High Current Jumpers... or... How to Safely Connect Batteries? - Page 1
Forum section: electronics / projects

author: gnuarm
author_rank: Super Contributor
timestamp: March 13, 2021, 11:07:12 pm
content: A device has a SLA battery capable of many amps of current and some 6,000 uF of capacitors. When the battery is connected it is going to create a rather large current surge, no? Any idea if this would be expected to cause trouble? I'm thinking it will result in some sparking at the connection point and not sure what in the electronics. In the path are a couple of FETs that will be immediately powered up in the on state. They are controlled by a power path chip from LT, LTC4416.I was thinking the caps can be charged up initially through a resistor on the board, even if it takes a few seconds. But what to use to bypass the resistor in operation? I have only found a small number of jumpers for 10 amps (normal operation) from Harwin. Ideally this should be over rated somewhat. A 16 amp jumper is several dollars and they don't tell you what to plug it into!I thought about using an installation procedure to manually connect the battery to the board initially through a resistor, but that seems a bit problematic. I also thought about using another connector like the battery connector (so rated for the current) as a jumper. Still, this requires the guy installing the battery to have read the procedures.I'd hate to add a FET just to act as a current limiter. It would be easy to do I think. One capacitor and a couple of resistors. Current limiter is the wrong name. Slow start would be more accurate.Then the last thought I had was to only connect the battery after power has been applied. The caps would then already charged up to nearly the battery voltage. The external 12V power pack probably limits the current anyway mitigating the cap charging problem.

author: bob91343
author_rank: Super Contributor
timestamp: March 13, 2021, 11:15:10 pm
content: It depends on the application. Charging the capacitors gradually before connecting the batteries is likely the safest approach. A giant inductor would be excellent electrically but impractical both mechanically and economically. A relay might help but a constant current drain. The cheapest solution would be a resistor with a shorting bar.

author: coppercone2
author_rank: Super Contributor
timestamp: March 14, 2021, 06:55:37 am
content: you know given how much those inrush limiting things fail I do wonder if a big ass inductor is actually a bad choice in the long run or if its short sighted thinking. That part of a circuit always sucks,your either1) mad cheap (lets get the PTC going)2) rupe goldberg3) get laughed at for not being cheap or using some rupe goldberg gimmic. Inductor is actually a nice solid state choice here. More solid state then any god damn semiconductor or resistor or relay you can put in there.I had a modest project in mind that i made along ago that used the typical short out the resistor thing. I think I am going to do do some research and see what it looks like with a inductor. Sometimes I wonder if the weird form factor and phantom space savings are worth it over just making a standard test equipment box for whatever doodad with a magnetic element.

author: gnuarm
author_rank: Super Contributor
timestamp: March 14, 2021, 10:21:51 am
content: I don't get how the inductor can work. Yeah, it might slow the current ramp, but there is a time variant load that the inductor might interfere with.If I have to go with something automatic, I'm thinking a simple pFET with a voltage divider on the gate with a capacitor to Vbatt on the gate. The voltage divider will pull the gate and capacitor toward a conducting state with a time constant when connected to the battery (although it's tough to get the RC slow enough as it crosses the threshold) and when the battery is removed the cap discharges in a fairly short time. It's just strange that we already have FETs in the path switching between the power pack and the battery on power fail. For some reason that can't or isn't being used for this purpose.

author: nctnico
author_rank: Super Contributor
timestamp: March 14, 2021, 11:36:53 am
content: In these cases you often see a resistor (a beefy one) and a relay which comes in after a delay. The trick is to make sure the relay coming on is always delayed which is where a power supervisor chip with variable delay (extra capacitor) like the LM8365 comes in handy. For 12V you can use relatively cheap automotive relays.

