Adjustable DC current limiting for low voltage battery charging - Page 1
Forum section: electronics / general

author: john61ct
author_rank: Regular Contributor
timestamp: June 19, 2019, 04:20:18 am
content: Given a battery that can pull a much higher current than the source suppliesvoltage setpoint (controlled elsewhere) between 2V and 5VI'm looking for a standalone current limiting device in between, adjustable between 1A and at least 10A, controlled by a pot/screw or MCU/PWM signal, ideally either?up to 50A would be better, but perhaps that would need to be a separate device / design?I am a noob, so an OTS device I could just buy would be ideal. Next choice a kit to assemble, if necessary I guess I pay someone to design and maybe build the first dozen units?Any and all suggestions, feedback welcome, including links to a more appropriate forum to post this query.Thanks in advance

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2019, 02:48:38 am
content: Do you mean input current limiting so that the battery does not load down the source?Linear Technology makes some simple charge controllers which support this. It can be added to an existing charge controller with a current sensing circuit on the input. Nothing would preclude having a constant output voltage, constant output current, and constant input current regulator.

author: john61ct
author_rank: Regular Contributor
timestamp: June 20, 2019, 03:35:11 am
content: The device must go between the battery and the PSU.But not only to protect the source necessarily.Yes, in the case of a "battery that can pull a much higher current than the source supplies"but not with, for example, 250A current available from the PSU and a small 30Ah battery, there I will want to limit the C-rate to say .2C or 6A,because, unlike say FLA, an LI battery is not self-limiting, will pull charge current at a much higher, more stressful rate than what is healthy for its longevity.While with a 1000Ah bank it is "safe" enough, but our current-limiting device may be needed to protect the power source, if it does not have robust current-limiting itself built into its design.The current source "upstream" of the PSU is not a concern, say the mains power circuit feeding an AC-to-DC rectifierthe total loads pulled from the rectifier/PSU will be kept below that circuit's capacity by our CL device downstream of the PSU.I will check out Linear Technology, thanks

author: john61ct
author_rank: Regular Contributor
timestamp: June 20, 2019, 04:08:01 am
content: OK, so I guess they make component chips only, not finished devices?Skimmed through the data sheet for LTC4000, enough to know I'd need hundreds of hours of close study to even start to understand it.All the application examples referenced LFP chemistry and 3-4S cell count, higher voltages than 1S, especially not sure with regard to LTO's sub-3V (2.6-2.8?) required setpoint.Also pretty low charge rates, but enough for say 20-30Ah cells and general prototyping purposes.Or would there be a way to use this chip to control higher charge currents, say 50A and above?

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2019, 04:50:46 am
content: Quote from: john61ct on June 20, 2019, 04:08:01 amThey make controllers which support this function. They have separate current limits for the input and output. The idea is not to overload the power source.QuoteIt could be done but honestly at higher currents, it is probably just as easy to add this function with an external current limit control loop. This comes down to having 3 control loops instead of 2 control loops.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 21, 2019, 01:08:54 am
content: Quote from: john61ct on June 19, 2019, 04:20:18 amA current limited power supply could do that.

author: john61ct
author_rank: Regular Contributor
timestamp: June 21, 2019, 01:39:09 am
content: Of course.The idea is per battery control, independent of the power source.Possibly dozens of batteries each at different sizes and SoC stages hence independently current-throttled, fed off one common buss, where only the voltage is regulated.

