Hi Welcome to the Eev blog an 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 uh 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, uh, build a gadget, you want to build in some rechargeable batteries. you have a couple of choices. One is your traditional nickel metal Hy dried, uh, rechargeable double A AAA whatever. Um, batteries, right? Well, they're a bit of a pain in the butt.

They're old hat chemistry. They're uh, a low terminal voltage, so you got to often wire a lot of them in series to get the voltage you want. Um, and they're just, you know, big pain in the butt. Not available in really nice tiny shapes and sizes like these: lithium ion cells.

Here's a Uh 230 Milah uh cell. Here's a 50 milliamp power cell. You can get these um from companies like uh, Power Stream.com and many, many other places um, hobbyist, uh, outlets for uh for remote control stuff all over the place. And as it turns out, they're cheap, readily available, and really easy to use in terms of uh, charging, circuitry and stuff like that.

So we'll go into it. and uh, you've seen your standard Nokia uh, 3.6v nominal uh 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.

Uh, and you can get those and they charge exactly the same way. So, but we're going to, uh, stick to the individual lithium ion. SL Lithium Polymer There's no real difference between Lithium ion and Lithium Polymer. Don't fall for it.

It's a bit of a marketing gimme. Okay, so we're going to take a look at these. how you can charge them, 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 millimet thick and they come in various sizes. You can actually get them, uh, under a millim 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 sizes. You just 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. SL Lithium Polymer As I said, no difference whatsoever.

Don't let anyone fool you. otherwise. okay, lithium ion cell. 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 Uh on: Uh ablea Uh nickel Metal, Standard 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, slopey 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. Uh, there are actually two different types. Don't confuse these with lithium ion and lithum polymer cuz they're the same thing. In fact, they're the Uh new type of anode material.

I might 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. Um, and it's the traditional type.

Older type uses a Coke material not not to be confused with the trademark Coke or the other type of Coke this actually comes from Mar Cole It's actually derived from that or the new modern ones. In fact, the vast majority of lithium ion batteries you can buy Uh 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 Uh.

The curve here is flatter than 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 Vols directly from the battery using the low Dropout voltage regulator. because if your circuit is 3.3 volts there, it is okay. But uh, you might use a linear low Dropout voltage regulator.

might have a Dropout voltage of 0.1 volts or even 0.2 volts. Even at 3.5 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 3vt level or something like that and a Coke anode.

The older type batteries are typically taken to be dead around. you know, 2 1/2 volts, 2.6 2.7 something like that. Um, even the Uh. Even some of the graphite And1s are determined to be dead at 2.7 Now, these curves will vary by manufacturer.

They'll vary by battery type um, slightly different. uh, process variations in the manufacturer of the the battery. All sorts of stuff. But these curves are going to be very similar and they're handy cuz you can power your 3.3v circuits directly from one of these little lithium ion cells.

Fantastic! Just something quick. I Forgot to add that the Xaes here is actually uh, time or capacity like from 0 to 100% the capacity of your battery or C as it's called Uh Or it could be 0 to 1 hour or 0 to 10 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? I'm 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, uh thing. So a lithium ion as well.

But really, uh, to charge them properly, you should sense the temperature of them as well to determine, as well as the Uh voltage on them to determine when to stop charging or when they're full. Now, with lithium ions, you're 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, uh, the temperature of them to determine when the charge when you're finished charging. These things and it's full.

Uh, really. a lot of the charger chips on the market which we'll go into, they do have built-in uh, temperature interfaces for temperature sensors which sense the cell, but really, that's just, um, 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 uh, to charge them is really easy. Any lithium ion or Lithium Polymer cell. They charge exactly the same way. As was said, there's only two differences.

One is, well, the only difference is the Uh charge voltage, which we'll have a look at here: 4.1 volts or 4.2 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, uh, well, all lithium ion charger cells on the market uh, will be 4 . 1 or 4.2 is the most common by far volts.

Plusus, at least 1% some are capable of going down to3 or 4% accuracy and I won't go into it. But you can look up the research yourself. Uh, lithium ion batteries. Their shelf life and their number of recharges is pretty much directly.

Um, or the biggest Factor directly proportional pretty much to what maximum voltage you actually charge them at and the charge rate as well. But that voltage? critical? Okay, you 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 too. 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 cuz it's interesting. For starters, Okay, now the Uh x, uh, 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 is the battery current or the cell current. now, uh, the current.

This is important. If this little thing here is a 50 milliamp battery which is what it is. Okay, then uh, that is called one C or one. that's the charge rate.

