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| Balance charging lithium cells |
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| paulca:
Something I have looked into before. It came down to, yes, you can do non-resistive balance charging but it's (1) much more complicated and (2) takes much longer. However, I wanted to point out that those who believe that you don't need to balance charge very often obviously do not use multi-cell packs in the real world very often. EVERY time you charge a multi-cell pack you MUST use a balancer. On a good pack you might only see 100mV difference during charge or discharge, however with a straight 12.6V charge across a 3 cell pack you could end up with 200mV inbalance and charge a single cell to 4.35V or higher easily. If you are high current charging the pack they tend to explode in flames at that voltage. My experience is in high current LiPos discharged at 10C and charged at 2C, 100mV imbalance is not uncommon and depending on use they can be much, much higher. Consider that LiPo alarms or other voltage monitors tend to take their running power form cell 1. It's also not completely uncommon to pull 8.4V off cell 1 and 2 on a 3S or 4S pack to power radio gear, although generally frowned upon, it's still done. Resistive balancing is loseful. The total amount of loss would need to be calculated. It would be interesting to see how much power is actually lost using a set of LiOns such as Adam Welch's solar shed setup. |
| thm_w:
--- Quote from: paulca on April 29, 2019, 04:00:09 pm ---Something I have looked into before. It came down to, yes, you can do non-resistive balance charging but it's (1) much more complicated and (2) takes much longer. However, I wanted to point out that those who believe that you don't need to balance charge very often obviously do not use multi-cell packs in the real world very often. EVERY time you charge a multi-cell pack you MUST use a balancer. On a good pack you might only see 100mV difference during charge or discharge, however with a straight 12.6V charge across a 3 cell pack you could end up with 200mV inbalance and charge a single cell to 4.35V or higher easily. If you are high current charging the pack they tend to explode in flames at that voltage. ... --- End quote --- But some real world packs do not use a balance circuit, as I was told by Siwastaja. Even in microchips multi-cell 18650 charger app-note they left it out. I just checked a two cell pack here I have been using for a few months (numerous charge and discharge): 3mV difference in cell voltages (3.880 3.877V). Of course the capacities are very close, I can't remember the specifics but I think within 50mAh. High current charging for sure may be a different story, this is only charged at 1A max. |
| Nitin25:
So which logic you have used to sense individual cell voltage. how to sense effectively individual cell voltage with microcontroller if there are more number of cells in series. As microcontroller works only 5 volt. |
| paulca:
--- Quote from: thm_w on April 29, 2019, 09:20:45 pm ---But some real world packs do not use a balance circuit, as I was told by Siwastaja. Even in microchips multi-cell 18650 charger app-note they left it out. --- End quote --- Yes, but this is cheating you. What will happen is as soon as the first cell reaches 3.0V the pack disconnects. As soon as a single cell reaches 4.20V the charger disconnects. This is exactly why laptop packs without balance circuits have such a short life span. It's obviously for one reason. It makes companies more money. How many laptop packs have been taken apart by cell refurbishers to find one single parallel cell set at 2.9V but the rest are absolutely fine, yet the pack is unservicable. |
| OM222O:
there seems to be different schools of thought here like always :-DD As I mentioned earlier the packs that I plan on making are DIY packs made from recycled laptop packs which as pointed out earlier, only have one or two bad cells but the rest are fine. so there will be no guarantee of capacities being equal or even the cells having exactly the same chemistry. A few pointed out that if a cell is not charging like the rest it would only mean that they are band and will fail soon so replacing them is best, but I'm still not sure how a pack can remain balanced when cells have different capacities :-// yes, I know I compared them to super caps which are different, but the main reason that they charge to different voltages and need balancing every time, is exactly because they have different capacities. maybe I should do some real world tests after I build the packs and do 10 full charge / discharge cycles using both methods and compare the voltage levels of each cell at the end of each charging / discharging cycle to see after the initial balancing, if the cells drift again or remain close together. |
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