author: ocset
author_rank: Super Contributor
timestamp: July 14, 2019, 12:52:30 pm
content: So you are wanting to charge lithium cells from the mains at 1A to 10A possibly more.How many lithiums in series, I think you said just one?...thats 4v max.You sound like you have lots of cells and some have different charge currents(?)You want to buy off shelf.So to my mind, what you need is an offtheshelf mains power supply giving you a DC output of around 10V or 12V or so (12 v is probably more common).Then you just wire in offtheshelf switchmode current regulators from the 12V bus to the cell(s). Use switchmode as its more efficient than linear mode.If you still need assistance, I will search the meanwell or recom etc websites for you?..(for the above hardwares). The current regulator must have the necessary voltage monitoring so that it doesn’t overvoltage the lithiums.If you are charging multiple lith cells in series, then you will need to watch that individual cells are not going overvoltage…..this may screw up your offtheshelf requirement…as you may need to add in the individual cell voltage monitoring circuitry yourself…….and this isn’t likely to be available off the shelf.If you are just charging single lith cells……..and many of them, then you can do it with multiple current regs (offtheshelf)…but to be honest, I dont know of lith cell chargers off the shelf…….i mean, they have to have all the necessary mothering circuitry to make sure that totally dead cells don’t consume charging current….and that shorted cells don’t consume current for ages…..and what current level to charge at if the lith cell is severely undervoltage….and I even think that if a lithium cell goes undervoltage too far, then you have to discard it….and so your monitoring circuitry has to flag this up for you………….if charging at >0.2C, then its adviseable to have temperature monitoring circuitry, which either stops charging till temp goes down below threshold, or pegs back charge rate till the temp gets in range.So really…battery charging, with all its requirement for mothering circuitry…often needs a custom design……….but at least you can get your 12v supply offthe shelf….after that, I feel you may need to have some custom electronics….but by all means look up offtheshelf lithium chargers to see whats out there.

author: john61ct
author_rank: Regular Contributor
timestamp: July 14, 2019, 01:58:07 pm
content: Thanks for responding!Quote from: treez on July 14, 2019, 12:52:30 pmTake AC out of the picture, rectifier (AC to DC charger, PSU) if needed is upstream, then DC-DC conversion handling the voltage regulation.Also, the cells will be regularly isolated for testing, and a BMS active while assembled in packs for normal use, so take those bits out too.The only topic here is the limiting of current amps, independent of those upstream devices, and yes on a 1S level only, say 2-5V max. No serial connections, individual cell voltage monitoring nor balancing required.That 1-10A range is just to start, keeping things "realistic" but still useful to test using real life cell sizes, 5-100Ah, and yes .2C rate is OK as a ceiling, depending on chemistry, agree temp monitoring is a good idea.I realize to get up to the next level of the design once it's been proven - say 8-60A range? that will require different components.> You want to buy off shelf.LOL sure would be nice if possible, but that seems a challenge, at this point looks like I need to learn to solder and pay for design work.

author: ocset
author_rank: Super Contributor
timestamp: July 14, 2019, 02:41:22 pm
content: OK, so it sounds like you have the AC to DC bit sorted…..but are you sure you have power factor correction here?…since above 75W it will need to be power factor corrected, though I think if you’re a company you can have poor power factor and just pay more for your electricity.If you don’t want that you can just buy a power factor corrected offline isolated PSU….or several if you want more power. (given that you don’t connect them in parallel unless the relevant mothering circuitry is in there)From what you say, and the current levels, it sounds to me that what you need is a synchronous buck converter for each cell…obviously with a current regulated output. You can make the current level variable by having the error amplifier external and just change the reference voltage into it in order to vary your current level as required. Also, there would be another error amplifier which kicks in as the cell voltage gets to the end voltage…….and then the buck converter just gets regulated to maintain that voltage.I am sure there are more exotic charge regimes for lihium, and others may chime in with them....ie cute ways of pressing the absolute maximum charge into the cell.Realistically, it would be best to have a microcontroller overseeing the process of charging…because say for example, you have a cell at 2v or so (just say) , that never increases voltage even when current is poured into it for several hours…then you want to stop charging that cell, as its obviously got some problem……also the micro can do stuff like read battery temperature monitors and peg back charge rate of the bucks to a suitable level.The sync buck is very easy to do, and linear.com have plenty of chips…but if you want a semi-offtheshelf solution, then vicor and synquor do some good modules which just need a bit of extra circuitry around them.Due to the fact that battery chargers usually need some "mothering".....this is often best done in software...i must admit i find the PIC micro's in C are easiest to program....there's a steep learning curve but short basic C code is very simple once you get into it. If you cant write long interwound programs, then you can get round it to an extent by using multiple different small microcontrollers each doing there own small simple task.