It's just called C or 1 C Okay now Lithium ion batteries. Most make sure you always check the data sheet for your cell, but most of them will uh charged around typically 0.5c So if this is 50 milliamp hours or yeah, 50 m battery capacity or 1C will 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, uh, lithium ion cells half C So that 100% means in this case, 25 milliamps from 0 to 25 milliamps and voltage. the green curve from 0. In this case, 4.2 volts.

Now it starts off at ignore this one cord 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 well, zero, right? It goes from zero to 100% 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, it depends on the capacity of 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 cell's already dead. Okay, at 2.8 volts there, let's say the battery is 2.8 volts.

When you start charging, it'll slowly rise like that. fairly sort of linear and then it starts to taper off like that until it eventually gets to 4.2 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 Char it changes modes from constant current charging into constant voltage charging where all it does is now.

instead of pushing a constant current into the battery, it maintains that uh, it goes constant voltage 4.2 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 4.2 Vol level. But what happens to the current 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 Uh set by a percentage of your charge current. So if charge current's 100%, this is why I called it 100% because this is what they call it in 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. It's fully charged. Bob's your uncle? Okay is typically taken at 10% of the full current.

So if this was 25 milliamps constant current charging level. once that, uh, once the current level got down to Uh 2.5 milliamps for this particular little tiny cell here, then bang It stops charging. and that's it. Fully Charged battery.

Woo! Piece of cake. Now that two-step process is what's required to really get full. Utilized the full capacity and the full life of your battery. But some cheap Chargers and very fast Chargers Quote marks.

We'll actually just totally skip this constant voltage uh process and just do constant current current and then stop when it gets to 4.2 volts and it's still going to have say, 80 or maybe even 90% of the batter's full capacity if you just do this mode. So this mode here may take an extra hour or something and you may only get an extra 10 or 20% out of it so you've really got to weigh up. Um, 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 finish 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 charger Ic's have this mode, some don't um, 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 manufactur in the data sheet, but typically around say, 2.8 volts, that batter is deemed to be really dead. Fully dead. It needs rejuvenating. Okay, so it needs to be, uh, fixed.

If you get a really completely dead cell that's only got one volt on it or half a volts or no volts on it Okay, you've really left your product on. It had no low voltage cutout the cell. It's completely killed the cell. You can rejuvenate it, but you can't just jump straight into 100% current because you'll You'll further damage the cell.

You'll ruin it. So what they do is they have a pre-charge Uh, a pre-charge process where it only charges it at typically 20% of the full capacity. Now, Uh, that value varies, as does this. Uh, pre.

as does this full charge value. These can vary. Some chips even allow you to adjust this and this as well as the charg current. and they your really flexible chips.

But uh, typically if if you plug your charger chip on and it measures that the voltage is less than 2.8 it 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, uh, very simplistically. what's inside a lithium ion charger battery chip. They can be incredibly simple.

So simple that they can only have uh, three terminals on them. Really? If 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 build-in voltage reference, and it's a fixed charge current. Some chips might charge it, say half an amp or 100 milliamps or something like that fixed. You can't change it and it all handles it internally.

Uh, 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. Oh, we, we'll 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 Uh, but it allows you to calculate okay.

I've got my little 50 Milah battery I Want to charge that at half C To be on the safe side? 25 milliamps I would plug 25 milliamps in the formula in the data sheet and that would give us a resistor value that allows this chip to charge constant current here of 25 milliamps and to do that, most Uh chips the fully integrated ones will have a built-in Uh current shunt sense resistor there with a little amplifier with a little Um differential amplifier there as well and a series pass transistor or series pass mosfet in there driven by an opamp. And you've seen these type of Uh circuit configurations before Now, this past 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 Um are typically lithium ion charger. Ic's are typically a linear type because they drive the pass transistor with a DC voltage.

It's a linear thing. Some will actually uh, 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.

These switch mode ons 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 uh, three or twostep charging process. instantly determine uh, the current flowing through the cell and they determine the voltage on the cell. They 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 you're ping a 3.3 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 count when you calculate this value up here.

So if our circuit was taking an extra 25 milliamps, uh, then our cell at half c25, we'd need to set this value to 50 mamps 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 iron charger chips is that you can leave them permanently connected to the cell like this. and when they're finished charging, they will actually, uh, stop. They won't draw any current back out of the cell like that, and they'll actually have a Dodes built across the Uh pass built into the pass transistor here to actually stop.

If if you're if you physically remove or short out, um, your charger input, it won't drain the battery back out of it and you can get specs for for the current that leaks back out of the batter battery. It's usually quite small in the order ofm, you know, microamps or or sub microamps or something like that. so you can really leave these things just permanently hooked on to your circuit under test. It's fantastic.

