Battery on power supply output to help service heavy load transients? - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: April 22, 2020, 07:55:34 pm
content: HiWe are doing a 12kW output power supply with....Vin = 700VDCCable resistance = 7.08 OhmsVout = 48VDCIts an SMPS made with DCDC modules.The load will have transients which are likely to be from no_load to 130% overload and vice versa.Overload is treated with an output current clamp, but this takes some time to act (approx. 150ms).In order to service these transients, (without the power supply tripping out) we are thinking of having a lithium battery simply connected to the output. (We need one there anyway to act as a power fail power source).Is this concept of using a battery on a power supply output a common way of meeting heavy load transients as described?There won’t be any interfacing circuitry (other than an initial inrush cct so that the battery doesn’t inrush to the power supply output caps upon connection) . The battery will start off being fully charged, so will only take a “trickle” charge from the power supply.We are thinking of picking a ~47V battery so that it settles to 48V and just gets trickle charged as the power supply supplies the load.

author: Gyro
author_rank: Super Contributor
timestamp: April 22, 2020, 08:19:32 pm
content: Lithium batteriesHATEbeing trickle or float charged. You could use Lead Acid though, although you would see a voltage drop between float charge and discharge voltages.P.S. To float charge Lead Acid at 48V you would need a specific number of cells, you couldn't simply put four 12V batteries in series (which would require about 55V). If you had 21 cells, they would float charge at 48V but only yield 42V (depending no state of charge and load) on discharge.P.P.S. I don't think there's any battery chemistry which would provide such a small difference (your 47/48V) between charge and discharge. BIG Supercaps?It would be much cheaper to rate your PSU for peak load though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2020, 06:32:34 am
content: Duhh, why is the power supply not rated for its load? That's literally the first thing you design when designing a power supply...Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 23, 2020, 12:30:40 pm
content: What you suggest can work if the power supply is current limited. Otherwise if the output impedance of the power supply is too low, it will overload holding up the output voltage preventing the battery from supplying any current.The power supply should be current limited anyway to avoid damaging the battery, and itself, during charging.

author: duak
author_rank: Super Contributor
timestamp: April 24, 2020, 03:58:40 am
content: Treez,Since you have a battery in your system and you're working with 800 VDC potentials, you may want to consider interlocks, disconnects and fuses:https://www.gigavac.com/catalog/power-products/minitactorhttps://www.gigavac.com/catalog/power-products/manual-disconnect-switcheshttps://www.gigavac.com/catalog/power-products/fusesHere's some ap notes:https://www.gigavac.com/application-notes/index-for-power-productsIf you don't know already, switching medium voltage DC is a whole 'nother thing from AC. Contacts weld when closing and burn when trying to interrupt a current.Cheers,

author: ocset
author_rank: Super Contributor
timestamp: April 24, 2020, 05:12:39 am
content: QuoteThanks, the power supply is rated to supply its nominal maximum load in steady state, which is 12kW.Its these overload transients where it goes from no_load to 130% overload for a very short time that we want the power supply to ride through. (not trip out)

author: Nusa
author_rank: Super Contributor
timestamp: April 24, 2020, 09:35:42 am
content: If it's a quality power supply, it might already have the surge capacity you require. That should be part of the specs if you're specifying it. If it's off the shelf, then you should have specs that tell you what it has.The easiest way to cover your bases is probably to put a 15kW unit in the box you spec to provide 12kW service.

author: BravoV
author_rank: Super Contributor
timestamp: April 24, 2020, 12:10:13 pm
content: I guess this is one of the reasons as OP always whines about, that all UK's power supplies designer job is gone to Far East.

author: bdunham7
author_rank: Super Contributor
timestamp: April 28, 2020, 03:26:35 am
content: I see nobody has asked this directly--what voltage tolerance do you have during this 'overload' and what type of load is creating this? A motor? Something else?

author: richard.cs
author_rank: Super Contributor
timestamp: April 28, 2020, 09:33:52 am
content: Quote from: Gyro on April 22, 2020, 08:19:32 pmIt is a little more complex than this. Lithium cells have almost zero leakage current, so if you float them you get to a point where no current flows at all. At that point the cell can't "know" if it's being float charged or not, the voltage is constant and the current is zero (consider also paralleling cells, each cell sees a voltage source on its terminals). Still, we have the practical observation that they don't seem to last long when we do it. I have a suspicion it's more related to what happens when you then thermally cycle the cell whilst float charging it, under those conditions the cell potential (open circuit voltage effectively) wibbles gently up and down past the float voltage and then currentwillflow. I don't have any evidence for this, but a similar mechanism is recognised when paralleling primary cells.In any case it seems that in practice youcanget away with float charging them provided that you keep the float voltage modest, 100-200 mV below the normal fully charged voltage. I have encountered designs that do this, and done it once or twice myself without problems.

author: bdunham7
author_rank: Super Contributor
timestamp: April 28, 2020, 08:55:36 pm
content: Quote from: treez on April 22, 2020, 07:55:34 pmTo answer your question directly, it is how automotive electrical systems are managed. However, the the alternator (generator) of an automobile is current limited and more or less functions as a voltage-regulated current source, but with a time-constant of several hundred milliseconds. Thus when the load suddenly increases or decreases, which it does in normal use, the lead-acid battery supplies or absorbs what can be very large transients--a hundred amperes or more in some cases.If you want to research other solutions, look at electric cars which still have 12 volt electrical systems and lead-acid batteries. In my electric car there is a more or less typical 12 volt battery and a 1.2kW voltage converter that steps down the ~350VDC to a temperature-dependent 13.8-14.4 volts. I don't know the topology of the converter or how the current limiting is implemented, but I don't think any typical SMPS that I'm familiar with would be a good substitute.In your case, I suppose you could theoretically use Li-Ion battery packs configured so that the cell voltage was as close to 3.92 volts as possible, but you would need an aggressive BMS/cell balancing system to keep them all even, as well as significant safety systems--and you'd still have to limit the SMPS current output. With that much energy in a field system I think it would be quite dangerous without extensive control and safety systems. I don't know of any systems that work this way at anything more than laptop-sized systems. I've never seen the original design of the Boeing 787 system, though. I hope it wasn't that recklessly crude.

