What's up with my Lipos?

[paul devereux]

I operate on a tight budget - I've only got two Lipos ( 3s 2200s) and an ImaxB3 charger. A few months ago I had about 10-20% power left in each battery at the end of a 13 minute flight, now I am lucky to get 9 minutes before the power drops off. I normally do two flights a day as 20 minutes is enough for me (I am just learning). The  batteries each take about 3 or 4 hours before the three green lights show it is charged. Obviously I'll get a new Lipo or two, but I wondered if this drop off in power is normal?

[leccyflyer]

Personally I don't "do" percentages of remaining capacity, I'm more interested at the voltage at the end of the flight. That said, if you are routinely running your batteries down to 10% of their capacity that is going to shorten battery life and yes, that sort of reduction in flight time can occur.

 

I'm puzzled why you are charging your batteries at  less than 1/3C though - what is the charge rate of your battery charger?

[paul devereux]

2 minutes ago, leccyflyer said:

Personally I don't "do" percentages of remaining capacity, I'm more interested at the voltage at the end of the flight. That said, if you are routinely running your batteries down to 10% of their capacity that is going to shorten battery life and yes, that sort of reduction in flight time can occur.

 

I'm puzzled why you are charging your batteries at  less than 1/3C though - what is the charge rate of your battery charger?

This is the specs of the ImaxB3:

• Charging current: 850 mA
• Input voltage: 110-240 V
• Cells in battery: 2-3S
• Charging power: 10W

iMax B3 charger | RC Factory (rc-factory.eu)

Edited May 25, 2023 by paul devereux

[Frank Skilbeck]

Unfortunately a drop off in performance of Lipos is pretty common, especially if they are regularly discharged to very low levels. But flight time is also governed by how much throttle you use, as you've become more experienced you might be giving it a bit more 'wellie' 

[leccyflyer]

That's a rather low charge rate, which some might advocate as being good for battery longevity, though I'm not convinced. I think the main reason for your reduced capacity is the running of the battery down to a low voltage. If you were getting 13 minute flights your power train must have a modest current draw, so you were likely extending your flights down to low voltages on the cells under load. Were you running the batteries until the low voltage cut out operated on each flight?

[PDB]

26 minutes ago, leccyflyer said:

Personally I don't "do" percentages of remaining capacity, I'm more interested at the voltage at the end of the flight.

As a newbie to electric flight, what sort of voltage would be considered acceptable at the end of a flight?

[steve too]

Use a charger that tells you how much it has put into the battery. Adjust your flight time such that you are not taking more than 2/3 of the nominal capacity out of the battery. Try to avoid discharging at more than 20x the nominal capacity. If you have telemetry, set a voltage alarm that is .1V below the lowest voltage that you get on a flight where you take 2/3 out of the pack.

[MattyB]

30 minutes ago, PDB said:

As a newbie to electric flight, what sort of voltage would be considered acceptable at the end of a flight?

I always aim for a voltage at rest of ~3.7V/cell after flight, certainly no lower than 3.6V/cell. This should ensure you can achieve decent cycle life on your lipos, providing you are not pushing them beyond their actual realistic C ratings (not the made up ones printed on the outside of the pack…!).

 

Some example typical discharge curves that may help…

 

 

Edited May 25, 2023 by MattyB

[leccyflyer]

34 minutes ago, PDB said:

As a newbie to electric flight, what sort of voltage would be considered acceptable at the end of a flight?

I aim to land with at least 3.7v/cell and set my timer accordingly.  For storage voltage I use 3.8v/cell and in the majority of cases the packs come back home without needing to put them on a storage charge.

[paul devereux]

52 minutes ago, leccyflyer said:

If you were getting 13 minute flights your power train must have a modest current draw, so you were likely extending your flights down to low voltages on the cells under load. Were you running the batteries until the low voltage cut out operated on each flight?

It's a Wot4 foam-e, I'd set the timer for 13 minutes, flying at about just over half throttle (quite close to the stall, actually). Since I have become more confident as a pilot I fly faster, about 80% throttle sometimes. I had noticed a drop-off in power just before the 13 minute timer sounded, which is why I reset the timer to 9 mins, now one battery lost power just before that.

[paul devereux]

17 minutes ago, MattyB said:

I always aim for a voltage at rest of ~3.7V/cell after flight, certainly no lower than 3.6V/cell. This should ensure you can achieve decent cycle life on your lipos, providing you are not pushing them beyond their actual realistic C ratings (not the made up ones printed on the outside of the pack…!).

 

Some example typical discharge curves that may help…

 

 

 

8 minutes ago, leccyflyer said:

I aim to land with at least 3.7v/cell and set my timer accordingly.  For storage voltage I use 3.8v/cell and in the majority of cases the packs come back home without needing to put them on a storage charge.

I don't really understand what 3.7v/cell means I'm afraid. Though I get @steve too saying use only 2/3rds of the battery power. I guess in practise that means getting new Lipos and leaving the timer at 9 mins- that would be using 2/3rds of the charge?