So if you've got one of, uh, if you've got a product that say goes into sleep mode all the time, it's got no onoff switch it just Wes up then you can just leave all this permanently attached and you 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 Powerst Stream.com which is a provider of, um, a whole bunch of Uh Batteries 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 there's some primary Lithium batteries, but look at these babies. Ultra thin, rechargeable lithium uh polymer lithium ion batteries 500 microns Point uh from .5 mm to 1 mm thick and you can bend them. If you've got a product which needs to be uh, you know, flexible and like you can't just put a square battery into it.

like if you got something that's mounted on your wrist, you want to wrap the whole battery around your risk wrist. No problems whatsoever. Awesome. But um, let's go into say the standard uh, uh, Lithium Polymer uh cells here and let's take a look at the whole range of them.

They're all nominally. um, don't worry about the nominal voltage, that's just the average voltage. They're all actually. um, the 4.

I Believe they're all the standard up 4.2 volt variety, but you'd have to read the data sheet for that. but you can get them in capacities as low as 8 to 12 milliamp 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 uh, battery capacities that small, so just be wary of that.

Um, it can actually be difficult because most lithium ion battery charger Ic's are actually optimized for you know, half an amp or an amp or two amps or something like that. And then it's a bit of a uh tradeoff between uh, the the circuitry inside is designed for those for for current and voltage current accuracy at those sort of currents. Yet if you try and charge them at very low currents like you know, if this is a 12 milliamp nominal cell, you would have to charge that at half 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 quite difficult, so just be wary of that if you do go that low. if you're designing Ultra tiny products. but uh, check out the size of these Dimensions 3x 99 mm 2x4 and you know, 18x 5.2 And there's countless, uh, different sizes and thicknesses and capacities and things like that. And here's the data sheet for that particular battery we just chose.

It was one completely at random and there it is. Uh, the charging voltage is 4.2 so it is a graphite type. Um, Anode plusus 03 volts. Uh, that is quite tight indeed.

That's why you have to have a very accurate, uh, Lithium dedicated lithium ion charging IC that has that sort of accuracy and then as you can see, it actually recommends a 0.5 um C constant charge rate for a standard charge. If you do want to do a fast charge, it can do it at one C and then the cut off. Um, you remember that actual percentage value we're talking about there? it is not Point uh 01 C All right, 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 Uh 50 Mah capacity battery and I'm going to charge that at 0.5c 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 Ic's 2529 of them and as you can see, here's the parametric table. There's different battery chemistry now unfortunately. dig 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 Uh, know the difference between uh those so it won't give you an extra charging voltage. It's got Supply voltage here, but it' be nice if um, you could actually choose 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 uh, like here. Now it hasn't popped up with. Strangely, it hasn't popped up with microchip. Microchip's 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 choose it. must be in those categories there cuz they're multi chemistry devices. just uh, be careful of that.

You can actually miss quite a few manufacturers if you don't select the uh, the correct um, uh, actual uh battery chemistry here. but we can just ignore that. We can just reset that and say I want microchip Parts cuz I know microchip parts are in stock I like them. They're cheap, They're small.

they work. um so I'm going to try those and as you can see, most of them are lithium ion based ones. but uh, let's go for the uh inst stock Parts shall we? And let's have a look. We've got uh, 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, uh, search by price if we were price sensitive or something like that. But the mCP 73, 812 M mCP 73831.

Um, you can actually get those for 42 cents each for 3,000 or 68 cents for one off. So they very cheap. They're in a five pin so uh, so 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 Uh Mcp73831. Let's take a look at the data sheet. 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 uh, it's a linear type uh device. It's 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 uh, it doesn't drain your battery on you. It's got high uh accuracy.

Pretty good. better than the standard 1% It's got plusus 75% there, which is really nice. I Like that you can get it in four different options for different uh, chemistry batteries, but we want the 4.2 Vol 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. They'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 data sheet now. Programable 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 Great! We need 25. It'll be within the ballpark on the graph as we'll see later.

Fantastic! It'll still maintain its current accuracy down to 15 milliamps. It's got selectable preconditioning um, that, uh, pre-charge that actual Rejuvenation charge 10, 20, 40 or you can actually disable that if you don't want it at all. And it's got selectable end of charge control too. Um, 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 Um and not a pin settable option. so just be careful of that. Larger pin count devices are more flexible. They will have uh, 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 it automatically Powers down.

