EEVblog #176 - Lithium Ion/Polymer Battery Charging Tutorial

hi welcome to the EEV blog and electronics engineering video blog of interest to anyone involved in electronics design I'm your host Dave Jones hi it's lithium-ion battery tutorial time why because these lithium-ion battery are cells that you can get these days from hobby suppliers professional suppliers whatever are great for designing into your next product or gadget that you want to build now when you build a gadget you want to building some rechargeable batteries you have a couple of choices one is your traditional nickel metal hydride rechargeable double-a triple-a whatever um batteries right well they're a bit of a pain in the butt they're old hat chemistry there are a low terminal voltage so you got often wire a lot of them in series to get the voltage you want um and they're just in a big painting the but not available in really nice tiny shapes and sizes like these lithium-ion cells he's a 230 million power uh so here's a 50 milliamp hour or so you can get these from companies like our power stream comm and many many other places aren't hobbyist outlets 404 remote control stuff all over the place and as it turns out that sheep really available and really easy to use in terms of our charging circuitry and stuff like that so we'll go into it and you've seen your standard at Nokia 3.6 volt nominal battery or actually this isn't quite a battery pack it's actually a single lithium-ion cell because a battery pack is multiple cells like these in series and you can get those and they charge right exactly the same way so but we're going to stick to the individual lithium ion / lithium polymer there's no real difference between lithium ion and lithium Palmer don't fall for it it's bit of a marketing gimmick okay so we're going to take a look at this how you can charge you how you can build them into your next product let's go and the great thing about these cells as I said is their size and shape take a look at these they're only a couple of millimeters thick and they come in various sizes you can actually get them under a millimeter thick they can you know they they're actually flexible they're absolutely amazing so if you're designing something really weird and unusual say I'm designing my new calculator watch or something like that right I would use one of these because they come in a whole variety hundreds of different shapes and size it is pick one that's optimized for your particular purpose fantastic now let's take a look at the standard characteristic discharge curve of a typical lithium ion / lithium polymer as I said no difference whatsoever don't let anyone fool you otherwise okay lithium ion cells that's just one cell remember not a battery pack a battery pack will have two or more cells in series but we're only going to consider the one cell now you'll notice this curve if you've seen some of my other tutorials on the add on double a nickel metal standard Dunn nickel metal hydride batteries alkaline batteries they all have a very similar characteristic curve like this they all start off at a particular voltage they sometimes they drop a bit quickly and then they have sort of a flat slope II kind of bit and then they drop off fairly abruptly at the end and lithium ion lithium polymer batteries are no different at all now there are actually two different types don't confuse these with lithium ion a lithium polymer because they're the same thing in fact they're the new type of anode material I won't go into the construction of batteries you can look those up yourself but the anode in there can use two different types of materials in lithium ion batteries the first one and the first one they ever used and it's the traditional type older type uses a coat material knock'em not to be confused with the trademark coke or the other type of coke this actually comes from Mark coal that's actually derived from that or the new modern ones in fact the vast majority of lithium I I on batteries you can buy and lithium-polymer batteries will have a graphite anode now the advantage of the graphite anode one as you can see it starts off and maintains a higher voltage for a longer amount of time and this is flatter the curve here is flatter then the coke anode one and it drops off much later in voltage as well so the advantage of that is that you can power your 3.3 volt circuit a modern circuit microcontroller whatever at 3.3 volts directly from the battery using the low dropout voltage regulator because if your circuit is 3.3 volts there it is okay but you might use a linear low dropout voltage regulator might have a dropout voltage is naught point one volts or even naught point two volts even at three point 5 volts dropout voltage you're using most of the capacity of that battery it's fantastic now the difference between a graphite one is typically determined to be dead at around about 3 volt level or something like that and a coke anode the older type mark batteries are typically taken to be dead around you know two and a half volts two point six two point seven something like that and even the even some of the graphite and a ones are determined to be dead at two point seven now these curves will vary by manufacturer they'll vary by battery type slightly different that process variations in the manufacture of the battery all sorts of stuff but these curls are going to be very similar and they're handy because your power your 3.3 volt circuits directly from one of these little lithium-ion cells fantastic just something quick I forgot to add that the x-axis here is actually our time or capacity like from zero to a hundred percent the capacity of your battery or C as it's called or it could be zero to one hour or zero to ten hours in terms of time makes no difference but