[leccyflyer]

3.7v/cell is the voltage measured for each cell,  via the balance lead. You must not let the voltage/cell drop below 3.3v/cell or you rosk damaging the batteries.  Measuring the voltage of each cell in a pack is a simple means of monitoring the health of the battery - once the battery begins to degrade and get "tired" through use and abuse, there will be an inbalance between cells - they will have a slightly different voltage which eventually manifests itself as reduced capacity, since the battery is only as good as the weakest cell. That would be compatible with what you are seeing in very long charge times - your charger is trying it's best to balance the cells and get them all to 4.20v- so has a long period with very low charge current to try and achieve that.

[steve too]

@paul devereux Get a charger with a display and a cheap checker which shows you the cell voltages when you plug it in the balance lead.

 

PS This sort of checker will do, but ignore the percentage. 

https://www.componentshop.co.uk/g-t-power-digital-battery-capacity-checker.html.html

PPS Basic checker.

https://www.robotbirds.co.uk/lithium-battery-voltage-checker-2-6s.html

Edited May 25, 2023 by steve too

[leccyflyer]

13 minutes ago, paul devereux said:

It's a Wot4 foam-e, I'd set the timer for 13 minutes, flying at about just over half throttle (quite close to the stall, actually). Since I have become more confident as a pilot I fly faster, about 80% throttle sometimes. I had noticed a drop-off in power just before the 13 minute timer sounded, which is why I reset the timer to 9 mins, now one battery lost power just before that.

 

That's a very good flight time -13 minutes - for a Wot-4 Foam=E. Sounds like your throttle management is perfect fine. I have my timer set for 7 minutes on mine and land at something like 3.75-3.8v/cell on a quite ancient Turnigy 30C 2200mah 3s1p pack, but quite a lot of a typical flight is flat out.

[PDB]

4 minutes ago, paul devereux said:

 

I don't really understand what 3.7v/cell means I'm afraid. 

Probably my most used piece of field equipment, many rebadged variants of the same device available.

 

https://www.nexusmodels.co.uk/fusion-smart-guard-3-lithium-battery-checker-balancer-p-fs-bc07.html

 

For IC I use it to check the flight pack before each flight and if you connect a Lipo balance lead the total pack voltage is initially displayed, pressing the cell button displays the voltage level per cell which I do before and after each flight.

 

 

[leccyflyer]

As PDB says, some form of battery checker is really a vital accessory for monitoring how your lipos are fairing and they are quite inexpensive. They probably aren't lab quality instruments in terms of accuracy, but close enough for jazz and certainly good enough for monitoring if the battery has been over-discharged in flight. Personally I check the v/cell of my packs after every flight, then check them when I get home to see if any of them needs a storage charge.

[Cuban8]

So many variables associated with the performance of our Lipos and their longevity, that it's near impossible IMHO, to predict and give figures with any real certainty. Initial quality, how they're charged, how they're used and stored, ambient temperature, and probably several other factors as well.

Ignoring a few simple basic rules about their use does have a bad effect on them, and there's plenty of info covering those here and elsewhere.

Paul, don't beat yourself up over all of this - flight times will vary, as how can we be expected to repeat exactly the same flight pattern each and every time? For instance windy conditions will play havoc with duration compared to a nice calm day.

For information, my regime that I've used for ages is along the lines of......

Charge at 1-1.5C

Take battery manufacturers' claims of C ratings and capacity with a bushel of salt and use them accordingly. The reason that AFAIA, batteries don't feature in proper magazine tests and reviews along the lines of how IC motors used to be reviewed (Peter Chinn from the dim and distant past) and for obvious reasons.

Note battery temperature, post flight - if it's any more than tepid, then chances are you are pushing things and putting the battery under stress, or have a pack that is well past its best even when used gently.

Post flight capacity check - no lower than 20% remaining. This is a very rough indicator as is voltage measurement given by the little black battery checkers that most of us use on the field.

Lipos don't last forever and if you can get a couple of seasons of good regular use from them, I reckon that's doing pretty well.

 

 

Edited May 25, 2023 by Cuban8

[Simon Chaddock]

Paul

I appreciate you said you are on a tight budget but I would strongly suggest you get a better charger. Something like the ImaxB6AC. It is a mains charger although there is a cheaper version available if you have a suitable 18V power supply.

The big advantage is you can set the charge rate (usually1c that's 2.2A for a 2200mAh) for the size of battery and it tells you via an LCD screen what the individual cell voltages are all the time it is charging so you know what they started and finished at. It also shows how many mAh have gone in..

You should not leave a LiPo unattended. Its rare but a LiPo can catch fire during charging if anything goes wrong.

 

As has been said it is very tempting to fly on until the power falls away but if when under load the individual cell voltages of a LiPo fall below 3V it will permanently damage the cell a bit each time you do it. The simplest way is to note the cell voltage when at rest after the flight. If it is below 3.7V, there are simple battery checkers that do this, then that flight most likely took some life out of the battery. It takes some experimentation to find out what type and duration of flight is kindest to the battery. The high power to weight of a LiPo has some limitations. 

 

Your existing 2200mAh may now be past their best but a good charger and a battery checker will help you get the best out of any new ones!      

