Charging LiPos at 4C

[Duncan Disorderly]

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?

[Duncan Disorderly]

 

On 12/04/2024 at 17:07, Duncan Disorderly said:

these two batteries will each be at 500 cycles by the end of this month

 

My two 6S 3300mAh LiPos reached 500 cycles each today: 200 charges at 3.3A + 300 charges at 12A

 

This means 6S flying = 1½p/min, 12S flying = 3p/min, which breaks down as: LiPo replacement @ 500 cycles (70%), Li-ion replacement @ 4 years (20%), 75kWh electricity p/a to recharge the Li-ions (10%) - only I haven't had to replace any LiPos or Li-ions yet, but that's how the budget looks as of today

 

I'd have to fly a 6S plane for 3h20m before it cost more than the £3 of petrol to get to the field and back, so it's all academic now - if I could just stop buying planes

 

Anyway, it's not about the money, it's about charging at 4C, which they seem to be consistently shrugging off as a non-event

 

[Learner]

That's some impressive flying hours considering the weather we've had this winter.

[Wihtgar]

I struggle to get 50+ cycles out of my 2200maH lipos which are charged at 1C and have a very easy life. 

[Duncan Disorderly]

 

6 hours ago, Learner said:

That's some impressive flying hours considering the weather we've had this winter.

 

Yes - flying didn't fit around work very well, so I went freelance and now work fits in around flying. I can work when it's dark/windy/raining, so I rarely miss a flying day if there's one to be had. I'm a good pilot but it's all hard work and zero talent, so I have to put the hours in.

 

There are approx 15,000 manouevres in the Aresti book of aerobatic descriptions. I can learn - to a decent standard - about 3 per year, so if I want to reach 1% of them I'll have to go at it for 50 years... It's like playing a musical instrument - you get better all the time but you never reach "the end".

[Duncan Disorderly]

4 minutes ago, Wihtgar said:

I struggle to get 50+ cycles out of my 2200maH lipos which are charged at 1C and have a very easy life. 

 

I used to be you! I used to be able to take a perfectly decent LiPo and make it cylindrical...

 

It's why the first post in this thread is so long and boringly detailed - because I don't know which bit of my regime has suddenly stopped my battery malaise. I think it might be a combination of all of the things I listed but, weirdly, this all really took off when I started charging at 4C - the very act that everybody told me would ruin everything.

 

I think there's something in having them only holding a charge for 10 mins; there's definitely something in only charging them to 4.15v or so (this won't affect your flight times at all and may double your battery life at a single, simple stroke - and, if you think about it, there's absolutely no incentive for battery manufacturers to advise you to do this), and leaving 50% in at the end of the flight seems to really help too - but I appreciate that you need a charge box to do this effectively, or you run out of LiPos pretty quickly.

 

I think the charge box is the answer to electric flight - since having it I haven't bought a single LiPo and have had hundreds of flights. That's with a charging regime where I've actually tried to kill the LiPo with 12A. You've already got a charger and my 40Ah of Li-ions cost less than £100 (I made 2 packs, so £200, but just scale it to your needs - at 3S you could get away with a £30 Li-ion pack), which means I only have to buy 2 LiPos for any plane I own, and can fly all day pretty much non-stop. It then allows you to put into practise a MUCH better regime for charging, because they're not sat holding charge for 24 hours and you're not trying to drag every last millivolt out of them - you just give them a quick 50% topup and off you go again.

 

Also - throttle! Most people use way too much. Which is fine if that's what you like doing, but it will cost.

 

Also - high C batteries - maybe try one?

[Edgeflyer]

Which li_ion cells are you using for your charge box? 

[Ron Gray]

I use the ones in my car 😉
 

But ever since the initial post on increasing the charge rate I now only charge to 4.17v and charge at 3C.

Edited April 27 by Ron Gray

[Wihtgar]

Do you guys use chargers that you can set the final charging voltage or do you monitor and stop at the charger at the appropriate time?

[Edgeflyer]

I can set final voltage but my charger is limited to 6 amps

[Ron Gray]

1 hour ago, Wihtgar said:

Do you guys use chargers that you can set the final charging voltage or do you monitor and stop at the charger at the appropriate time?

Final voltage 

[Duncan Disorderly]

14 hours ago, Edgeflyer said:

Which li_ion cells are you using for your charge box? 

 

LG M50LT 21700 from https://www.cellsupply.co.uk/21700-batteries

 

https://www.flyingtech.co.uk/product/isdt-air-8-dc-portable-smart-balance-charger-500w-20a-1-8s/ are marvellous little things

 

[Duncan Disorderly]

4 hours ago, Wihtgar said:

Do you guys use chargers that you can set the final charging voltage or do you monitor and stop at the charger at the appropriate time?

 

As per Ron, final voltage

 

https://www.flyingtech.co.uk/product/isdt-air-8-dc-portable-smart-balance-charger-500w-20a-1-8s/

[Duncan Disorderly]

8 hours ago, Ron Gray said:

I use the ones in my car 😉
 

But ever since the initial post on increasing the charge rate I now only charge to 4.17v and charge at 3C.

 

How is it going? Very interested to know - good or bad. Russ and I are a very small sample size so it'd be good to increase the depth of the knowledge pool.

 

Those car packs are amazing. 96S 72P or something. Keep you flying for a whole year!

[Ron Gray]

So far I've seen no difference in performance in my batteries which range from humble 3s 1300mAh to 6s 5000mAh ones, although I haven't really done a lot of charging of the big packs as they tend to be used in my EDFs which haven't been flown a lot recently. The ones that have been used a lot are my 4s 3700mAh ones and they are just the same as before I started the new charging regime. So I will continue doing what I'm doing and charging at the field using the V2L from my EV and maybe even push it a bit and go to 4C!

[Duncan Disorderly]

13 minutes ago, Ron Gray said:

maybe even push it a bit and go to 4C!

 

Go for it! Are they high C rated? Russ is currently using Zippy 40C 5S in pairs to make 10S 4000 and they're happy too at 15A, so it's not just the heavy duty Turnigy, and has got me wondering how low the C can go...

[Ron Gray]

The 4S 3700mAh are 40C and the 6S 5000mAh are 70C.