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| Using BOD 2.7V AVR ATTiny85 as Li-ion 18650 4.2V battery discharge protection? |
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| Siwastaja:
The output diode makes the voltage regulation suck beyond laughable. Too much variation in Vf in function of temperature, current and unit variation. You can make it work safely using worst-case values but then you are trading off a lot of cell capacity. Yes, LM317 can be used in a charger circuit, but not alone / as is. Addition of CC is a good start. You still lack starting voltage and temperature qualification check. You can kind of ignore the temperature issue and manage that manually, but voltage check is very important and easy to forget to do manually. I'd either do this properly, or do it even more cheaply, i.e., use the existing microcontroller to connect the solar cell directly to the cell, using a single FET and cell & panel voltage sensing, and write perfect software that never crashes :D. |
| beduino:
--- Quote from: Siwastaja on October 05, 2019, 02:19:28 pm ---The output diode makes the voltage regulation suck beyond laughable. Too much variation in Vf in function of temperature, current and unit variation. --- End quote --- I used BAT85 (200mA) schottky diode (thanks to ~100mA charge current limit) and voltage drop on fully charged capacitor 220uF during the tests was only ~0.11V, since on LM317 output I've 4.13V and after diode on charged capacitor 4.02V without any load. Not such a big deal whether battery will be charged to 4.13V or slightly below 4V under a few mA load. I used detailed design procedure shown in LM317 datasheet including IADJ current 50uA and it is nice that calculated theoretical output voltage 4.11V is very close to measured 4.13V :-+ --- Quote from: Siwastaja on October 05, 2019, 02:19:28 pm ---Yes, LM317 can be used in a charger circuit, but not alone / as is. --- End quote --- Yep, you need calculator, too :-DMM |
| Siwastaja:
--- Quote from: beduino on October 05, 2019, 10:25:11 pm --- --- Quote from: Siwastaja on October 05, 2019, 02:19:28 pm ---The output diode makes the voltage regulation suck beyond laughable. Too much variation in Vf in function of temperature, current and unit variation. --- End quote --- I used BAT85 (200mA) schottky diode (thanks to ~100mA charge current limit) and voltage drop on fully charged capacitor 220uF during the tests was only ~0.11V, since on LM317 output I've 4.13V and after diode on charged capacitor 4.02V without any load. --- End quote --- Yes, and BAT85 (Nexperia datasheet) is defined to have up to 800mV drop at 100mA at room temperature. There is no minimum defined. At 10mV, the max at Tamb=25 is still 400mV. Quite a difference to your measured 0.11V. Taking temperature variation into account, basically it can be anything between around 0.1 and 1.0V, roughly. Hence, the regulation sucks. Measuring single-unit values on lab bench is design by luck. For li-ion voltage regulation, do not design by luck, using unregulated elements (such as diode drops). Close the loop against actual voltage reference, reading at the cell terminals. Of course this doesn't matter. It's likely good enough. Sometimes the cell may charge fully, sometimes to approx. 80%, sometimes slowly, sometimes more quickly. You have no control, and you won't know, but it's likely within acceptable limits for you. --- Quote from: beduino on October 05, 2019, 10:25:11 pm --- --- Quote from: Siwastaja on October 05, 2019, 02:19:28 pm ---Yes, LM317 can be used in a charger circuit, but not alone / as is. --- End quote --- Yep, you need calculator, too :-DMM --- End quote --- And a bunch of external components to deal with the current limiting, and the output diode. And you still have no pre-condition check. You have a nice fireball generator. An overdischarged cell is full of dissolved copper, ready to grow dendrites to short out the cell when full charging current is applied. An integrated lithium ion charging IC would do all this much easier. I'm all for thinking outside of box and trying dangerous things, but forcing an LM317 do it isn't very innovative, either. |
| beduino:
I've found those articles very usefull: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries --- Quote from: Siwastaja on October 06, 2019, 07:38:11 am ---Sometimes the cell may charge fully, sometimes to approx. 80%, sometimes slowly, sometimes more quickly. You have no control, and you won't know, but it's likely within acceptable limits for you. --- End quote --- It will never fully charge while maximum charge voltage is limited by LM317 to 4.11V and BAT85 diode limits additionally even under small load, but this is what I'm loking for - more charge cycles at very low charge current. https://batteryuniversity.com/index.php/learn/article/bu_808b_what_causes_li_ion_to_die --- Quote ---Similar to an EV, Li-ion in satellites must also endure a lifespan of 8 years and more. To achieve this, the cells are charged to only 3.90V/cell and lower. An interesting discovery was made by NASA in that Li-ion dwelling above 4.10V/cell tend to decompose due to electrolyte oxidation on the cathode, while those charged to lower voltages lose capacity due to the SEI buildup on the anode. NASA reports that once Li-ion passes the 8 year mark after having delivered about 40,000 cycles in a satellite, cell deterioration caused by this phenomenon progresses quickly. Charging to 3.92V/cell appears to provide the best compromise in term of maximum longevity, but this reduces the capacity to only about 60 percent. --- End quote --- --- Quote from: Siwastaja on October 06, 2019, 07:38:11 am ---An overdischarged cell is full of dissolved copper, ready to grow dendrites to short out the cell when full charging current is applied. --- End quote --- Charging current is limited to ~100mA, so as you noticed at this point voltage drop on BAT85 might be around 800mV, so at such load we can have around 3.3V on batery which is around 30% capacity, but at loads at the range of 4mA (eg. 8MHz ATTiny85 in active state on its internal oscilator) charge current is stil close to 16mA based on 0.11V voltage drop on 6.8 ohm current limiting resistor, so we will see during the a few day tests how close cell voltage could be to 4V achived with electrolite capacitor without any load. --- Quote from: Siwastaja on October 06, 2019, 07:38:11 am ---An integrated lithium ion charging IC would do all this much easier. --- End quote --- Another story is charging in low temperatures and I'm not sure how many of those available li-ion charging IC deal with such conditions. http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures --- Quote ---Charging is indeed possible with most lithium-ion cells but only at very low currents. According to research papers, the allowable charge rate at –30°C (–22°F) is 0.02C. At this low current, the charge time would stretch to over 50 hours, a time that is deemed impractical. There are, however, specialty Li-ions that can charge down to –10°C (14°F) at a reduced rate. --- End quote --- While, I do not need too high charging currents, maybe even this circuit could charge li-ion in temperatures below zero since 0.02C, where eg.: C: 2500mA is 50mA at –30°C, so I can limit this charging current to this level just by using 2 times bigger current limiting resistor and this is a beauty of such simple designs of course depending on application. This is not for life critical applications, but it might be used to keep running sensors in bike computer, where Li-ion 18650 cells can be hidden easily into a frame and only a few might be needed. It is acceptable to charge only 60% if it prolongs battery life. http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries --- Quote ---Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 4.0V/cell should deliver 1,200–2,000 and 3.90V/cell should provide 2,400–4,000 cycles. --- End quote --- |
| Siwastaja:
Your issue is, you are assuming things and overlooking obvious advice, and trusting a well-known and widely debunked fake site (yes, their content quality has definitely gone up, but still, be careful.) For example, this graph which plots the voltage from 0 and starts the charging from 2.5V gives a false impression that this would be something that happens normally, while it isn't. The fact that your current is limited to "only" 100mA is meaningless. It's still in massive violation of the approved post-overdischarge conditioning cycle. The actual process varies, but it still requires voltage pre-checks, charging at around C/100 only, and adding timeouts and voltage checks mid-process. But just don't implement conditioning. It's optional and can only save a small subset of failed cells. Don't make the cells fail in the first place, and detect failed cells to prevent fire. Never just connect a 18650 cell to a 100mA, 4.1V CC-CV source. Check the voltage first. If < about 2.5V, discard the cell. If you want to implement the conditioning cycle, learn to do it properly. Yes, a 100mA current source is less likely to blow the cell up, than, say, 1000mA current source, but it can do it. All it needs to do is to plate the dissolved copper within the separator, and you have an internal short. This can happen at 100mA, given enough time. And yes, integrated li-ion charger ICs should have temperature qualification check. If not, they are crappy and dangerous you just need to implement it yourself :). The ones I have used do it. But for your own hobby project, it's not that dangerous to skip the temperature check. Especially at such low charge rates, and less than full charge voltage, which reduces the lithium plating risk at low temperatures. I'd say you are OK down to, say, -10 degC. |
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