 

 

Edited May 25, 2023 by Simon Chaddock

[Cuban8]

5 minutes ago, Simon Chaddock said:

Paul

I appreciate you said you are on a tight budget but I would strongly suggest you get a better charger. Something like the ImaxB6AC. It is a mains charger although there is a cheaper version available if you have a suitable 18V power supply.

The big advantage is you can set the charge rate for the size of battery and it tells you via an LCD screen what the individual cell voltages are all the time it is charging so you know what they started and finished at. It also shows how many mAh have gone in..

You should not leave a LiPo unattended. Its rare but a LiPo can catch fire during charging if anything goes wrong.

 

As has been said it is very tempting to fly on until the power falls away but if when under load the individual cell voltages of a LiPo fall below 3V it will permanently damage the cell a bit each time you do it. The simplest way is to note the cell voltage when at rest after the flight. If it is below 3.7V, there are simple battery checkers that do this, then that flight most likely took some life out of the battery. It takes some experimentation to find out what type and duration of flight is kindest to the battery. The high power to weight of a LiPo has some limitations. 

 

Your existing 2200mAh may now be past their best but a good charger and a battery checker will help you get the best out of any new ones!      

 

 

Absolutely correct. The Imax B3 charger is hopeless and you really need one of their four button chargers that can be picked up for not a lot of money, especially the non-mains type.

[Cuban8]

Imax B6 available on Ebay for under twenty quid including postage - perfectly good bit of kit, I use one myself for my smaller (2200 - 3000ish) 3 cell packs.

Mains version (which I also have) available for about thirty quid.

Battery checkers from 4-Max for about twelve quid or half that on Ebay if you can wait  for delivery from China. Plenty of reviews and tutorials on YouTube regarding Lipo use and charging etc.

Edited May 25, 2023 by Cuban8

[RedBaron]

To check your new and old batteries, there really is a strong case to check the internal resistance of each cell and it there is any significant deviation in resistance between cells then its bin time. A new battery is cheaper than a new model. There are specialised resistance checkers like the "ESR-Meter" but I believe that some modern chargers have this function.

Keep records but always check at the same ambient temperature else past data wont help, small temp changes equals different resistance measurements. 

[Cuban8]

A couple of meters (V and A) and some simple arithmetic will also do the job - but I'm not sure if the info gleaned either by a dedicated instrument or a couple of DVMs will be overly helpful to the average modeller beyond other more simple practical measures.  Interesting to know though - but never felt the need to bother.

[MattyB]

4 hours ago, leccyflyer said:

 

That's a very good flight time -13 minutes - for a Wot-4 Foam=E. Sounds like your throttle management is perfect fine. I have my timer set for 7 minutes on mine and land at something like 3.75-3.8v/cell on a quite ancient Turnigy 30C 2200mah 3s1p pack, but quite a lot of a typical flight is flat out.

 

Based on the fact it appears the OP has damaged his packs, I'd say it's more in the "too long" category  - as noted by @Simon Chaddock above, if he is flying until he's noticing a significant drop in power, that is well past the optimum point for pack life.

 

My method for setting the timer for a first flight of a tractor sport aerobat or trainer:

• Run up the motor at full power ont he wattmeter and measure peak current (e.g. 30A)
• Calculate 70% of that (e.g. 30 * 0.7 = 21A)
• Work out what 2/3 of your battery capacity is (e.g. for a 2200mah, it's 2/3 * 2200 = ~1470mAh, or 1.47Ah)
• Work out how long you could run at 70% of peak amps and use 2/3 of your battery's capacity (e.g. (1.47/21) * 60 = 4.2 minutes)

Note - This is a very conservative calculation suitable for a maiden flight, and should result in your coming down with plenty left in your pack. Once you get down, measure the individual cell voiltages, compare those against a std lipo curve to work out roughly how much of the pack you have used, and adjust your timer accordingly.

Edited May 25, 2023 by MattyB

[kevin b]

Obviously Paul (like me) isn't into technical electronics.

I think the best advice is to set your timer to about 7-8 minutes Paul.

If you timer is set to 9 minutes it will probably be nearer 11 minutes before you get on the ground.

Check the lipo at the end of the flight and try to aim for 35-40% left at the end of the flight.

Better to have more batteries and a lot of short flights, rather than long flights and dead batteries.

That's what I do and some of my batteries are nearly 10 years old !

You probably will need to upgrade your charger as well if you are buying more batteries.

Preferably one which will charge several at the same time.

Also best to charge at 1C. That means for a 2200ma lipo you charge it at 2.2 amps and a 1500ma lipo 1.5 amps.

at the moment Ripmax are doing some good deals on 22ooma lipos (cheaper than buying from China)

I have a couple and they are ok for general sport flying.

Edited May 25, 2023 by kevin b

[paul devereux]

Thanks to everyone who responded. Sub-10 mins doesn't give much flight time per battery does it?  I'm a bit surprised it is so short. Though obviously worth it to get more longevity from the batteries. I didn't realise I should have aimed to keep a minimum charge either, I thought it was better to drain them and then recharge like the Tx battery.