that is the standard characteristic curve of a battery the voltage of the cell versus time or capacity so how do you charge one of these lithium-ion cells glad you asked it's pretty darn easy regardless of how complicated all this stuff here looks it turns out to be pretty simple so stick with me now the when you charge the traditional nickel metal hydride batteries and stuff like that they're a bit of a pain because um they're an exothermic thing so a lithium ion as well but really are to charge improperly you should sense the temperature of them as well to determine as well as the voltage on them to determine when to stop charging or when they're full now with lithium ions is supposed to do the same thing but these small capacity ones if you charge them at a low enough current you don't have to worry about sensing are the temperature of them to determine when the charge when you're finished charging these things and it's full really a lot of the charger chips on the market which we'll go into they do have built in temperature interfaces for temperature sensors which sends the cell but really that's just not to detect that the cell is finished charging just to really actually protect it from gross overloads and things like that if it shorts out something goes horribly wrong something like that so ah to charge them is really easy any lithium ion or lithium polymer cell they charge exactly the same way as were said there's only two differences one is a well the only difference is the charge voltage which we'll have a look at here for point one volts or four point two that value is very critical it's got to be within like 1% or something like that so that's why you'll find that most it will all lithium-ion charge at cells on the market will be four point one or four point two is the most common by far volts plus - at least 1% some are capable of going down 2.3 or 0.4 percent accuracy and I won't go into it but you can look up the research yourself lithium-ion batteries their shelf life and their number of recharges is pretty much directly all the biggest factor they're directly proportional pretty much to maximum voltage you actually charge them at and the charge rate as well but that voltage critical okay you've got to get it right the chips handle it for you so don't worry about it just giving you some background info now to charge a lithium-ion battery uses what's called a constant current and constant voltage process or CC CV process constant current constant voltage it's a two-step process I've got three steps here but the first step is actually optional so we're only going to deal with step number one and step number two and it's really quite easy and the chip does handle it all for you but I'm just explaining how it works if well you really should know because it's interesting for starters okay now the X the y axis here we've got volts in green okay so the green curve here is the battery voltage the terminal voltage of the battery during the charging process this is time on the x axis and the blue curve here is the battery current or the cell current now are the current this is important if this little thing here is a 50 milli amp hour battery which is what it is okay then that is called 1c or 1 that's the charge rate it's just called C or 1c okay now lithium-ion batteries most make sure you always check the datasheet for your cell but most of them will be charged around typically not 0.5 C so if this is 50 milliamp hours or 50 milli amp hour battery capacity or 1c we'll charge it at half that rate or 25 milliamps so from so this blue curve which is the battery current 100% actually means 25 milliamps or half C some of them can be charged at 1 C if you want to charge them faster some maybe you can even charge them faster than that but we're not going to go into it a typical thing for one of these low capacity lithium-ion cells 1/2 C so 100 means in this case 25 milliamps from 0 to 25 milli amps and voltage with the green curve from 0 in this case 4.2 volts now it starts off at ignore this one chord pre-process okay we'll go into that later but starts off with a constant current process as you can see the charger just starts goes from what zero right it goes from zero to a hundred percent charging capacity or half C 25 milliamps so it sits there for I don't know it might sit there for an hour whatever okay depends on the capacity of your battery and during that time it's pushing a constant current into the battery it's a constant current process and as that happens the cell voltage assuming the cells already dead okay at two point eight volts there let's say the battery's two point eight volts when you start charging it'll slowly rise like that fairly sort of linear and that it starts to taper off like that until it eventually gets to four point two volts which is the upper threshold or the float charge voltage threshold and it goes by many different names in the data sheets and stuff like that but that's the float charge voltage and once it hits that point that very critical voltage point got to be within like 1% or better then it chart it changes modes from constant current charging in to constant voltage charging where all it does is now instead of pushing a constant current into the battery it maintains that it goes constant voltage for point two just like a voltage regulator in fact it works exactly like a linear regulator as we'll check out down here and it keeps it at that four point two volt level but what happens to the current I hear you ask well it actually starts to drop and taper off and it takes quite some time until it gets down to a threshold level down here which is actually set by a percentage of your charge card so if charge cards 100 percent this is what I called hundred percent because this is