How to calculate the maximum charge of the battery that it can give from a fully charged state to a fully discharged one. I have been trying to solve this problem for 2 months. The drone itself is not. Maybe someone knows How to calculate the maximum charge of the battery? And sorry about my English. I don't know him well. Or some formulas for finding the electrical capacity of C?
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1$\begingroup$ Welcome to Robotics:SE. What has your research shown you so far? You might find it helpful to review the site tour and Help Centre and, in particular, How to Ask. $\endgroup$– sempaiscubaCommented Jan 12, 2020 at 1:52
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$\begingroup$ what will you do with the information? $\endgroup$– jsotolaCommented Jan 12, 2020 at 2:43
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2 Answers
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If you want to learn about batteries and their characteristics, you should probably go to the BatteryUniversity to learn a few things before you start to do calculations, because if you miss interpret stated parameters as facts, you will get wrong results.
https://batteryuniversity.com/learn/
The specific mAh you can get from a battery depends on the voltage difference from charged to discharged state you are willing to use and at what current you do it (careful it can be dangerous or damaging to discharge to low or charge to high as well as overdrawing current). The way to be sure is to measure when you charge or discharge. Stated capacities on batteries are estimnates at best, depending on battery type it can be the most achivable mAh or the minimum value which is always achived ... check your datasheet of the battery for this(if your battery has no datasheet then the only option is measuring).
There is some websites and youtube channels doing battery tests, only if you test a cell you can know exactly how it performs.
LIION:
https://lygte-info.dk/review/batteries2012/Common18650Summary%20UK.html http://www.dampfakkus.de/akku_liste-nach-groesse.php?size=18650
LIPO:
very difficult to find tests because the batches of batteries even from the same vendor have different performance characteristics. Find some here:
https://rightbattery.com/lipo-liion-life-batteries/
NiMH/NiCd
https://rightbattery.com/nicd-nimh-batteries/
I hope this gives you the right pointers on where to start.
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The mAh rating should be written on the battery. For Lipo batteries typically used in drones, the stated mAh is usually a fairly decent estimate of the actual, though that varies some depending on the brand and quality of the battery supplier and the current load and age of the battery. Even if there is no label on the battery, the mAh rating is typically printed on the side of the cells. If there is no label printed with mAh capacity, or you don't trust the stated mAh, you can discharge the battery till the resting voltage is about 3.7 volts per cell under no draw, and then charge it back. Your charger should tell you how many mAh were put back into the pack, and you can typically multiply this by about 1.2 to get an estimate of the total mAh available in the pack. It's not 100% accurate, but it's a safe ball-park. Regardless, the number you get will be a good value to use when estimating functional capacity for flight time estimates.
You typically won't want to draw your battery much below about 3.3-3.5v per cell under load, and it should recover to around 3.7v resting with no load. You don't want to use much more than about 80-85% of the overall mAh capcity of the battery. A resting voltage of 3.7v should get you right there, but if you have way of measuring mAh consumption via a current sensor, it will be much more reliable than relying on voltage alone, as voltage can fluctuate dramatically depending on the current draw load on the battery and the quality of the cells.
Another thing to be aware of is as your battery ages, the useful capacity of the cells, as well as the safe current draw, can reduce fairly dramatically. You'll need to continually monitor the health of your packs throughout their life, and if you start to see a dramatic drop in performance, stop using them and replace them immediately.
Keep in mind this is true only for LiPo batteries we typically use. If you want to use Lithium Ion, the safe discharge voltage and discharge rates are very different, and the claimed mAh ratings can be dramatically off depending on the reliability of your source for the Li-Ion cells. Some 18650 style Li-Ion cells have been found to contain powder filler and a much smaller cell inside, so be aware of those caveats. Li-Ion cells are also typically safe to draw down to around 2.5v per cell rather than the 3.3ish of LiPos, but that as well as the max recomended current draw can vary depending on the manufacturer. If you have Li-Ion cells, be sure to check the manufacturer supplied specifications if you can find them, and make sure your cells are genuine. Knockoffs are a problem in the Li-Ion world. In general Li-Ion is only suited to applications that have both low current draw and low transient spikes, and there is much more variation in quality and specifications.
For both types of cells, keep in mind the current being drawn from the battery can dramatically impact the ability of the battery to deliver its full capacity. The higher the current draw the less usable capacity you will have regardless of the true pack capacity. This is why it's always the best idea to confirm both the current delivery performance, and the mAh rating if you have any doubts, and why running both a current sensor and voltage sensor in the air is ideal.
As far as identifying other characteristics of a battery, using a reliable "internal resistance" meter can tell you a lot about the battery performance and how much current you can safely draw from it. I use the Wayne Giles meter, and you can find a good discussion on the theory and how to use the meter here: https://www.rcgroups.com/forums/showthread.php?1323465-ESR-IR-Meter-fo-Lipos
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$\begingroup$ be warned this is only true for LiPo-Batteries and some things in this post are missleading or half true. mAh stated can be way off (only if you test the batteries you can tell). $\endgroup$ Commented Jan 15, 2020 at 9:43
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$\begingroup$ I've tested hundreds of lipo batteries and have never once found dramatic variation from the stated mAh. The Lipos we typically use for drones (which this question was referencing) are typically very close to stated. The ones where I've found variances have been slightly larger than stated, so going with the stated mAh is a very safe bet. $\endgroup$ Commented Jan 16, 2020 at 20:30
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$\begingroup$ Depending on how much current you draw from them this is a bold statement, they often state higher C ratings which the cells can't handle. This can make a battery have way less mAh at that high load. However if you buy reputable brands this is a non issue. About the LiPo being THE drone battery, for professional applications Li-Ion has some advantages as they have 40% more energy density. FPV drone flyers stick with their LiPo (power hungry setups), but i use Li-Ion for my FPV quad and it works fine. Many drones in the professional space use Li-Ion. $\endgroup$ Commented Jan 20, 2020 at 14:11
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$\begingroup$ Oh yeah, the C ratings are all over the place. 55C-65C is essentially the highest actual constant rating that can be achieved, but you see batteries in the 100s of Cs advertised. Biggest gimick in the battery game. Hence my recommendation for the Giles meter. $\endgroup$ Commented Jan 20, 2020 at 22:21
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$\begingroup$ I would also suggest that though we've seen a rise in DIY Li-Ion packs and a very few commercially available Li-Ion packs (the Titan, etc) for long range multi-rotors, the application of Li-Ion in the drone industry as a whole is very limited and very narrow. Even though large drones have a sustained current draw that might be within the limits of what a Li-Ion pack can deliver, the larger motors and props create large current spikes when the RPMs change during stabilization and other rapid orientation changes that precludes Li-Ion from common usage. $\endgroup$ Commented Jan 21, 2020 at 13:28