Battery Capacity Checking

[Andrew McKelvey 1]

I have just reinvested a healthy chunk of money in buying some new li-po batteries in the belief that the old ones I have may no longer be up to it. aka 4cell 3300mAh 60C as compared to the same with 20C and being about 6 years old or more.

The comparison using my wattmeter with fully charged batteries in my plane setup gives nearly a 100 w difference , ie a difference of about 6 - 8A draw at full throttle and the older battery being about a volt or more down on the new one. I know this is down to internal resistance of the battery which has worsened with age and I could do comparison checks of my older battery's against new one but I am reluctant to do a load of testing with an 12x6 prop wizzing round at full chat just to provide the load plus the wind factor isn't good in the model room.

 

The questions!

 

1 How do you test an battery for capacity and obtain comparative results

2 If you manage to compute the internal resistance of said item how do you determine a pass or fail

3 Is there a method or a bit of technology I can use that will mean I am not holding down my plane and applying full throttle and trying to read the numbers on my wattmeter.

 

Your thought please will be gratefully received, I have more than enough electrical test equipment to do the measurements I just dont know what the results mean other than the old battery makes the plane unflyable and the new ones make it a pleasure to fly.

[John Stainforth]

10 minutes ago, Andrew McKelvey 1 said:

I have just reinvested a healthy chunk of money in buying some new li-po batteries in the belief that the old ones I have may no longer be up to it. aka 4cell 3300mAh 60C as compared to the same with 20C and being about 6 years old or more.

The comparison using my wattmeter with fully charged batteries in my plane setup gives nearly a 100 w difference , ie a difference of about 6 - 8A draw at full throttle and the older battery being about a volt or more down on the new one. I know this is down to internal resistance of the battery which has worsened with age and I could do comparison checks of my older battery's against new one but I am reluctant to do a load of testing with an 12x6 prop wizzing round at full chat just to provide the load plus the wind factor isn't good in the model room.

 

The questions!

 

1 How do you test an battery for capacity and obtain comparative results

2 If you manage to compute the internal resistance of said item how do you determine a pass or fail

3 Is there a method or a bit of technology I can use that will mean I am not holding down my plane and applying full throttle and trying to read the numbers on my wattmeter.

 

Your thought please will be gratefully received, I have more than enough electrical test equipment to do the measurements I just dont know what the results mean other than the old battery makes the plane unflyable and the new ones make it a pleasure to fly.

Six years for a LiPo! Is that common? I've not achieved anything like that.

[Dickw]

47 minutes ago, Andrew McKelvey 1 said:

............................

The questions!

 

1 How do you test an battery for capacity and obtain comparative results

2 If you manage to compute the internal resistance of said item how do you determine a pass or fail

3 Is there a method or a bit of technology I can use that will mean I am not holding down my plane and applying full throttle and trying to read the numbers on my wattmeter.

 

Your thought please will be gratefully received, I have more than enough electrical test equipment to do the measurements I just dont know what the results mean other than the old battery makes the plane unflyable and the new ones make it a pleasure to fly.

1. Most of our battery chargers have a discharge ability. If I discharge packs at the same current setting it will allow comparison of the relative capacity between packs.

2. My chargers will also measure internal resistance. I don't really have a pass or fail value, but use the better cells in more critical applications. Packs are scrapped when they won't do any of the jobs I need doing rather than any fixed value.

3. Yes, full throttle while handling a meter can be "fun" 😀. I gave up doing that some years ago and invested in data logging. On board loggers are available, but if you have a telemetry radio set with logging in the Tx that is now more common using current etc. sensors in the model. Just run the motor or fly the model and read the data later.

I have attached an example of a flight with my TN Gnat mini EDF yesterday to show what is possible.

 

Dick

 

[Andrew McKelvey 1]

I didn’t fly for about three of those due to the pandemic so they where stored with a storage charge and never abused before hand. I have one of those data loggers that got a write up in RCM&E a few years ago and have used that, my charger will only discharge at a few amps, I was hoping to emulate a real life situation and pull about 40A and see what that did to voltage etc but apart from giving me some electrical values how does that equate to how good the battery is?

 I have only ever binned one battery that puffed up severely, they seem to get demoted to powering hand drills etc but discovered that Li-On cells from laptop packs do a good job there after the ni-mh and ni-cads pack in. Requires surgery with a dremel but at least keeps then out of the landfill.