what they call it and data sheets when you look at it the value that it stops charging at is deemed to be full okay so this value down here your battery is full its fully charged Bob's your uncle okay is typically taken at 10 percent of the full current so if this was twenty five milliamps constant current charging level once that once the current level got down to two point five milliamps for this particular little tiny cell here then bang it stops charging and that's it fully charged battery well piece of cake now that two-step process is what's required to really get full utilize the full capacity and the full life of your battery but some cheap charges and very fast charges quote marks will actually just totally skip this constant voltage our process and just do cost occurring current and then stop when it gets to four point two volts and it's still going to have say eighty or maybe even 90% of the battery's full capacity if you just do this mode so this mode here may take an extra hour or something and you're may only get an extra ten or twenty percent out of it so you've really got a way up you know the pros and cons of actually doing that but all good battery charge at lithium-ion battery charger chips will be a two-step process and they'll only stop when they're finished this constant voltage charge process now what's this first stage here I hear you ask well it's called the pre charge stage and this is used some lithium-ion battery charge ICS have this mode some don't arm but your good ones will this mode will only be used if the battery voltage when you first turn on that chip and it measures the battery voltage if it's less than the pre charge voltage threshold goes under different names depending on the manufacturer and the datasheet but typically around say 2.8 volts that battery is deemed to be really dead fully dead it needs rejuvenate okay so it needs to be up fixed if you get a really completely dead cell that's only got one volt on other half a volts or no volts on it okay you've really left your product on it had no low voltage cut out the cell it's completely killed the cell you can rejuvenate it but you can't just jump straight into a hundred percent current because you'll you further damage the cell you'll ruin it so what they do is they have a pre charge a preach are it only charges that are typically 20% of the full capacity now that value varies as does this are pre as does this F full charge value these can vary some chips even allow you to adjust this and this as well as the charge current and they're really flexible chips but typically if you plug your charge your chip on and it measures that the voltage is less than 2.8 we will only apply 20% of the current until such time as it reaches 2.8 and then it'll go into the next constant current process so what's this circuit down here well this is very simplistically what's inside a lithium-ion charger battery chip they can be incredibly simple so simple that they can only have three terminals on them really if they have a fixed there's an input terminal where you plug your charging voltage in there's an output terminal which goes to your battery and there's a ground if it's got its own building voltage reference and it's a fixed charge current some chips might charge it's a half an amp or 100 milliamps or something like that fixed you can't change it and it all handles it internally a more a slightly more advanced charger chip might have an extra pin which allows you to typically set the charge current with a single resistor because it that will actually be a percentage I will go into it anyway it allows you to set the charge current with that value resistor there's a little formula in there varies by the manufacturer and the type of chip bit allows you to calculate okay I've got my little 50 milliamp hour battery I want to charge that at half C to be on the safe side twenty five milliamps I would plug 25 milliamps in the formula in the datasheet and that would give us a resistor value that allows this chip to charge constant current here of twenty five milliamps and to do that most are chips the fully integrator ones will have a built in current shunt sensor resistor there with a little amplifier with a little differential amplifier there as well and a series pass transistor or a series pass MOSFET in there driven by an op-amp and you've seen these type of circuit configurations before now this pass transistor can depending on there's a lot of control circuitry in here and voltage references and stuff like that that go between the different modes but you don't have to worry about that with when you've got a pass transistor like this you can make it operate in constant current mode like this by determining the voltage drop across that current that shunt sense resistor there you can keep it at a fixed current and then when it switches into another mode it can work as a linear voltage regulator and that's why these are typically lithium ion charger ICS are typically a linear type because they drive the pass transistor with a DC voltage it's a linear thing some will actually drive this with a pulse width modulator okay and there you switch mode types but you can look at the data sheets to see the differences between those but most of the simple ones and and there's nothing wrong with them most of them will be of the linear type the switch mode ones are more advanced if you want to get greater efficiency based on various input voltages and stuff like that anyway these automatically charge the battery using this three or two step charging process instantly determine the current flowing through the cell and they determine the voltage on the cell they've got building voltage references and they do everything for you and you can just hang your circuitry via