author: Berni
author_rank: Super Contributor
timestamp: March 14, 2021, 11:46:37 am
content: The inductor works because it doesn't allow the current to change instantaneously. So when the connection is made the current is zero and has to ramp up. This gives more time for a better low resistance connection to form before the bulk of the current flows and reduces the overall peak current. However... the thing might ring a lot when this happens since the inductor stored up all the excess energy and will then recirculate it back and forth between itself and the caps a few times before it dies out. Also upon disconnection it makes things worse, since if a lot of current is flowing then the inductor will try to maintain it when disconnected, this causes a DC arc to form, making the air conductive, forcing the current to flow through it, making it hotter. Provided there is enough current this gets the air hot enough to draw out a large fat arc out to a centimeter or so. In the process making a bright flash while it melts and vaporizes the contacts. This is how stick welders work, little/no spark on contact but draws an arc once pulled apart.P channel FET works well for such a thing but has one issue that it does not know when to turn off. When there is no battery you can wait to see a voltage before you turn it on. But once it is on and you remove the battery the FET keeps conducting and providing voltage (I assume you can also charge an SLA battery as it sounds like it serves for backup) so it has no way to know that the battery has disappeared. But since you are only worried about the turn on, it sounds like it would work well. Perhaps the same circuit can even function as a reverse polarity protection in case someone connects the battery backwards(someone does that eventually).Connecting the battery via a resistor before use is actually used a lot in electric vehicles as those use large high voltage batteries that need to be disconnected when shut down for safety reasons. This disconnection is done using a large contactor, but the motor controller has very large capacitors on the input, so the contactor has a risk of welding its contacts from such a large surge. To mitigate this there is commonly a so called "pre charge cirucit" added that first connects the battery to the system via resistors to slowly charge up the capacitors, then switch the main contactor on once the caps are already full.If the sparking you are concerned about is on the battery terminals when connecting the battery then there is likely nothing to worry unless these batteries get swapped out often. Quite a few devices make a small spark when connecting a large battery, i am usually not too concerned about it, i just quickly brush the terminal over the battery a few times at first to make sure the sparking goes away before properly connecting it (In case there is a bad short that would set things on fire if i connected it).

author: Siwastaja
author_rank: Super Contributor
timestamp: March 14, 2021, 12:38:16 pm
content: 6000uF 12V bank is definitely in the territory of requiring a precharge circuit.No, inductor is not usable for inrush current limiting large capacitor banks. I have no idea why anyone would suggest that, it is obviously not used anywhere for this purpose. It would be one enormous inductor, and storing that much energy in the magnetic field then causes some concerns how discharging the inductor is handled in all situations.You can refine the inductor idea though; if you use a smaller inductor, enough to store not all but a tiny part of the precharge energy, then switch this inductor alternatively charging the output from input until certain current is reached, then let the inductor discharge to the output, then charge again and so on, you create a very useful active precharge circuit. This familiar circuit is also known as "buck converter". It needs current feedback and current limit; basically a CC supply which can finally saturate at 100% duty cycle. This circuit has the upside of allowing the precharge to exceed 50% efficiency.For a simpler approach, do what nctnico suggested. A very very very usual precharge circuit consists of a current-limiting resistor and a bypass switch and a bit of control logic. Precharge efficiency is 50%, but the amount of lost energy does not usually matter because you don't turn the thing on and off all the time. I highly recommend two things:1) thermal fuse electrically in series, thermally coupled to the precharge resistor. If the logic fails, large power dissipation finally blows the thermal fuse, preventing fire. Doesn't need to be resettable; if the logic fails, fix the logic. This is just for preventing a device failure escalating into a disaster.2) control circuit based on voltage difference or output voltage, not time alone. Do have a timeout, though. If the output has unexpected load, precharge never finishes properly. If time-based, voltage does not rise enough, causing high inrush anyway when the relay engages. If only voltage-based, the precharge in such case is infinite and the resistor heats up too much. Both voltage and time has to be measured. All of this is just a few lines of code on a microcontroller. I don't know if proper control ICs are available, though.If batteries in different State-Of-the-Charge are involved, such large amounts of energy need to be transferred that the resistor-based solution is typically out. The only remaining option will be the switch-mode CC converter (which supports 100% duty). This is complex and expensive enough so that whenever you can avoid the situation of having to parallel batteries on-the-field, do your best to avoid this. Batteries can be paralleled in controlled environment where you can separately charge/discharge them to the same terminal voltage, measure to double-check, then parallel and make sure the parallel connection does not disconnect by accident on the field.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 14, 2021, 01:27:20 pm
content: There are XT60 connectors that have a precharge contact and resistor built in. Perhaps it would be easiest to just fit one of those into your design?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 14, 2021, 03:00:24 pm
content: Quote from: NiHaoMike on March 14, 2021, 01:27:20 pmThis design shows genius-level simplicity and works well for the cases where precharge is only needed during inserting of connectors, i.e., the capacitors stay charged all the time without mechanical or semiconductor switches.

author: gnuarm
author_rank: Super Contributor
timestamp: March 14, 2021, 08:20:15 pm
content: Quote from: nctnico on March 14, 2021, 11:36:53 amYeah, the relay isn't going to work. It requires far too much current. But the supervisor is a good idea. Or a simple RC. Either one will do. I'm liking the pFET with an RC.