[Frank Skilbeck]

Dick, what is the data analysis program you are using, I used to have that on an old computer and I can't find it again. Thanks. 

Edited April 21, 2023 by Frank Skilbeck

[Dickw]

Andrew

If you pull 40A from a 3s Lipo without spinning a prop you are looking at something something between 450 and 500 watts which is a lot of heat to dispose of. I have seen large wire-wound resistors used to provide that sort of load  e.g use 3 in parallel RS components 1ohm 150 watt resistors part no 175-2462 (about £18 each) but that gets expensive if you want different loads and voltages. You could also use light bulbs to provide the load - e.g 10 x 50W 12 volt bulbs in parallel would also pull about 40A from 3s Lipo, but again you need different setups for different loads and voltages.

That is why I moved to using a data logger to just use the planned motor/prop/battery set up to do the testing with "on the fly" as shown in my previous post. You can also do the testing on the ground but at least it is only the model you are holding and not a meter as well.

 

You want to "pull about 40A and see what that does to the voltage". The voltage will drop on load in line with Ohms law and so if your charger can also measure battery IR you can calculate the voltage drop to the ESC - i.e. it will be battery off load voltage minus (battery IR' x 'current').

 

You ask about electrical values “how does that equate to how good the battery is?”

Well in general the lower the IR the better the battery, but since IR varies with battery size, state of charge and temperature, you can only really compare similar batteries to see which is better.

 

For competition use which battery is better can be important, but for general flying I just watch the plane and see if it is performing as expected. Of course, if you use in flight data logging you can compare the performance of a battery over time and see if it is deteriorating.

Dick

 

Edited April 21, 2023 by Dickw

[Dickw]

2 minutes ago, Frank Skilbeck said:

Dick, what is the data analysis program you are using, I used to have that on an old computer and I can find it again. Thanks. 

Frank

That is DataExplorer.

https://www.nongnu.org/dataexplorer/download.html

 

Dick

[Andrew McKelvey 1]

DickW I think I may have found an answer on the t internet, There is a dc test to calculate battery internal resistance,  open circuit voltage (OCV) with no load, followed by the second reading with a load; Ohm’s law calculates the resistance value (voltage difference divided by current equals resistance).= internal resistance. My guess would be the higher the load current the more accurate the result? I also found a video of a guy explaining the in’s and out’s of lipo’s, not sure his qualifications but he sounded like he knew what he was on about. His take on it was New battery’s of various capacities return an resistance of below 10micro ohms and continue to be usable up to 20 micro ohms and any thing above that should be retired. I normally use a 12v 60w headlamp bulb to discharge any unused lipos back to storage voltage (actually x2 to increase the current) The effect of the higher internal resistance reduces terminal voltage under load, the prop spins slower and causes more heat to be generated in the battery which makes it hotter and thus not what we want. This has taken most of the day so now I need to test my older lipo’s, thanks for the input guys

[Dickw]

I think you are getting there 👍

He is probably talking about "per cell" IR rather than per pack, so if you measure your pack IR as suggested you need to divide by the number of cells in series to get the cell IR.

Dick

[Frank Skilbeck]

3 hours ago, Dickw said:

Frank

That is DataExplorer.

https://www.nongnu.org/dataexplorer/download.html

 

Dick

Dick, thanks, it didn't come up when I typed data explorer into the search engine.

[Frank Skilbeck]

2 hours ago, Andrew McKelvey 1 said:

DickW I think I may have found an answer on the t internet, There is a dc test to calculate battery internal resistance,  open circuit voltage (OCV) with no load, followed by the second reading with a load; Ohm’s law calculates the resistance value (voltage difference divided by current equals resistance).= internal resistance. My guess would be the higher the load current the more accurate the result? I also found a video of a guy explaining the in’s and out’s of lipo’s, not sure his qualifications but he sounded like he knew what he was on about. His take on it was New battery’s of various capacities return an resistance of below 10micro ohms and continue to be usable up to 20 micro ohms and any thing above that should be retired. I normally use a 12v 60w headlamp bulb to discharge any unused lipos back to storage voltage (actually x2 to increase the current) The effect of the higher internal resistance reduces terminal voltage under load, the prop spins slower and causes more heat to be generated in the battery which makes it hotter and thus not what we want. This has taken most of the day so now I need to test my older lipo’s, thanks for the input guys

Bear in mind that the OCV is linked to the state of discharge of the pack, so you'd need the average of the starting OCV and finishing OCV, but if you are only doing it for a couple of seconds then the difference will be negligible.