a low dropout voltage regulator as we mentioned before if your power of three three volt circuit no problems at all hang it straight off but always remember when you're charging that your circuit will also take a certain amount of current as well so you have to take that into account when you calculate this value up here so if our circuit was taking at extra 25 milliamps then our cell at half C 25 we'd need to set this value 250 milliamps to cater for the current down into the cell and also to power the circuit under test and the good thing about most of these lithium-ion charger chips is that you can leave them them permanently connected to the cell like this and when they finish charging they will actually stop they won't draw any current back out of the cell like that and they'll actually have our diodes built across the pass and built into the pass transistor here to actually stop if you're if you physically remove or short out your charger input it won't drain the battery back out of it and you can get specs for the current that leaks back out of the Babri battery it's usually quite small in the order of them you know micro amps or a sub micro amps or something like that so you can really leave these things just permanently hooked on to circuit under test it's fantastic so if you've got one of if you've got a product that say goes into sleep mode all the time it's got no on/off switch it just wakes up then you can just leave all this permanently attached and you've got no power switch whatsoever brilliant okay let's take a quick look at some lithium-ion batteries that you can get on the market I'm using that power stream comm which is a provider of a whole bunch of battery cells some of the largest selection on the market so let's go into batteries and packs down here and check out some of these now there's a decimate primary lithium batteries but look at these babies ultra-thin rechargeable lithium polymer / lithium ion batteries 500 microns point out from 0.5 millimeters to one millimeter thick and you can bend them if you've got a product which needs to be you know flexible like you can't just put a square battery into it like if you've got something that's mounted on your wrist you want to wrap the whole battery around your risk wrist no problems whatsoever awesome but let's go into say the standard lithium polymer cells here and let's take a look at the whole range of them they're all nominally don't worry about the nominal voltage that's just the average voltage they're all actually the four point I believe they're all these standard at 4.2 valve variety but you'd have to read the datasheet for that being get them in capacities as low as 8 to 12 milli amp hours really tiny stuff but let me tell you it is very very difficult actually to find a lithium ion battery charger chip that actually handles our battery capacities that small so just be wary of that it can actually be difficult because most lithium ion battery charger ICS are actually optimized for you know half an amp or an ampere or two amps or something like that and then it's a bit of a trade-off between the circuitry inside is designed for those current and voltage and current accuracy at those sort of currents yet if you try and not charge them at very low currents like if this is a 12 milli amp hour nominal cell you would have charged at a 1/2 C or 6 milliamps then the current accuracy of those battery charger chips is going to be very difficult to get at 6 milliamp hours and I've tried to find some and trust me they can be are quite difficult so just be wary of that if you do go that low if you're designing ultra tiny products but check out the size of these dimensions 3 by 9 millimeters 2 by 4 and you know 18 by 5.2 and there's countless are different sizes and thicknesses and capacities and things like that and here's the datasheet for that particular battery we just chose it was one completely at random and there it is are the charging voltage is 4.2 so it is a graphite type anode plus minus 0.03 volts at that so quite tight indeed that's why you have to have a very accurate lithium dedicated lithium-ion charging IC that has that sort of accuracy and then as you can see it actually recommends a nought point five C constant charge rate for a standard charge if you do want to do a fast charge it can do it at 1 C and then the cutoff you remember that actual percentage value we'll talking about there is no point O one C alright let's do a quick search here using digi-key for a suitable battery charger IC for that example battery we were using before the little 50 milli amp hour capacity battery and I'm going to charge that at naught point 5 C or 25 milliamps so let's type battery charger into digi-key search here and see what we get if we scroll down here we've got battery management ICS 2529 of them and as you can see here's the parametric table there's a different battery chemistry now unfortunately digi-key don't let you um select the charge voltage because it doesn't really know even if you go drill deeper into the specific lithium-ion batteries here which we can actually do but it still doesn't know the difference between those so it won't give you an extra charging voltage it's got supply voltage here but it'd be nice if fun you could actually choose that 4.1 or 4.2 volts but it doesn't do that but most I know most are going to be 4.2 anyway so let's choose a manufacturer which we are like here now it hasn't popped up with strangely it hasn't popped up with microchip microchips actually the one I wanted that's a bit of a fail there maybe there's an extra ah there we go I didn't actually I choose it must be in those categories there because they're multi chemistry devices just start be careful that you can actually miss quite a few manufacturers