author: john61ct
author_rank: Regular Contributor
timestamp: July 14, 2019, 03:32:33 pm
content: Quote from: treez on July 14, 2019, 02:41:22 pmYes the upstream AC rectifiers all seem to have good PFC, but I admit I'm not too concerned about overall efficiency at this point.The rig is mobile, so when AC is involved, fed by arbitrary sources including campsite / marina / household outlets, but also generators.But often alternator output, even solar when needed - that's when overall efficiency becomes a concern - so DC from the start.But I don't want these "upstream issues" to divert the central topic of this thread.> From what you say, and the current levels, it sounds to me that what you need is a synchronous buck converter for each cell…obviously with a current regulated output.So I interpret this to meanthe current-limiting functionality **must** be integrated with the voltage regulation.Is that really the case?I interpreted David'sQuote from: David Hess on June 20, 2019, 04:50:46 amto imply not necessarily.____To simplify, let's say I have a TDK Lambda SWS600L-3 capable of feeding a "charge buss" at 3.48V up to 120A.The Rn devices are my black box, current limiting regulators under discussion here, aka "CL node controllers"The cells all require the same voltage input from the charge buss, but may be different Ah sizes or otherwise require different current rates.And BTW, in another charge session, I may tweak the voltage up to 3.62V.I am happy to incorporate a microcontroller into the design of these controllers **if needed**, but would prefer greater reliability, robust simplicity if possible, so let's assume temperature protection is taken care of elsewhere if that helps.These controllers can also act as stop-charge contactors, even if their Amps setpoint isn't itself controlled by mcu.> linear.com have plenty of chips…but if you want a semi-offtheshelf solution, then vicor and synquor do some good modules which just need a bit of extra circuitry around themThanks, I don't know what an error amplifier is yet either, will investigate.

author: ocset
author_rank: Super Contributor
timestamp: July 14, 2019, 07:49:38 pm
content: An Error amplifier is something which helps get a control system control properly.A battery charger is a control system. In other words…..you tell the charger what charge current you want (demand), and then it trys to control the actual current to that charge current. Ie the actual value is made equal to the demand value……..if the actual charge current does not equal to the demanded charge current, then there is an error….therefore you need an error amplifier…which amplifies the error, so that the modulator is more capable of getting rid of it…and allowing your actual current to be equal to your demanded current.Attached is the schematic and LTspice simulation of a synchronous buck battery charger. It charges a battery with a 4 amp current from a 12v input voltage. But if the battery voltage goes above 3v, then the voltage error amplifier kicks in and makes it supply whatever current keeps it at 3v….so the voltage regulation takes precedence over the current regulation whenever the voltage goes over the threshold.All these levels of current and voltage can simply be changed by adjusting the reference voltage into the relevant error amplifier.I used a dummy resistor for the load instead of a battery….but the same can be done with a battery.LTspice is a free simulator from linear.com and you can download it and run the simulation if you want.This kind of thing can be very quickly put together on a PCB (or PWB whatever you may call it)

author: ocset
author_rank: Super Contributor
timestamp: July 14, 2019, 08:07:36 pm
content: The synchronous buck is a good choice because its very efficient, and avoids losses associated with the Buck diode…..this is especially useful for low voltage outputs..ie a single lithium cell.Forgive me, but You seem to be making your bus voltage almost equal to the actual intended battery voltage. There is no need for this……in fact, since the bus is your distribution system, you can reduce i^2.r losses in it by making the bus voltage as high as possible…and then the current in it gets lower…..so you can use thinner conductor wire which is cheaper….and results in less i^2r loss for you. For your intended output voltage, you could use up to say 24V as your bus voltage….just that you have to watch for low duty cycles and low on times of the buck. However, you can actually control it and reduce the frequency of the buck for lower voltages so that you can still work from higher input voltages….there a lot of ways to do thisHowever, 12v is an ok input voltage for your app.If you really wanted, and you have long cable runs…you could distribute at 100V or more , and then have an imtermediate step down converter to step down to a more suitable input voltage for the buck near to the load.I understand you will be getting this done yourself...but give me a shout if youy want to hire myself out for this...otherwise, good luck and ill keep sending my ideas here if you have further questions...im sure others here will be helping you out too.

