Help with charge controller selection - Page 1
Forum section: electronics / projects

author: techy101
author_rank: Unknown
timestamp: March 22, 2016, 06:21:24 pm
content: Long time lurker/video watcher, third time poster...I'm working on a senior design project for school and have run into a bit of a stumbling block. The project is an open source, low cost myoelectric prosthetic hand collaboration between our CE group and an ME group. We are going quite a different route from many of the existing open source designs and not using an Arduino/ATmega or RC servos, but instead are working with an STM32L4 and finger mechanisms controlled by self-locking screw drive and geared DC motors.Anyhow, I suppose the background doesn't really pertain to the actual question at hand.We have decided on going with 2S2P LiPo batteries to get the nominal 7.4V supply as a single cell at 3.7V would require too much current draw for all of the motors. The intent for the design is to recharge the hand without removing the battery itself.The trouble I have been having is with selection of a battery charge controller. In our design the charge controller would need to be able to supply 4A at 6.8V, which is a worst case scenario. Previously in a personal project I have used the TI BQ24073 to handle a single LiPo cell that could be recharged via USB. The design worked well and I’ve been a bit caught off guard at what feels like the difficulty moving over to a similar chip that would support a 2S cell configuration. I found quite a few dedicated charging IC’s that would work, but very few that will both manage the battery and supply power to the rest of the circuit from either the AC adapter input or the battery as needed. Due to safety concerns with the use of Lithium batteries we want to use an off the shelf charging IC. We have already sourced a battery pack that can meet our voltage, capacity, and discharge rate needs.The IC that really caught my eye was the Maxim MAX1873, but looking through the datasheet left one point of concern on page 2:Parameter: VL Output VoltageConditions: 6.0V < Vdcin < 28VMin: 5.15V, Typ: 5.4V, Max: 5.65VMy understanding of this is that even though the AC adapter will be 9V and battery will be ~7.4V the voltage at my load (which I’d intended to have as a pair of buck converters, 6V for the motors and 3.3V for the MCU) will only be ~5.4V instead of the ~7.4 that I had been working with.My specific questions are:1. Am I correct in my understanding of the load output voltage from this chip?2. Would anyone have suggestions for alternate IC’s that might better meet our needs?3. If my approach is flawed, could I get some help in clarification/correction?Thank you,Matthew

author: Buriedcode
author_rank: Super Contributor
timestamp: March 22, 2016, 06:58:09 pm
content: I believe 'VL' in the specs is the output of the IC's internal regulator that powers its logic. It isn't meant for external use so I don't think you need to worry about it.Also, Lithium batteries (often lithium polymer) have an 'average' cell voltage of 3.7V, but are charged up to 4.2V, and generally ~3.1V considered depleted. The cells should have internal protection circuitry that'll disconnect them once they are below about 3.0V. So the peak voltage from your batteries is 8.4V, minimum 6v. You will need a minimum of 9VDCfor the charger to charge two lipo cells. (2X 4.2V + overhead of current sensing and drop across MOSFET). As you are powering your device from the charger input whilst it is charging, your load will have to cope with 9V.1. No. It doesn't appear to regulate the 'load' voltage from the source input. It uses that input to power the system load whilst also charging the battery. It uses a current sense resistor to determine how much power in total (charging the battery, AND supplying power to the load) is drawn from the source input. This is so you can set a limit of how much total current is drawn from the charging source (so full current charge of the battery plus full current from the load, can be quite high).2. I'll have a look, maxim tend to do very nice charge management IC's that both control the charge algorithm, as providing power to the load from the charge input. That device simply switches the load to the input, but there are others that use a second buck converter so you can power the load with a fixed voltage at say 6V whilst using a 9V charger input. It would be better to use a higher voltage for the charger input, and use an IC that has a second buck for the load (so 15V input -> 8.4V for the battery + 6-9V for load). As it is cheaper and easier to find higher voltage adapters, than ones with higher current.3. I don't think your approach is flawed. It makes sense to use a battery charger IC dedicated for this purpose, especially as there are so many to choose from, from the cheap lienar regulators that just 'charge' the battery, all the way to multiple switching regulators that can provide several voltage rails, monitor discharge rate and disconnect battery on failure.However, you must determine certain parameters. Min/Max voltage your 'load' can cope with. The maximum current draw from your load.For worst case 6.8V @ 4A, during charge, your charger input will have to supply a fair bit of current.