if you don't select the the correct the actual battery chemistry here but we can just ignore that we can just reset that and say I want microchip parts saito microchip parts are in stock I like them they're cheap they're small they work so I'm going to try those and as you can see most of them are lithium ion based ones but let's go for the in stock parts shall we and let's have a look we've got 80 items well let's just view those I'm happy with that and what's first first cab off the rank here we could actually search by price if we were price sensitive or something like that but the mCP 73 8 1 2 mm CP 73 83 1 you can actually get those for 42 cents each for 3 thousand or 68 cents for one off so they're very cheap they're in a 5 pin Esso SOT 23 package and that's incredibly small simple obviously and there's 21,000 in stock I'm happy with that I'm actually going to check out the mCP 73 8 3 1 let's take a look at the datasheet they call it a miniature single cell fully integrated lithium ion lithium polymer charge management controller fantastic it's a linear one it says it's an integrated it's a linear type art device has got an integrated pass transistor it's got integrated current sets and it's got reverse discharge protection which we also mentioned which is great so when you disconnect the input it doesn't drain your battery on you it's got high accuracy pretty good better than the standard 1% it's got plus minus 0.75 percent there which is really nice I like that you can get it in four different options for different dark chemistry batteries but we want the 4.2 volt device just make sure that you order the right one some of them aren't pin selectable in fact most of them won't be there'll be a fixed voltage so just make sure you do get the 4.2 Volt or whichever voltage for your particular cell which you'll get on the datasheet now programmable current range now here's where I mentioned before not all of them will go down to a low current for very low capacity batteries but this one says it'll handle from 15 milliamps up to 500 milliamps great we need 25 it'll be within the ballpark on the graph as we'll see later fantastic and still maintain its current accuracy down to 15 milliamps it's got selectable preconditioning that pre-charged that that actual rejuvenation charge 10 20 40 or you can actually disable that if you don't want to at all and it's got selectable end of charge control too but because as we'll see down here there are hardly any pins on it at all I think those options will actually be a factory option and not a and not a pin settleable option so just be careful of that larger pin count devices are more flexible they will have these they will often have these settings on a separate pin with a separate program resistor you just choose the right value resistor and you can set your end charge control to anything you like but I don't think this device will have that anyway it's got thermal regulation automatically powers down it's nice it's in a and you can get it in a tiny two millimeter by three millimeter DFM or an easier to use five pin SOT 23 fantastic I like it and this is the typical application this is how simple it is here your voltage input from your charger decoupling cap your output voltage you've got to have a decoupling cap on there otherwise it can oscillate just like any linear or low dropout voltage regulator can same thing here the internal charging circuitry is the same similar circuitry to what's used and it will be an unstable loop unless you add the output the recommended value of output capacitance so just make sure you do that and it's got ground pin and a programing pin which allows you to set the programming current and it's got a stat output which can drive an LED to presumably tell you that it's finished charging and here's the internal circuitry for it it's not much at all but as you can see your input pin here your battery output pin here here's your pass transistor with the internal blocking diode so it stops discharging from the battery there's another smaller pass transistor there reference voltage generate a whole bunch of a bunch of comparators for your different modes your preconditioning moe your termination mode the end of charge and all that sort of stuff and that and there's your upstart output pin that's only available on the seven three eight three one the seven three eight three two presumably doesn't have that pin if you don't want it you can probably save half a cent there or something like that and as you can see there's not really much in them at all voltage a couple of constant current generators and things like that they're pretty simplistic devices because they don't really have to do much at all apart from that transition from a constant current mode into a constant voltage mode and to do that doesn't require much circuitry at all it's supply voltage range from three point seven five to six volts brilliant not a problem let's look through some of the other stats here as you can see the regulated output voltage four point two volts from there's there's the different part numbers that you can buy with the different charging float voltages make sure you get the right one don't want to wait goof that up at all otherwise you'll be in big trouble and you'll damage your cell and there's the current regulation it looks like it's got dot plus minus 10% current regulation there which isn't too bad the precondition current is set to 10% now the program resistor 2k to 10k I don't know what what's going on there the precondition current this seems weird they've got the the same condition over here yet different values I think that's a datasheet mistake anyway I'm not sure what's going on there aha here it is I've scrolled down to the product identification system right