[Andrew McKelvey 1]

The video I watched showed typical resistance values for a new battery 4 cell of between 6 and 8 micro ohms, shown on the charger per cell, so adding them together is more than 20 micro ohm so that would write off a new battery so I took it as a value for each cell of over 20 would be enough to write off a battery of cells. My chargers don’t calculate the cell resistance, probably because they are circa 10 years old and reasonably cheap.

Edited April 21, 2023 by Andrew McKelvey 1

[Simon Chaddock]

The other consideration is what capacity the battery really had when it was new.

There was a period when the marked capacity (like c rating) was possibly a bit optimistic and either simply could not be achieved or only by discharging the battery at a low rate and to a level that caused permanent damage.

The only true test would be measure a particular discharge capacity to a given voltage when it was new and compare it to the same test done x years later.

 

I have an example of a 7 year old 5000 mAh 2s that is 25% lighter than a modern equivalent with the same c rating. I seriously doubt the original ever had the claimed capacity. It certainly dropped to 3.7V/cell well before the claimed capacity was discharged even at a tiny 0.1 c (one amp) rate.

 

The electric vehicle battery industry are beginning to claim measured figures for the number of discharge cycles to a specific capacity but then their batteries are continually monitored from new and charged/discharged at low rates compared to an aircraft LiPo.    

 

    

[Cuban8]

On 21/04/2023 at 10:19, Andrew McKelvey 1 said:

I have just reinvested a healthy chunk of money in buying some new li-po batteries in the belief that the old ones I have may no longer be up to it. aka 4cell 3300mAh 60C as compared to the same with 20C and being about 6 years old or more.

The comparison using my wattmeter with fully charged batteries in my plane setup gives nearly a 100 w difference , ie a difference of about 6 - 8A draw at full throttle and the older battery being about a volt or more down on the new one. I know this is down to internal resistance of the battery which has worsened with age and I could do comparison checks of my older battery's against new one but I am reluctant to do a load of testing with an 12x6 prop wizzing round at full chat just to provide the load plus the wind factor isn't good in the model room.

 

The questions!

 

1 How do you test an battery for capacity and obtain comparative results

2 If you manage to compute the internal resistance of said item how do you determine a pass or fail

3 Is there a method or a bit of technology I can use that will mean I am not holding down my plane and applying full throttle and trying to read the numbers on my wattmeter.

 

Your thought please will be gratefully received, I have more than enough electrical test equipment to do the measurements I just dont know what the results mean other than the old battery makes the plane unflyable and the new ones make it a pleasure to fly.

As a retired electronics engineer, I'm all for a scientific approach to problem solving and analysis but I do often wonder at the way many posters get themselves tied up into knots and become stressed out when it comes to lipos. Needlessly, in my view.

Worrying to the Nth degree over a few millivolts of mis-balance between cells or creating highly detailed discharge and internal resistance curves is IMHO way beyond what most hobbyist/Sunday flyers really need. Lipos do deteriorate over time and and well used cells will fall away in performance, much like a worn and tired old glow motor, but we don't usually worry about those  and subject them to all manner of tests.

Personally I'd be inclined to keep things simple, a model's ongoing performance will give you loads of info about how its battery is managing - eyes, ears and a wristwatch will be just as useful as will checking the system with a Watt meter from time to time. You said yourself, "I have more than enough electrical test equipment to do the measurements I just dont know what the results mean other than the old battery makes the plane unflyable and the new ones make it a pleasure to fly"

QED.

 

Enjoy your flying above all else.

 

[Andrew McKelvey 1]

Someone somewhere thinks there is validity in being able to measure cell resistance, my question is what do you do with the answer, no one has gone near an answer yet

[Masher]

I measure all new batteries and make a note of the result. If I found any discrepancies or wide variation I would question why. Either go through a few cycles of charge/discharge and/or speak to the supplier. Assuming all the cells are similar and about right (less than 10mOhm) I put a date label on the battery and use it. Then I can refer back to original readings if ever the battery seems less than normal in flight. Just like 'puffiness' internal resistance is a good measure of battery quality and condition. I too am a retired electronics engineer so this little routine keeps my technical side happy. However guess what? I don't think I've ever gone back and checked against original figures ........ just check what goes in and out of the battery versus typical flight you soon know if something isn't right