author: cpuerror
author_rank: Regular Contributor
timestamp: April 30, 2020, 02:08:06 pm
content: I think you would be much better off using a motor to spin a large flywheel. When a transient occurs, the motor will act as a generator and return energy back into the system. A 100kg flywheel spinning at 4000rpm will be sufficient for this amount of load.

author: ocset
author_rank: Super Contributor
timestamp: May 03, 2020, 02:31:02 pm
content: Thanks,So, to summarise, we are doing a ~12kW power supply that comprises 42 DCM3623T50M53C2T00 modules in parallel…each one outputting 50V and 6.7A at maximum load.The load is motors (BLDC + Inverter).Battery in parallel with loadThe load also has a 72Ah Lithium Battery in parallel with it. There is no Battery Management System between Battery and Load or between Battery and Power supply connection. Usually, the load runs directly off the battery with no power supply…But we are adding a power supply so that the load can run for longer…The problem we face is that we are not allowed to add a Battery Management System.Situation of Battery overcurrent chargeingAs such, there is a chance that the power supply could start putting more than 90A into the Battery ,which is not permitted. (could overheat it).The reason that this could happen is that the DCM3623T50M53C2T00 DCDC modules decrease their Vout with Iout. As such, a lengthy period on say, half load, following a previous lengthy period on full load , would make the power supply vout rise and potentially put more than 90A into the Battery.Solution: Hack DCM3623T50M53C2T00 to make it constant Vout…We can solve this by buying DCM3623T50M53C2T70 DCDC modules. These have a constant vout over the load current range. However, we have already bought DCM3623T50M53C2T00 DCDC modules, and cannot afford to now buy the ’70 version.Therefore, our contractor has elected to “hack” the DCM3623T50M53C2T00 DCDC modules so as to make them behave like the DCM3623T50M53C2T70 modules. (ie, have constant vout vs iout) .This will be done as in the attached. As you can see, a microcontroller constantly reads the load current, and outputs a voltage to the DCM3623T50M53C2T00’s TRIM pin so as to keep the vout at 50V, no matter what is the load current.Feedback loops fighting each other…The problem with this is that we also have a current clamp connected to the TRIM pin. (This is to stop any module from overly hogging current in the parallel array). The challenge is to set the feedback loop bandwidth of the microcontroller system to a bandwidth such that it doesn’t fight with the feedback loop of the current clamp. There is already an internal feedback loop inside the DCM3623T50M53C2T00 to regulate vout as described. So this is quite a lot of feedback loops we now have, all closed on essentially the same thing….Would you agree that we must set the microcontroller’s feedback loop bandwidth to be 10x slower than the current clamps bandwidth?(The current clamp’s feedback loop bandwidth already had to be reduced in order to stop it fighting with the internal feedback loop of the DCM3623T50M53C2T00)DCM3623T50M53C2T00 datasheet:http://www.vicorpower.com/documents/datasheets/DCM3623x50M53C2yzz_ds.pdfDCM3623T50M53C2T70 datasheet:http://www.vicorpower.com/documents/datasheets/DCM3623x50M53C2y7z_ds.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: May 03, 2020, 05:59:03 pm
content: Thanks for providing the DC module p/n and specs.It appears that the model you actually have is designed with parallel operation in mind and achieves that to some degree simply by the nature of it's inherent output sag. By eliminating the output sag you have defeated the inherent parallel balancing feature. I don't know the answer to your feedback loop bandwidth question, but I did notice that the modules have a specified input of nominal 28VDC, with an acceptable range of 16-50VDC. I'm afraid to ask this--but I must: Is the plan to connect all 42 of the modules inputs together in series and connect them to your 800VDC, all without any method of balancing the input voltages? Has any of this been connected up and tested? Does the module manufacturer approve of this method or did anyone ask them?I'm also curious about the '72 AH Li-ion battery'. Can you tell us how many cells and what chemistry? And by not being permitted to add a BMS, do you mean that the battery pack itself is manufactured without BMS or cell balancing--it is just a bunch of cells that someone connected in series?

author: ocset
author_rank: Super Contributor
timestamp: May 03, 2020, 07:00:24 pm
content: Thanks,We are not putting them all in series across 800V.That much i can say.

author: bdunham7
author_rank: Super Contributor
timestamp: May 03, 2020, 08:04:34 pm
content: If there is any issue of professional responsibility or licensing for you and things are as described, I'm sure you are better off swabbing floors than participating in that shitshow. I'm still curious how you (hopefully:they) would use those modules with 800VDC if not all in series, or perhaps a 2 x 21 array.

author: wizard69
author_rank: Super Contributor
timestamp: May 03, 2020, 08:41:55 pm
content: The first thing that comes to mind here is: Why such a complex power supply for BLDC motors?It is still pretty common in motor control systems to use a suitably sized transformer, a Bridge rectifier and a series of caps. Simple. If the BLDC motor controllers are well designed they should be able to handle a relatively wide DC levels from such a supply. That is voltage dips and and such from load and line transients shouldn't be a problem.That doesn't solve your standby power problem but that is another issue separate from the issue of the regulated supplies.Quote from: treez on May 03, 2020, 02:31:02 pm