author: techy101
author_rank: Unknown
timestamp: March 22, 2016, 08:42:57 pm
content: Buriedcode, thank you for such a detailed response!I had actually rounded the 4A value at 6.8V so I'll need to go back and get some better numbers; as well I had misunderstood the correct minimum battery voltage.I would love to have a fixed voltage output from this chip; that's how the single-cell previous project had functioned. Do you know if there is a proper name for such a device? For now I've been trying to drill down through battery charger's and have seen very little about fixed output voltage since most of them end up being dedicated charging chips.I hadn't thought about the AC adapter current requirements, but it makes complete sense to try and use a higher voltage adapter.Regards,Matthew

author: AndrewDojo
author_rank: Contributor
timestamp: March 22, 2016, 09:07:09 pm
content: Techy101, you might consider an alternative battery chemistry.The issues of thermal runaway in Lipo batteries and their enthusiastic exothermic consequences are well documented. I find the concept of having such a battery strapped or attached to a person disturbing.For this (and other) reasons, I prefer LiFePO4 cells. They do not show thermal runaway.There are design consequences to changing the battery chemistry.* Cell voltage is lower. Nominally 3.3V, max 3.5-3.6V (depends on battery supplier).* Cutoff voltage can be lower, but I would still use 3V as most of the energy has been released.* LiFePO4 cells have a lower energy density, which means that they will be bigger by some amount for the same amount of energy storage.* LiFePO4 cells can have a much higher current handling capacity (both discharge and charge) - again it depends on battery supplier.* LiFePO4 have a much higher recharge cycle. LiPo are generally 300-500 charge cycles. LiFePO4's can exceed 2000 cycles.Off-the-shelf LiPo charging solutions are common. Solutions for LiFePO4's are harder to come by, but do exist. Generally, it is a voltage scaled version of LiPo.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 22, 2016, 09:27:49 pm
content: Quote from: AndrewDojo on March 22, 2016, 09:07:09 pmHorribly dangerous and utter bullshit forum legend! I am debunking this shit all the time, but here we go again.Almost everyone has such a battery strapped or attached to a person almost 24/7 today (look at your phones and mobile devices), yet they show very little problem.The reason? The absolutely huge R&D made by a few gigantic li-ion experts; notably Sony, Panasonic, Samsung and LG. A lot of safety effort has gone in the internal construction of the cell, such as shutdown separators, current interrupt devices, overcurrent and overtemperature protection devices etc, as well as the chemistry itself.LFP chemistry you give as a "safe" alternative has remained niche due to it's horribly low energy density, and no proper safety R&D has occured. Today, LFP cells seem to be produced only by relatively small, mostly Chinese factories with no proper quality control and no proper cell-level safety mechanisms at the first place. The kicker? A much safer cathode chemistry (LFP) is still not safe enough so that these important factors could be safely ignored; but false sense of security happens at the industry level. Academic tests of short-circuiting and severely overcharging shows that commercial off-the-shelf LFP cells do exhibit thermal runaways (or at least catastrophic failures with heavy self-heating and explosion like gas venting) in situations where off-the-shelf LCO cells do not, due to the working protection (on cell manufacturing level) on the latter.Conclusion? If you want proper energy density of li-ion and want to really maximize safety, go for industry-trusted brand li-ion cells from the few top manufacturers - the ones I listed above, plus maybe a few others. Know what you are buying. Don't get anything niche or "special" chemistry, and don't trust any marketing claims. Only go for things thatactuallyare in the pockets of millions of people all the time. Trust the industry experience. Don't use hobby batteries in such a product.Trust me. I know this shit.PS. "Lipo" today means the packaging (form factor; how the internal guts are wound together and sealed), nothing more; nothing to do with chemistry. The original meaning (chemistry-related) didn't make it and was lost over a decade ago. Most LFP cells on the market are also "lipos". So every time someone is "comparing" "LiFePO4" with "lipo", it's an instant and automatic bullshit flag. Industry experts try to avoid the word altogether, because it lacks meaning and is confusing. It's best to think that such a thing as "lipo" doesn't exist at all. The more correct term is "pouch cell", and for the cathode chemistries, LCO and NCA are currently prevailing in high energy density mass products. They can be compared with LFP.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 22, 2016, 11:03:08 pm
content: Whilst this thread is about finding a charger IC, we must remember the guy is in the design/prototype phase.Yes, lipo's can go 'boom' and cause serious injury, but time and again those instances were from very cheap, sealed, cells with few, if any, safety features that are often mandatory. The high current draw (which implies larger capacity, which implies higher charging current) does increase risk, but if the enclosure the cells are in have vents/weak points, their own safety circuit (most do) and well designed charger, I do not see there being a problem with using Lipo's. As the previous poster said, they aren't brand new technology anymore, and completely ubiquitous so we're bound to hear more 'horror stories'.They are also widely available, can provide the currents mentioned, and there's support all over the web. Quadcopters (I refuse to call them drones!) require very heavy current and most use lipo's.If you build the power supply (regulators and charger) or a separate board, then you can redesign that part for a different chemistry later should you have concerns. But for a proof of concept, nothing wrong with it.