author: nctnico
author_rank: Super Contributor
timestamp: March 14, 2021, 08:45:37 pm
content: Quote from: gnuarm on March 14, 2021, 08:20:15 pmRC is not going to work reliably. You will need some circuitry to discharge the capacitor in case the voltage drops. All this logic is contained withing a power supervisor chip. Power supervisor chips are not trivial to design so the chance you can come up with a good circuit by yourself is not high.

author: gnuarm
author_rank: Super Contributor
timestamp: March 14, 2021, 09:18:34 pm
content: The XT90-S connector is a good idea if I can get people to accept it. It is only in a mid cable format and this cable already has connectors at either end, so a bit clunky, but workable.These things also seem to be a Chinese product that doesn't have a standard exactly. I can't even tell for sure who the original maker is, possibly a company called Amass. Actually, after digging a bit there appears to be a board mounted male connector, but it is horizontal format and I'm pretty sure that won't work with our board. We would need a vertical connector.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 15, 2021, 10:36:41 pm
content: There are exist ICs for this purpose. Google 'inrush current mosfet controller'.For example, LM5069 etc.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 16, 2021, 08:05:22 am
content: I have used XT-90 (and XT-30) connectors soldered to a PCB. Just be aware that the diameter of the "wire side" of the XT-90 pins vary between manufacturers so it may be difficult to do the correct footprint. Obviously, it's not designed to be used like this so you are on your own. I have found no problems, but gluing the plastic down to the PCB would definitely help reduce stress to the solder joints.I like the XT connectors because of their large "current density" and simple and robust design compared to Anderson Powerpoles, for example. I like the fact the contact relies on the spring force of the metal pin itself, and nothing relies on the plastic, so even if the connector heats up and the plastic softens, this does not lead into thermal runaway like with Known Good Western designs.Just be aware the nameplate current value is more like Absolute Maximum Rating. I use XT-90 up to maybe 50A continuous. Oh well, the current density is still great, it's of great value. The only thing I miss is locking mechanism.If you are concerned about reliability and traceability, you should try to communicate with Amass directly to guarantee only getting genuine parts.I have ordered whatever clones I happen to get, they all do work, at least with derated current...

author: gnuarm
author_rank: Super Contributor
timestamp: March 16, 2021, 10:32:01 am
content: Yeah, I do have concerns about using these connectors. As far as I could tell there are either no official distribution channels or Amass sells through Alibaba. I have next to no confidence in Alibaba vendors.I've had a chance to consider the board mount option. Because the horizontal mount which is intended for mounting, so has proper support, is not a good match for our design I expect this will be used in the cable to the battery. It's probably better that way since the board can be a bit tricky to access depending on where the connector ends up on the board.I've had a conversation with the guy who has designed the power board and he is in denial about the issue altogether indicating there is no problem. He claims "parasitics" will prevent the big surge. I'm not sure what parasitics he is talking about. The battery, capacitor and FET resistances are in the design. Nothing is included for the one connection that would already be connected, but that won't be a large factor. Parasitic inductance would be pH I expect but even if it were uH it would not be a factor. The simulation shows the FET being overcurrent for 600 uS.I'm pretty tired of working on this project. Fed up is more like it. It is led by a project manager from Dyson who is very much out of his league because of the highly technical nature of this project. Literally every part of it seems to be done by the seat of the pants often using the phrase, "it seems to work". In engineering that is equivalent to, "I can do that, hold my beer".

author: Siwastaja
author_rank: Super Contributor
timestamp: March 16, 2021, 10:51:56 am
content: Inrush due to large capacitor bank is catashrophic if there is any semiconductor switch. Beware. I had to issue a recall in a project where I had just designed a TI BQ series power path part following the appnote directly without giving it though on my own. And it had only some 3000uF of standard ESR electrolytic capacitance and maybe some 100uF ceramic. The FET was a big one. Blows short circuit when a large off-the-shelf 24V supply (assumably with quite some output capacitance) is connected. Didn't blow with a smaller supply during initial testing.I have seen the same failure mode in a few laptops and/or their docking stations. The P-channel pass MOSFET blows short circuit due to inrush.Parasitic R limits current, but FETs usually don't have that great of SOA.Active control circuitry is usually needed beyond a thousand uF. Analyse the SOA of all components in power path.Blow the whistle whenever an incompetent designer causes danger. You have to do their work first, though, by doing full design calculation and analysis, i.e., what they should have done to begin with, to be able to prove their work is shit.