author: john61ct
author_rank: Regular Contributor
timestamp: July 14, 2019, 09:28:28 pm
content: So, again, **must** the current-limiting functionality be integrated with the voltage regulation functionality?What I was hoping for was **pure pass-through of the voltage level**, as accurately as possible - or at least as predictably as possible,for the device in question to only be "throttling" the **current** flow demanded by the cells..Quote from: john61ct on July 14, 2019, 03:32:33 pmBut if integration with the buck conversion really is the **only** way to go, so be it.I think a 48 nominal bus makes most sense on the DC side, but of course, as you say stepping down to 12V (13.is required anyway, so either could be the input available.I'll start over with searching for COTS units, but at 2-5V adjustable output, the pricing-per-Wattage output for those also featuring adjustable current, have so far been **very** high!

author: ocset
author_rank: Super Contributor
timestamp: July 15, 2019, 05:15:03 am
content: QuoteYes

author: john61ct
author_rank: Regular Contributor
timestamp: July 15, 2019, 07:23:14 am
content: OK thanks for clarifying that.So, so say the "top-level" DC buss voltage where the kW live is at 48V.And the bottom level output of the Rn black box devices aka "CL node controllers" are now each synchronous buck converters / 1S battery chargers, with adjustable voltage setpoints in the range 2.35 and 4.39V, and adjustable current-limiting from 0.1V up to (6-10A, whatever is reasonable for prototyping, to scale up past 100A later).Precision (and accuracy) in voltage regulation is critical, ballpark is enough for the current capping, failure mode for both should ideally be going lower rather than higher.So next question.I assume stepping down from 48V directly with these (large n-count) node chargers will be more difficult and expensive, given that "intermediate" robust enterprise-quality DC-DC buck converters are not expensive.Given the bottom level, lower-amp output target voltage is in the 2-4V range, would a "closer" 5V at the intermediate bus be "better" than a higher 12V?Better meaning cheaper, cooler, more energy efficient, higher power possible and/or less stressful on our components?If not in a big way, then nominal 12V would certainly make for greater inputs flexibility.The idea of mcu control is now more compelling, with feedback loops on cells, both voltage and current acceptance rates, opening up the option of varying both "live" during the charge cycle to test out the impact of different approaches balancing max longevity vs short charge times, throttling C-rate based on rising temperatures etc.Another question - why is "output vs input isolation" so common only for voltage source converters, very rare to get that feature and adjustable current limiting in the same unit? Is it inherently very difficult, or just adds costs?

