm
content: Quote from: treez on May 03, 2020, 03:03:37 pmI'll limit myself to your specific question in order to minimize my frustration at the evolution of your flaming disaster that you aredevelopingcobbling together.First, your question would be most appropriately directed at the manufacturer of the connector. Second, if you are asking us, it would be helpful if you actually specified the exact connector you have in mind and how often you plan on unplugging it. However, if want a blind stab by someone with a bit of experience, here it is:1) Unlike a contactor (heavy duty relay), the action of manual connection depends on who is doing the plugging in and how fast they do it. A prompt snapping in by a strong person vs a weak wiggling in by a weak one just might have different results.2) By your specs your system has a steady state maximum output of 250ADC. Your output capacitance is 9000uF if I understand correctly. That is less than the capacitors in my audio amplifier in my living room and at a lower voltage to boot. I'm not going to do any math since I don't have any specifics anyway, but it just isn't that much energy.3) With 48VDC, there just isn't that much arcing on connection (but see #1), but there will be if you disconnect with high current. Since your system doesn't appear to have any current flowing when it is disconnected (please, please tell me that at least that is true) I suspect that the damage to any connector that can safely handle the specified load will be very minimal. Probably the least of your worries.

author: coppercone2
author_rank: Super Contributor
timestamp: May 03, 2020, 05:20:36 pm
content: but who the fuck plugs in something that hot without a load switch in series? just why? 12kw wtf its not like a 9V battery

author: bdunham7
author_rank: Super Contributor
timestamp: May 03, 2020, 05:36:50 pm
content: Quote from: coppercone2 on May 03, 2020, 05:20:36 pmIf I understand correctly it isn't 'hot'. The power supplies are turned off as the connection is made. I still think you have a point, although IMO the disconnect switch should be on the battery pack. Suppose there was a malfunction and the power supply was shorted? I hope they at least have high energy fuses inline inside the battery pack.

author: coppercone2
author_rank: Super Contributor
timestamp: May 03, 2020, 06:25:27 pm
content: you are right about the capacitance though, 60V @ 10mF does spark but its not that bad.but still 250A at 48VDC is.. a hard core welder. And the voltage is higher then a welder.. but it is good for doing 1/4 inch steel in a single pass I think. (rule is 1 amp per 0.001 inch). Copper has a much higher thermal conductivity so it will be vastly different.What the capacitor might do is let a stronger arc start faster if its jiggled during mating but I am not sure if its significant. (more energy will be released to conductive plasma from a 'brush up').I think you want to minimize the plasma formation before its quenched by voltage drop from being shunted by contact area conduction.