author: gnuarm
author_rank: Super Contributor
timestamp: March 16, 2021, 12:01:09 pm
content: Quote from: Siwastaja on March 16, 2021, 10:51:56 amThis is a volunteer project for an open source ventilator. People are working in their spare time or like me are retired... mostly. The problem is largely the guy running the show not managing it like a proper project. I had to browbeat him to get a design review on the power board and it was barely done at all with no follow up presentation of fixes for the issues noted at the time. Even more importantly, we have been working for the better part of a year much of it ignoring my pleas for REQUIREMENTS! Yes, we've never done a requirements analysis or delegation to design units. It's all done by the seat of our pants. The power board had chips on it that served no useful purpose other than increasing profits at Linear Technology.In this case the designer is waving his hands and talking about having designed devices with larger caps and bigger surges, etc., etc... rather than simply doing the analysis to show I am wrong.I know that I tend to be a thorn in the sides of other engineers because I spot these sorts of issues and make noise. Once I let such an issue slide when I should have screamed bloody murder. When I approached the lead engineer with the facts of the matter his response was, "It's already been through verification testing." That was when I realized I should work on my own. lol I started with the ventilator thing because a group needed a power fail alarm circuit designed as a separate board. Through many steps that has evolved into a very complex effort involving measuring many parameters in an FPGA. That initial alarm design had a supercapacitor with a current limiter to deal with the surge, but minimize the charge up time. Ironic, no?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 16, 2021, 01:31:33 pm
content: Oh, the analysis is quite simple.Ignore L.Sum up all worst-case component resistances. Power source, wires, PCB tracks, fuses, switches, capacitor ESR. Worst case here means smallest. Note, especially with electrolytic caps, even if they are well known for fairly high ESR, it can be surprisingly low. If unsure about R, do not assume, research.Calculate inrush current to 0V cap bank assuming highest possible input voltage from the lowest-resistance power source possible. You can calculate the RC time constant as well, or you can run the numbers manually for a few different durations, like average currents during first 1µs, 10µs, 100µs, 1ms, 10ms and 100ms. If your math skills relating to exponent function is rusty, run Spice. This way you can relate to the SOA curves will get to next.Then look at the SOA curve (under that name for transistors; similar curves, or at least 2-3 points in textual form, exist for relays, etc.) for each and every component. Such curves / tables / textual representations convey you numbers like: 1µs max. 1000A, 10µs max 500A, 1ms max 100A, and so on. You may need to interpolate/extrapolate. Be careful about the conditions, for example a FET datasheet might give SOA at Tj=25 degC.Now you verify that you have ample margin, like at least 50%. The more extrapolation, assumptions, etc. you have to make in the process, the more you need to leave margin.Typically takes an hour or two from the designer unless the power path component count is very large and tricky.

author: gnuarm
author_rank: Super Contributor
timestamp: March 16, 2021, 05:01:54 pm
content: I already had a spice sim circuit. I just needed to plug in accurate values for the battery ESR (0.019) and cap ESR (0.060 per cap, 6 caps). The FETs already have full data. Figure 36 inches of 16 gauge is 12 mohm, two sets of connectors 10 mohm each. Even with the various other small resistances, the surge current will be far above the max 36 amps of the FETs. Power is 275 W avg over 250 us and 190 W avg over 500 us. There is a chart in the FET data sheet for single pulse rating and even with all the parasitics, this is around the limit. Given the exponential shape of the current and power pulses adding a small amount of resistance spreads the pulse but still puts it right at the limit for one time pulse dissipation. An additional 100 mohm dissipates 1.6 W in normal operation and does a little to reduce the surge power, 100 W avg over 500 us. A 250 mohm resistor gives 58 W avg over 500 us with 4 W dissipation in normal operation. This is pretty close to half the rated power curve of just over 100 W.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 16, 2021, 05:08:51 pm
content: It seems you have a clear case here, you have not only speculated butprovedthe thing will very likely just blow up suddenly. What else can you do? I mean, if someone still wants to make that bomb the only thing I can see doing is to get your name off the project and go on for new challenges.Yeah, precharge circuits are PITA, it feels like some "unnecessary" janitor work and it's unsexy. I understand it, I always hope the circuit works without one, but when you run the numbers and they clearly show you need one, what else can you do.But no matter how fancy high tech you have, if the most basic plumbing blows up, it's all worthless. In cases like this, having a team with more than one competent engineer helps, you can share the tasks and the "co-pilots" cover for you (meaning, find the problems and help fix them, not cover it up).