author: ocset
author_rank: Super Contributor
timestamp: July 15, 2019, 07:20:35 pm
content: A sync Buck could be done to get you directly from 48V to 2.3-4.4v.The cell, if healthy, wont spend long at 2.3v when charging anyway…..and will quickly get up to around 3.5v or so I would have thought.But lets take VBatt = 2.3VThen Vin/Vout = 2.3V/48V = 0.048 (= D, the duty cycle if the buck is in continuous mode)So that gives the Buck an On-time of 0.048 * T.(where T is the Buck’s switching period)Lets take a switching frequency of 100khz…then T = 10us…..So your Buck’s on-time is D.T. = 479ns…….this is above most sync buck chips minimum on time, so it would be easily do-able.However, its fashionable these days to operate low voltage Bucks at >200khz……which is good because you can then (to an extent) use smaller inductors…..at these higher switching frequencies, the minimum available Duty cycle increases ( due to the fact that “minimum on time” is generally constant) which means its less possible to use a higher bus like 48V….but at 100khz switching frequency, you could just Buck down from 48V to 2.3-4.4v and be done with it.Some would argue that a 48V input buck would need to use higher voltage fets, which have a higher rds(on)…..so would condemn the extra inefficiency compared with Bucking down from say 12v to 2.3-4.4v………..however, I would say that your distribution bus would be more efficient at 48V, due to lower i^2.R losses….so I would go for the 48V to 2.3-4.4v solution. I just don’t think it would be worth interposing another converter to drop from 48v to 12v…and then bucking down from 12v to 2.3-4.4v with another converter.As an adjunct, I would say that there are certain types of sync Buck’s which can even be operated at very low duty cycle at high frequency…..(so you could do 48v to 2.3v at f(sw)>300khz say)…..these are controlled by an algorithm called “emulated current mode control”….if you google that you will probably find some sync buck chips which implement that for you….i must admit I would say it was easy but personally I have never had to bother with it.Another option to get from 48V to 2.2-4.4v would be to use a PRM module/VTM module combination from vicor.com. These are high quality resonant mode modules which are very efficient…a little more efficient than a sync buck solution……..i don’t know how much the vicor parts cost……I once used them for a company whose product was so expensive that the cost of the vicor modules was irrelevant ( I was bucking down from 48v to 1.5v at 80 Amps)……….but just to mention, whilst looking through the vicor website, I did notice something else that’s’s relevant here……..some Quasi-resonant mode buck modules ….as I remember it, you just had to add the inductor and a few other bits. So that’s something else.But anyway, the general solution for your application would be a sync Buck to get from 48V to 2.3-4.4v….of the general form that I showed in the post above (#11).I am sure you are aware that 48V output offline power supplies are very common offtheshelf parts even at high power levels……I take it that this is why you seem to have chosen 48v as your distribution bus.Regarding bucking from 5v to 2.3-4.4v….or from 12v to 2.3-4.4v……….i don’t think the difference would be significant…….so I would just pick 12v input as it would mean less input current.There is a general rule (but often broken) in electrical systems that you should keep the voltage as high as possible for as far as possible…to reduce i^2.R losses. Having said that…cars and lorries use 12v or 24v, partly because its less wear on relays at the lower voltages.One advantage of current regulated sync bucks is that the outputs of many of them can be paralleled in order to provide an overall very high current supply….as you know, paralleling voltage output regulated converters is more involved, and needs more “mothering” circuitry to manage it.Its quite common to use multiple small converters rather than one huge converter to do a job…..this is because the small ones tend to be able to use more “abundantly available” parts. Also, if you want an all surface mount solution, then you would need to use small-ish parts…..since Big chunky SMD parts tend to rip off the PCB in transit……..even thru-hole inductors need PCB eyelets when they get big.QuoteThere’s no reason why you cant also have isolation and a current regulated output (cf voltage regulated output)…or isolation and a combination current/voltage regulated output....and its not difficult.**************************************************************Anyway, you imply above that it would take hundreds of hours to get up to speed with Buck converters etc.Some years ago, I taught the 17 year old son of the MD of the company “eta plus” a few hours to design Buck converters. Then he was off on his own designing them…..i could do the same for yourself….and we could build this huge charging system between us. It sounds like it has tech/environmental type “browny points” so you could get the US State to sponsor it.The general design of a sync Buck power stage ……inductor choice, peak/pedestal current , choice of F(sw) etc takes a few minutes………then it takes a little longer to thumb through the catalogs etc and find the cheapest componets etc…that might feed back into some re- tweaking of the previous stage etc etc……selecting the PWM controller can take a while if you’re fussy for the most optimal one as all the datasheets need reviewing……..The PCB layout probably takes the longest…including putting components into the PCB library etc.If you wish Here is my SMPS course.https://drive.google.com/open?id=1cmk81KxXA7-EliHHWr8ZimpEp7HCMIcYif you wish Here is the general maths needed to design stuff like sync bucks…https://massey276.wixsite.com/maths

author: john61ct
author_rank: Regular Contributor
timestamp: July 21, 2019, 06:45:42 pm
content: Vicor AppNoteConstant Current Control for DC-DC Convertershttp://www.vicorpower.com/documents/application_notes/an_ConstantCurrent.pdf