author: techy101
author_rank: Unknown
timestamp: March 23, 2016, 01:08:03 am
content: I've been looking at the MAX1873 closer and have another question. I understand how the battery would charge through the PMOS, but I don't understand how the battery itself could power the system load (as named in the typical operating circuit). Have I been mistaken that this chip can both:1. Power the device (system load) from an external input while charging the battery (input current limit notwithstanding)and2. Power the device (system load) from the battery itself when the external power has been removed?Thanks again for all the help!Matthew

author: Buriedcode
author_rank: Super Contributor
timestamp: March 23, 2016, 02:21:36 pm
content: 1. Yes. The PMOS is the input to the buck converter for the battery charging, and will draw current from the charger input, whilst the system load will also draw current from the charger input. CSSP and CSSN are pins for differential measurement of a current sense resistor. This resistor is before both the charging power path, and system load power path - so the chip knows the total current drawn from the charger input. I believe you can set this current limit to match that of your AC-DC power supply you're using for power. That way, if your current limit is say, 4A, and you're charging at 2A, if the system load draws more than 2A, the chip will reduce the charging current for the battery so the total input doesn't exceed your 4A limit.2. When the charging input is removed, the DCIN pin voltage drops to less than the BATT voltage, and the chip shuts down. The PMOS is kept 'on' (no switching) so current flows from the battery, through the inductor, and through the PMOS. The voltage drop across the PMOS is likely to be very small, and as you mentioned you'll be charging between 2-4A, the inductor will have to have a low DCR anyway, so the voltage drop across that will likely be small too.The only issue I can see is that the path from charger input to system, whilst being powered, is not regulated. So the voltage your system load will see is roughly the same as the input voltage. As many systems operate at 5V, the 'system load' would usually be another buck converter, that accepts a wide range of input voltages, say, 7-20V. In that instance, if you powered it with say, 15V, the system load would still regulate it down to 5V for the system, whilst regulating to ~8.4V (two cells) for the battery. When powered from the battery, the system would get ~8.4V and again, regulate this down to 5V.The point of using a higher input voltage is that, as its a switching converter, this reduces the current required. An input voltage that is only slightly higher than the required voltage for the battery means the buck converters step down capability isn't really used, So input current is more-or-less the same as output current - we lose the benefit of being able to use higher voltage for lower current. The downside of a higher input voltage is of course, our system load will be powered by that. So if charging with 12, from battery it'll get ~6.2V to 8.4V, but when charging it'll get 12V (minus the sense resistor drop). This is fine if the system load has a large input voltage range, but a nightmare if you have things like motors powered directly from it.Your choice of 9V is wise as this is quite close to the 8.4V max batt voltage the system will see, and the buck regulator is still needed because a depleted battery can go to 6.2V. Whilst 9 to 6.2V is only a 2.8V drop - at 2-4A charge, thats 5.6-11.2W a 'linear' charger would have to dissipate.So I would say this chip is a good choice, albeit, perhaps the minimum (accurate, efficient charging). One *could* add another buck converter between charge in, and system load, but this would start to mess with how the chip reacts to input current, and would only really allow you to use any voltage between 9 and 28V whilst having system load being 9V during charge, and batt voltage otherwise.

author: techy101
author_rank: Unknown
timestamp: March 23, 2016, 07:42:35 pm
content: Yet again thank youThis has been extremely helpful.Regarding the next stage of this system, it's actually a pair of buck converters. There's a TI TPS62177 for the 3.3V microcontroller rail and a TI TPS54622 for the 6V motor rail.Thinking about it, both of these devices are able to run on 12V as well as the battery voltage, so I was thinking that the system could operate properly by using the 12V adapter instead of a 9V.In the actual use case we're assuming that there wouldn't be a time where a user was both wearing and charging the prosthetic simultaneously, however for development and bench testing we do expect to have it plugged in during operation. Either way it will be nice to not have to swap a battery back and forth between the device and the charger.On a side note, this seems to have somewhat perplexed one of my professors. It's viewed as unnecessary complexity, but we see it as a major design decision for a final (albeit somewhat fake since it's a school senior design project) product. Actually the entire power supply design is kind of antithetical to the school environment. It's frustrating that we are discouraged from anything that can't be bread-boarded, which it seems that many of these newer designs can't as they're not DIP. The breakout boards are handy for some, but these power IC's seem to be extremely fussy (at least according to the implementation portions of the datasheets) about support component placement which wouldn't be possible using a breakout.This power supply is part of a larger prototype board that we're getting made pretty soon. We want to be able to fully test our motor controllers (MAX18470), including current feedback. Our adviser isn't discouraging us from, but isn't really in favor of putting the power supply on the same board, but it will be one of two spins and chances to test the design, so it seems worth it to jump on it early for the prototype opportunity.Anywho, just a bit of a rant about what seems to be a disconnect between the academic environment and, for lack of a better term, the real world. I understand the professors viewpoint, but many of them seem to be completely unaware of these "newer" and more advanced devices. This ends up making us justify each decision to use a smaller SMD component over the accepted through-hole designs that many groups go to.End of rant, but I was a bit curious as to other opinions on the seeming disconnect between the academic and industry approaches...

