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Following on from my previous thread about the Li-ion chargebox that I use for fast-charging my LiPos at the patch, I thought it may be useful to do a long term 4C charge rate test on the LiPos that I'm using: cheap-as-chips Turnigy Heavy Duty 6S 3300mAh 60C that were about £38 each when I bought them in 2020. As I tend to have lots of flights each time I go flying, and often fly alone or with only one or two others, my recently introduced charging-at-the-field regime relies on charging at an elevated C rate for a fun day's flying rather than a day hanging about waiting for batteries to charge. A lot of people have told me that this will result in reduced battery life but none of them can say how many cycles I should be getting, nor by how much this will be reduced when charging at high C rates (and I will ask these questions). It seems that nobody's actually tried it to find out, outside of RC buggy bashing where it's absolutely the norm, and much of the info I get quoted at me is from 15-20 years ago when things were definitely different. I want current, empirical evidence! So I went and got me some. I fly aerobatics, so at times I'm using a decent wodge of throttle. In F3A circles it seems that 100 cycles is considered about normal and 200 cycles is good. I had a pair of 3300mAh 6S packs that had about 200 cycles already on them at a 1C charge rate, so decided to sacrifice them in the name of science and immediately formulated a totally unscientific test: that from now on they shall be charged at 12 amps until they die. It's a minute sample size. There's no control. It's anecdotal. It's a really rubbish test but it is a real world evidence gathering exercise, done by Joe Punter having a rummage and sharing what comes up for whatever it may be worth. As my aerobatics aren't as power intensive as they are in F3A I figured that I should aim for double the number of cycles, so the aim of this shoddy, so-called "test" was to put another 200 cycles on each battery, only this time charging at 12A not 3.3A. (Mods - feel free to change the thread title to "Charging LiPos at 3.6363636363636363C" but I don't think it's quite as snappy) The rules: LiPos are only charged to 4.17v (aka "90%" but please use voltages!) and discharged to 3.80v (aka "50%"), which gives me a 7m30s flight +/- 0.05v, with roughly 1750mAh going back in. They are charged immediately before flying and then flown back down to 3.80v, so the whole charge/discharge cycle is about 20 minutes. On Johnny Nomates flying days they are put on to charge as soon as they come out of the plane while I go and fly what's just come off the charger - so no "cooling down" period - then rinse and repeat until brain-fade or nightfall sets in. The idea with the chargebox was that I could fly almost non-stop almost all day on any plane in my vast all-electric hangar while only owning two (sets of) LiPos for that plane. If only I'd thought of it before buying several LiPos for each plane I own... Anyway, going forward, it's definitely the way I'm doing it from now and I'll just have to try and insert a memory block about the dozens and dozens and dozens of LiPos that I never needed to buy over the years. If you're new to electric flight: put together a chargebox and buy one high-C LiPo per plane you own (but only if it takes different batteries to the rest of your hangar!) - it's the best advice I can possibly give you and will save you hundreds of £s - maybe thousands if you ever seriously move on to 10S or 12S - and keep you flying when everyone else is going home because "I've used up my battery". What's that saying... "The answers to all of your questions are written at the bottom of a 100 gallon drum of fuel." (there is no electric version of that quote, so it shall remain forever I.C.) I digress. I started the test in August and so far (11th April '24) they've done: 440 flights in a 6S 60" EF Slick - last year's pulled-the-short-straw autumn/winter hack & the most bonkers plane I've ever flown. Currently modelling the "several oz of mud" scheme. 10 flights in a 12S 74" EF Laser (10x2 packs) 50 flights in a 12S 74" EF Edge (50x2 packs) = 560 cycles ÷ 2 batts = 280 cycles each @ 12A, plus what they started with back in the days of 1C charging, so these two batteries will each be at 500 cycles by the end of this month and there's simply loads of life left in them yet. If this is the reduced battery life that people are telling me about then I'm fine with it - it's 1½p per (6S) flying minute, including charging up the Li-ions, and that's coming down all the time until they finally expire. They're not showing any signs of stress at all yet, though I never bother trying to measure capacity because it's going to take a long time for me to notice any reduction, given that I only use the 50% in the middle anyway. Internal Resistance is 1.1 to 1.2mΩ per cell (@19°C @24v pack voltage), which is what they settled down to after around 20 cycles and have remained at ever since. So, unless I've fortuitously bought the Willy Wonka Golden Ticket LiPos that are full of magic beans, I'm beginning to come to the conclusion that charging at high C rates is doing very little, if any, harm at all, and that battery damage is far more likely to come from charging all the way to 4.2v, poor throttle management, using the wrong C rating for your flying style, or "It says there's 14% left, so that's okay" which almost certainly means that you've actually just ruined your battery (I said use voltages!). I'm not having a go - I've been all of these people, which is why I've bought so many LiPos. You pays your money... I have another test going on, with four Zippy 6S 4500mAh 40C batteries, that's parked at the moment on 40 x 15A cycles each (not flying those planes just now) that is showing identical results, though I'm not drawing any firm conclusions until getting 200 extra cycles on them, which if they manage it will take them over 600 cycles. Tests are ongoing and I'll report back with more interesting info/tedious waffle. A future test will be to try this with a new battery - is it the 200 cycles at 1C that's somehow making it resilient to a higher C charge later on (chemically unlikely, I'd say), or can it sustain this kind of "abuse" from new? In the meantime, I'm trying to find out - ideally in layman's terms (no equations!) - whether the asymmetric process of reversible intercalation is at a fixed or variable ratio. In other words, does a high C rating for discharge automatically equal a proportionately high C rating for charging too? Surface area of the lattice seems to be key, which explains why high C batteries are bigger and heavier, but I'm not sure if the processes are exactly the same, just reversed, for insertion and extraction to/from the lattice because I'm rubbish at chemistry. Any electrochemical engineers out there?
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