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| Simplest way to add li ion to your projects without fire risk |
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| Beamin:
--- Quote from: KL27x on November 05, 2018, 06:50:37 am ---When I first started playing with li ion, anyone I asked said I was going to burn my house down. But if you can make a stable voltage regulator, you can charge a li ion battery. 1. A li ion cell needs to be limited to 4.2V on the terminals. 2. A li ion cell has to be limited to a certain amount of charge current. 3. Don't charge a cell that has gone under 2.7V at full current. If you want to recharge it at all, you ought to tickle it with something like 1/20 C until the voltage is up around 3V before stepping on the gas. The best way to prevent this is to use output protection/cutout, so the cell gets disconnected when the voltage drops below the cutoff threshhold you need. (For higher relative draw for the cell's available output rating, you might want to cut out at 2.7V. For a lower relative draw, you might want to cut out at 3V.) The amount of current depends on the cell, but IME 99% of li ion cells can handle 1C charge current. (Some can handle 100C+) So if you do not want to use a dedicated charging IC, a very simple way to charge a li ion cell is to make a 4.2V power supply, using an LM317. And then put a series power resistor on the output in order to limit the max current. Example, if the battery has 1Ah capacity, 1C charge rate is 1A. 4.2V-2.7 (for a dead cell) is 1.5V differential. 1A = 1.5V/R. R= 1.5V/1A. 1.5 ohm resistor in series will ensure that the max charge rate is less than 1A if the cell is completely dead. The internal resistance of the cell will mean that the charge rate is actually lower than 1A. The last third of charging will take longer than it should, but it will be safe in my opinion. As long as you can build a reliable constant voltage supply that you trust. But you can buy charging IC and boards relatively cheap. And if it is too much trouble to add it to any/all of your devices, you can just put a charging port in the device and charge it with your bench PSU. Or a single charging board with a modest or adjustable charge rate. But you still have to ensure that you don't over-drain the cell. You should be able to do that in a variety of more complicated ways, if you want to. But again, there are voltage detector IC's that are made exactly to do this. Just connect to a suitable logic level power FET and you have a simple cutout. --- End 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. |
| CatalinaWOW:
I don't have any problem with KL27x s answer with two additional caveats. First, this applies to single cells. There are other problems when charging series or parallel cells. Second, there are slight variations in the do not exceed voltage for different Lithium chemistries. His advice is specifically for the type called Lithium polymer, or Lipo. Finally, even if you do all this right there is more chance of burning your house down with these than with other battery chemistries. Watch your batteries for signs of damage and don't leave them unattended without additional precautions. Are you really sure that your charger design can handle a big power line surge or other rare event? |
| Beamin:
--- Quote from: Mr. Scram on November 05, 2018, 10:12:14 pm ---Does it have to be lithium iron? Other lithium chemistries are much more stable, though pose their own engineering trade-offs. --- End quote --- Other chems are not rechargeable that I know of and I doubt any of my junk bin batteries are anything different or exotic. |
| Mr. Scram:
--- Quote from: Beamin on November 05, 2018, 11:47:40 pm ---Other chems are not rechargeable that I know of and I doubt any of my junk bin batteries are anything different or exotic. --- End quote --- There are plenty of secondary cell lithium chemistries to play with, most of which are safer than lithium ion. That's just the one we got stuck with because it's both fairly cheap and energy dense. https://www.mpoweruk.com/lithiumS.htm |
| kjr18:
It kind of depends on what cells you have, etc. If your cells have built-in protection, you can skip that. For charging, tp4056 can do it with just two or three other parts. If you need higher voltage than battery voltage, boost circuit is needed, lower than cell voltage, some linear or buck regulator. Some cheap chinesium products depend on built-in protection to terminate charging, you can recognize these by their power supply, typically 4.2V and around 500ma output. The simplest way would be use of simple building blocks: charger/protection board, buck converter/linear reg, boost converter. You don't need anything more. You can even make some small boards yourself (if you are space restricted), or buy some. |
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