It's nice. it's in a you can get it in a tiny 2 mm by 3 mm Dfm or an easier to use five pin. So 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 got to have a decou in 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. uh, similar circuitry to what's used and it will be an unstable Loop unless you add the output the recommended value of output capacitor. So just make sure you do that. And it's got ground pin and a programming 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 uh, battery output pin Here here's your pass transistor with the internal blocking uh, diode so it stops discharging from the battery. there's another, uh, smaller pass transistor there. reference voltage generate a whole bunch of com whole bunch of uh, comparators for your different modes, your preconditioning mode, your termination mode, your end of charge, and all that sort of stuff.

and uh, that. uh. And there's your start output pin that's only available on the Um 73831. The 7383 2 presumably doesn't have that pin.

If you don't want it, you can probably save half a scent there or something like that. and as you can see, there's not really much in them at all Voltage you coup couple of constant current generators and things like that. They're pretty simplistic devices because they don't really have to do much um at all apart from transition from a constant current mode into a constant voltage mode. And to do that doesn't requ require much circuitry at all.

It's Supply voltage range from 3.75 to 6 volts. Brilliant. Not a problem. Uh, let's look through some of the other stats here as you can see the regulated output voltage 4.2 volts.

Um, from there's there's the different part numbers that you can uh buy with the different uh charging float voltages. make sure you get the right one, don't want to uh, 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 plusus 10% current regulation there which isn't too bad. Uh, 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 data sheet mistake. Anyway, not sure what's going on there. Aha, here it is I've uh, scroll down to the product identification system right at the end of the data sheet and this clears up the confusion we uh saw before with the Uh with the pre and post current termination ratios that were it said were 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 Uh option. The options are AC ad Atdc and they give you various Um options for the pre- and post charge termination and other things. So you got to make sure that you order exactly the right part.

Otherwise you know you could easily end up with uh being actually delivered or ordering the wrong part and that could slip into your product and you can wonder why. Your battery. um, uh, 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 66.5% that's um, much higher than the 20% I said before, but many chips use different um, lots of different Uh value default ratios for that sort of Um thing. Now, the charge termination ratio by default is 5% and the charge termination once it reaches 5% as we saw on that curve. Um, it will actually turn off and you finish charging. pass transistor on.

Resistance Um, there's the battery discharge leakage. Okay, so when it's finished charging, it will only take 0.15 microamps and under the various conditions. Um, so it doesn't take much current at all. Once the charge is complete and it's still got the input voltage on there, it takes up to 5.5 or maybe even as much as minus5 microamps from your battery.

So just take that into account. This isn't the lowest power device I've seen in terms of Um 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 Uh charge current on the Y AES in milliamps versus the programming resistor and as you can see, they give a range in the Um data sheet above for 2 to 67k or something like that. But as you can see, it is not a linear type thing so you can't just arbitarily put in like a 100K 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 down 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 to consider when you're uh, choosing these chips for low value, low capacity, ultra low capacity batteries. and last of all, I'm just going to take a quick look at a more flexible Uh charging IC the St Micro L 6924 uh D it's you'll find that's got uh, It says it's got programmable pre-charge current, programmable end of charge current, programmable pre-charge voltage threshold, and it's got a programmable charge timer as well which will be a backup device uh, just in case the voltage cutout doesn't work, it'll have a fixed time and then cut off just as a secondary Uh safety feature and it's also got an NTC or PTC themister uh temperature interface which will limit the charge current if the Uh if the temperature of the battery goes up past a certain setting. Now, let's take a look at the Um. You can see here that it's got different different resistors on here to charge Uh to change those various Um aspects of the charging cycle, the pre and the post uh charge current.

Now, if we go down here and take a look at uh, the internal block diagram, it's got. Um, there's the uh, there's the past transistor as well. There's V in here V out on the right here which goes the battery. It's got current detection.

uh, fault detection logic. Um, there's there's a diode that actually blocks it as well. Um, it's got a gas gauge uh function as well. And this is what a lot of Uh devices will have if they actually use the If.

If they use the resistor to set the charge current it, it actually drives a voltage a current through that resistor which is proportional to your Char charge current. So you can hook that up to an ADC on your microcontroller and you can actually uh, log how much uh current is going into your battery during charging. It's quite nice, so that's just a more flexible um IC that just allows you to do a a fair few more things than uh than the micro ship one we saw before. So if you really, um, have to, you know, get a really precise Uh 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 uh go through all the various aspects and you would design it uh, properly so that uh your 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. Program all 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 dumbass three terminal ones. take your pick.

but lithium ion battery charging is pretty simple with these dedicated ic's. So next time you designing a product and you want to build in a recharging solution, use lithium ion. The Uh cell are incredibly versatile in not shape and size, low cost. The chips are dirt cheap, readily available, easy to use.

Go for it. Hope you enjoyed it. See you.