Electronics > Beginners
Simplest way to add li ion to your projects without fire risk
Rick Law:
--- Quote from: Beamin on November 04, 2018, 02:03:03 pm ---So is this the simplest way to add li ion batteries to your projects that would normally run on batteries like nicd or nimh in a low cost simple solution?
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--- Quote from: Audioguru on November 04, 2018, 02:35:04 pm ---The DW01 IC is called a Lithium battery protection IC in Taiwan but maybe it is actually a crude charger IC that is missing many details of a real charger IC.
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Since you are looking to use your stock of lithium battery to power something, a battery protection circuit is exactly what would be desirable. Instead of rolling your own, why not just buy a protection circuit board. Boat loads of them out there most are based on the DW01 (or copies).
But, you didn't say anything about what you like the battery to power... a radio? an RC race car? a flashlight? a clock? (etc. etc.) And, what is the requirement (voltage, current, etc.) of the thing you want to power? No one can come up with a good solution without first knowing what is the problem to be solved.
Most rechargeable lithium cells are 3.7v, but NiMH cells are 1.2v cells. Unless you are powering something that had 3 NiMH cells (3.6v), you have a voltage mismatch. There are boost or buck converters and boost plus buck converters out there to boost (increase) or buck (decrease) the voltage to your exact need. You can build one too... Buck circuit, boost circuit, boost-then-buck circuit... Boost+buck is the funnest, don't matter the input voltage (within limits), you can output the entire range from below your supplied voltage to above your supplied voltage (within limits).
Since you want to power something that initially has NiMH or NiCD, one can assume the current draw is not too high. You still need to know what it is. I have a boost+buck that can do around 500mA, nice, simple and small. But if I try to use that for something requiring 1A or more, I can make a fire place out of my work bench. So, you need to know more about the load before you decide if you need a car, a van, a truck, or a train.
KL27x:
--- Quote ---I don't know if I trust myself to make that work. From watching my bench power supply with a bunch of USB powered things li ion, it stays constant current within the 0.001mA range then the last 20% of the time the current goes up slightly then down, as it approaches 0 the charger just stops suddenly. That seems very specific to use a LM317 and resistor.
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The USB charge can be described as both current and voltage limited. If you limit the voltage to 4.2V per cell, then the only way it can exceed its own max charge rate is when it is discharged significantly. For sake of argument, let's say a given cell must be discharged at least halfway before connecting it to 4.2V constant voltage source will allow for a dangerously high current.
So for this particular cell, if it were completely discharged, the USB charge circuit/IC (that is custom tailored to charge it as fast as safely possible) would be limiting the charge current to a given amount up until the cell was halfway charged. At this point, there is no longer any need to limit the current. At this point, the charge current will naturally taper off because of ohms law. 4.2V-Vbat is going to decrease. And the internal resistance of this particular battery will cause it to draw only the max safe current at this halfway point; thereafter, the current will taper off as Vbat rises.
I am speculating that the current on the device you witnessed "rise at the last 20%" is because it used a simple series resistor or somesuch to initially limit the current rather than an active current control. Once Vbat reaches a safe level, it switched this passive limiting device out of circuit so that is would no longer unnecessarily reduce charge current. Thus the current draw spiked a little at this point.
The only reason a USB battery charger has to turn off at all is if the voltage is a bit higher than needed. If you used a charge voltage say 0.1V higher than the max voltage the cell can handle, you can get that last 10% of capacity to charge a bit faster. But you need to turn off the charger when Vbat reaches 4.2V. If you charge with 4.2V or less, there is no reason you can't just leave it on there, indefinitely. Also, the main reason the USB charge circuit has to detect when the cell is charged is so it can flip an output line which can be connected to an indicator light. So the user can watch a red LED turn to green, and know the cell is done charging. :)
The graph you will find in a typical (good active) charging IC datasheet represents the current passed in order to safely and practically charge a cell in the fastest time. There is no need to follow this curve. There is nothing magic about it. Your current graph can look totally different, if you are ok with a longer charge time. You just have to limit max current and keep voltage under and up to 4.2V.
Yes, there are different chemstries needing different voltages. And batteries with multiple cells in series will need some extra precautions (for instance, I charge one particular 4S pack I use to only 16.6V, instead of 16.8V for starters). But there is no special precaution (I'm aware of) to charge li ion cells in parallel. For all intents and purposes, charging 2, 3, or 100 (matching) li ion cells connected and used in parallel is no different than charging a single cell that happens to have the same construction but is 2, 3, or 100 times larger in size. The reason NiMh cells are squirrely to charge in parallel is because Vbat is not a function of SoC. As you charge a NiMh cell, Vbat will rise, then fall, then rise, again. At a given Vbat, there are multiple states of charge. In li ion, Vbat is a function of SoC. Thusly, if one cell has more charge than the other, this means Vbat is slightly higher. And current will flow from that cell to the other. Simply put, li ion cells are self-balancing, in parallel.
CatalinaWOW:
Cells in parallel or series are not a problem if they are identical in capacity, initial state of charge and aging characteristics. If not, cells in parallel will not share current equally and if precautions are not taken one may exceed it's charge rate limits. Series cells will not all charge to the same state. Eventually some cells may overcharge before the string reaches the charged voltage for the string.
You can often go a long time before encountering these problems. Whether you worry about them depends on your risk tolerance.
Beamin:
--- Quote from: blueskull on November 06, 2018, 10:26:33 pm ---Ask for 5V input and let the user to provide a power bank. Leaves you with no liability and certification cost.
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Well when I build my hobby projects I would be the one supplying the power bank. Also leaves me with cost of certification and take all the liability. I wonder if I can sign a contract with myself to limit my liability to me, myself, and maybe even, I!
My scenario is vauge because the application is general. Always building things that run off 3 or 5 sometimes 9 volts. Have tons of 3.7V liion cells around from taking things apart. They are all one cell 3.7 but since they came out of all different consumer electronics the capacity is variable depending on which fits or is at the top of the junk batter box. So I wanted to order a bunch or cheap/simple chips so that next time I build something I can connect a barrel jack or preferably a usb charger to it and charge up the project and not worry about the battery blowing up. Back with nicads this was simple: connect your batteries to your project, just put a trickle charger to a barrel jack in the back of your projects, plug in when needed. Much like when you set your cordless phone down in its cradle. I'm not worried with output voltage or building buck/boost converters because that will be done already with the project, just a simple charging IC or circuit where I add to junk box 1 cell liion.
Why did this dude get banned? He had 10,000 posts thats a lot of work for trolling.
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