author: Buriedcode
author_rank: Super Contributor
timestamp: March 23, 2016, 11:40:12 pm
content: Quote from: techy101 on March 23, 2016, 07:42:35 pmThose are very nice converters!Quote from: techy101 on March 23, 2016, 07:42:35 pmNice. 12V is a good standard, and any bench supply will have that as an output, along with current limiting for testing.Yes, a bitch to prototype with, but with almost any SMPS 'bread-boarding' is a no-go. So as you'll have to get a PCB made up anyway, why not go the whole nine and use the chips that are best for the job? I'll agree that for a senior project, one can 'over-engineer' aspects of it, and get bogged down with sub circuits and specifics - which use up valuable time that should be spent developing the main 'proof of concept'. It could be you can't get the power supplies to work as desired, so best to have a back-up plan. Perhaps your prof doesn't want you to design every tiny detail, instead wants the focus on the concept as a whole. But if you're on schedule, I say give it a go if it will make the device more portable and therefore easier to test/demonstrate.I would also agree that putting the power supplies on separate boards is a good idea - keeping it modular. This may make the prototype clunky, but prevents all manner of hiccups... what if you make a mistake in the design for one of the PSU's? whole board has to be redone. With each board as a module you have: the battery charger module, with DC in, power out, and the battery connected. Your two other supplies on separate boards which I assume just have power in, power out, and maybe the odd control/monitor signal. (also, might be worth using the Iout pin of charger to monitor charge current from your system, only uses one ADC pin on a micro). Makes life so much easier to populate, and test, individually, whilst still minimizing the connections between boards. You could even made all three boards the same form factor with the same mounting holes to 'stack' them - but then that might curb heat dissipation.With that said, my final year project (way back in 2005..) I was met with similar resistance (see what I did there?). I had a habit of over-doing things, and I paid for it by spending far too much time on it, but without using SMD devices, I was limited to the sort of tech that was available a decade previously. It's not like you're being marked on 'who can use the coolest, most modern devices', but the convenience of smaller form factor of newer devices *and* the fact you made your own test boards for them shows initiative.On a side note, having 3 separate boards for the power supplies doesn't mean to say you have to order 3 boards. I regularly put several designs on a larger PCB, with lines of holes to separate them to break them off myself. Also, bolting them to a larger backplane gives you the form factor of a single board, with the convenience of removing/unplugging one for repairs/testing. Along with screw terminals for the power connections all makes life easier in the long run.

author: AndrewDojo
author_rank: Contributor
timestamp: March 24, 2016, 12:50:49 am
content: Quote from: Siwastaja on March 22, 2016, 09:27:49 pmThank you for your kind correction.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 24, 2016, 07:37:34 am
content: Quote from: techy101 on March 23, 2016, 07:42:35 pmSorry for off-topic, but...Sounds horrible, yet too familiar. One of the apparent reasons I was fired from a research job (related to battery systems; now I do independent research) at Uni here was some kind of "overqualification" thing. While I was employed to make something special no one else there wasn't able to do to begin with, very soon I found thatIwas constantly criticized becauseotherswere not qualified enough to easily understand my design choices (mostly basic EE stuff, nothing special for most people on this forum, for example!) Instead of trusting my design choices,oremploying other qualified people to get a more level team they wanted, they decided to get rid of the "problem" the easy way. They somehow expected that the results still come to them, but the project has been completely stalled since then. These people go on on masturbating at their powerpoint slides about what "they can" do, but it is not encouraged to studyhowthese things can be done in reality.I'm not surprised to hear about a professor that requires DIP packages and breadboards. I'm surprised Arduino didn't pop up, but yet again, that would require them to acquire "trends" faster.If they understood absolutely anything about their field, or even read eevblog forum once in a while, or ANYTHING like that, things like this wouldn't happen; but it appears that today, a 10-year old electronic hobbyist can be much more qualified and have better scientific skills than professors at Unis.