at the end of the data sheet and this clears up the confusion we saw before with the with the pre and post current termination ratios that were it said work programmable well they're programmable as factory options so up here you've got the part number you've got to order exactly the right option the options are AC a DAT DC and they give you various options for the pre and post a charge termination and other things so you're going to make sure that you order exactly the right part otherwise you could easily end up with a being actually delivered or ordering the wrong part and that can sleep into your product and you can wonder why your battery a battery charge performance isn't as good as your prototype and your testing showed because you might have the wrong part something to be wary of the precondition voltage on this is quite high it's a sixty six point five percent that's much higher than the twenty percent I said before but many chips use different lots of different value default ratios for that sort of thing now the charge termination ratio by default is five percent and the charge termination once it reaches five percent as we saw on that curve it will actually turn off and you finish charging pass-transistor on resistance there's the battery discharge leakage okay so when it's finished charging it will only take point one five micro amps and under the various conditions so it doesn't take much current at all once the charge is complete and still got the input voltage on there it takes up to five point five or maybe even as much as minus 15 micro amps from your battery so just take that into account this isn't the lowest power device I've seen in terms of off state leakage current and if we look at some of the characteristic curves here these are very interesting now this is an important one here this is the charge current on the y-axis in milliamps versus the programming resistor and as you can see they give a range in the datasheet above 4 to 267 K or something like that but as you can see it is not a linear type thing so you can't just arbitrarily put in like a hundred K resistor or a 1 Meg resistor and get really low charge values because then the current accuracy is going to be all over the place and it's it's not characterized on this curve so really it looks like that value there if you extrapolate across there it goes that well it tells you above that it was 15 milliamps and that's sure enough on the graph it looks like about 15 milliamps that's really something to consider when you're choosing these chips for low value low capacity ultra-low capacity batteries and lust of all I'm just going to take a quick look at a more flexible are charging I see the St micro l 69 24 D it's you'll find that's got it says it's got programmable pre charged current programmable end of charge current programmable pre charge voltage threshold and it's got a programmable charged timer as well which will be a backup device just in case the voltage cutout doesn't work it'll have a fixed time and then cut off just as a secondary safety feature and it's also got an NTC or PTC thermistor temperature interface which will limit the charge card if the if the temperature of the battery goes up past a certain setting now let's take a look at the you can see here that it's got different different resistors on here to charge to change those various aspects of the charging cycle the pre and the post charge current now if we go down here and take a look at the internal block diagram it's got there's the there's the pass transistor as well there's V in here V out on the right here which goes the battery it's got current detection fault detection logic that there's there's a diet that actually blocks it as well it's got a gas gauge function as well and this is what a lot of devices will have if they actually use the if they use the resistor to set the charge current it actually drives a voltage a current through that resistor which is proportional D card charge current so you can hook that up to an ADC on your microcontroller and you can actually log how much current is going into your battery during charging it's quite nice so that's just a more flexible I see that just allows you to do a fair few more things than than the microchip one we saw before so if you really have to my you know get a really precise value of charge and capacity and long life in a in a professionally designed product you would use a more advanced IC like this and you would go through all the various aspects and you would design it properly so that you're in your built in battery would have the longest life possible and if we take a look at the final application demo circuit down here as you can see it's just got programmable these these resistors here program all the various aspects of the charging cycles so there you go that's a more advanced one there's simple ones available some real dumb ass three terminal ones take your pick but lithium-ion battery charging is pretty simple with these dedicated IC so next time you're designing a product and you want to build in a recharging solution use lithium ion the cells are incredibly versatile in our shape and size low-cost the chips are dirt cheap readily available easy to use go for it hope you enjoyed it see ya

Channel: EEVblog

Views: 351,857

Rating: 4.9079003 out of 5

Keywords: lithium, ion, li-Ion, li-po, lipo, LiFePo4, polymer, battery, charger, charging, tutorial, how, to, microchip, digikey, project, diy, maker, hacker, electronics, product, design, mah, capacity, datasheet, curve, graph, millamps, amp, hours, mosfet, blocking, diode, resistor, charge, rate, set, program, pass, transistor, schematic, parametric, spec, specification

Id: A6mKd5_-abk

Length: 39min 17sec (2357 seconds)

Published: Wed Jun 08 2011