Lipo capacity check?

[SIMON CRAGG]

Does anybody know how to check the capacity of a Lipo?

 

For instance, I would like to check my stock of 6s 3300 lipos, to establish how good / bad they are.

 

Two are at least three years old.

 

Keep it simple please!

[Philip Lewis 3]

Discharge on a charger then balance charge it and see how much goes back in. When new it should have been the stated capacity but the amount of charge it will take will reduce quite rapidly with age. A 4,000 mah can quite easily become a 3,000 mah at a few years old. If you can get the battery near to discharged before using the charger to do the final bit or it will take an age to discharge it on a charger.  

[SIMON CRAGG]

Thx, discharge to 3.2v per cell?

 

[Philip Lewis 3]

Whatever your charger discharges to, unless it's faulty it won't damage the LiPo but it will be something around that number.

[RedBaron]

Possibly more important than capacity is to measure the internal resistance of each cell. It is best to keep records as changes to the internal resistance are a good indicator of when to bin. Always measure IR at the same ambient temperature as previously as temperature of the battery affects the reading for IR

[MattyB]

IR and how well the cells balance is far more instructive than a low C discharge IMO. I have seen plenty of lipos that "pass" that kind of check, but measure their IR and individual cell voltages off charge and you can see a different story that will manifest pretty clearly in the air in terms of performance...

[Martin Harris - Moderator]

An interesting subject.  As an example. I flew 2 flights today [yesterday technically] on a 5000mAh 4S own-branded pack from a well known supplier.  It is marked as 35C continuous and 65C bursts and although not new has had relatively few charge/discharge cycles and is tight with no sign of ballooning.

 

Measured consumption on the two flights were 2911mAh and 2429mAh - both were showing less than 20% capacity on a checker and terminal voltages around 3.7v per cell immediately post flight. During charging after the first flight [with some residual warmth in the pack] IRs were close to 3mOhms on all cells.  From memory, the pack took in the region of 3500mAh and a monitored charge this evening took 3.03mAh on the charger and showed 3.059mAh on a Wattmeter in series with the output as a check which correlates very closely.  Charging the stone cold pack straight from an evening in the car showed IRs of 4mOhms +- 0.1 for the cells suggesting a nicely balanced pack in pretty reasonable condition.

 

Admittedly none of the equipment was to NPL calibration standards but it indicates to me that the stated capacities of our cells are not necessarily accurate.  I would expect to see more energy input than output but I'd say this fairly new 5000mAh pack has a real world capacity of somewhere in the order of 3000mAh.  Although I only checked this one pack, the results are in line with casual observation of my other packs charged on a variety of chargers over the years.

[leccyflyer]

17 hours ago, SIMON CRAGG said:

Does anybody know how to check the capacity of a Lipo?

 

For instance, I would like to check my stock of 6s 3300 lipos, to establish how good / bad they are.

 

Two are at least three years old.

 

Keep it simple please!

Put pack in model.

Fly model until you notice the performance has dropped off enough that you are no longer having fun*.

Land.

Check time.

Determine whether you are happy with the fun:time ratio.

That will illustrate how good/bad the lipos are, in the only test that really matters. 🙂

 

* Is highly subjective and strongly dependent on the pilot and model being flown. Unless in a competition, it's really the most important measurement to consider.

[GrumpyGnome]

I'm with Leccyfler. I shove them on balance charge, wack 'em into their 'charged' carrier, put one picked at random into plane, fly until power drops, put in 'discharged' carrier, put one picked at random etc. etc.

I rarely discharge on charger and can't remember using storage charge.

Some have lasted years with no issues. Some are a tad puffed, a few have noticeably less 'oomph' so they get relegated to lower power-hungry planes until useless.

 

Many of my chums have a far better charge/discharge/storage regime but they don't seem to have a noticeably longer life, or consistent performance. They also religiously check batteries after each flight and say, for example "5 minutes and 64% left" - but I've never received, when asked, what that actually means - is it at 64% of 4.2v left? Or some measure based on the recommended discharge voltage? Or something else. Also, the time is based on a basic timer that stops/starts at n% throttle, so that's a pretty meaningless measure as well.

 

I do treat them a little like consumables - I'll happily spend £20 for some glow fuel that'll last 6 weeks or so, so if my £15 LiPo lasts 12 months, it feels ok.

 

Here is my usual caveat...... I don't have anything above 4S; I only have a couple of small edfs and my prop planes aren't hugely power-hungry; I don't get to LVC on my prop' planes but regularly do with my little edfs; ALL of my packs come from the budget end of the price spectrum.

 

YMMV 🙂

[Mastercrashman]

I’ve looked at a number of videos and internal resistance IR is considered to be a good indicator of battery health.
 

Readings of 20 milliohm and above per cell would be a call for battery retirement.  Readings in the high teens would be an indicator that the battery is nearing retirement.

 

With my stock of 3S batteries, however, I find those readings to be at odds with the visual battery conditions.  I’ve got Turnigy and Zippy batteries over 3 years old slightly bloated with readings of less than 10 milliohm per cell. On the other hand I’ve got Gens Ace two years old and Overlander one year old no swelling at all, giving good performance and the internal resistances are in the high teens per cell.  I do have 3 batteries which I’ve found to be shockingly around 30 milliohm per cell which give reasonable performance but I’ve retired them.

 

I always balance charge my batteries and now reduce them to storage charge when I’m not flying, the latter I’ve only started to do quite recently. 

 

On his website, Painless360, discusses the issue in a short video titled “Why is Battery Internal Resistance Important?” Its worth a look.

[John Lee]

Those figures are too simplistic.

 

With cells in good health in general the larger capacity the smaller is its IR; so you need to know the starting point in order to monitor a trend. eg I have had 5000MaH cells measuring 2mΩ whilst the little indoor 100MaH cells show about 100mΩ on my charger, but both were new and in good health. 

 

Having said that I don't worry about it too much & am with Leccyflier in that I just use them until I notice an unacceptable drop in performance in flight.

[Mastercrashman]

Perhaps it’s more important that IC values for each cell are similar rather than setting a benchmark limit??

 

 

 

 

 

[Simon Chaddock]

The IR has less impact on cell 'performance' than the heat generated during a high rate discharge.

The heat dissipation for the middle cells in a battery has little to do with air flow but with the rate of heat conduction through the other cells. Only the outer cells can have any significant convection heat dissipation from air flow. 

The heat generated is proportional to battery capacity and cell IR but the rate of heat loss is determined by the surface area to capacity ratio of just the outer surface of the outer cells. A big 6s battery will be much more likely to suffer performance degradation from heat caused by a high IR than say a small 800mAh 3s.

Edited October 28, 2022 by Simon Chaddock

[leccyflyer]

You see all sorts of definitive statements about IR values online - such as that batteries are on their way out if their IR is above 20milliohms/cell and need to be retired - and again, I'd revert to that fun:time ratio I referred to earlier. Firstly in my experience the IR has quite a strong relationship to battery size and plenty of my smaller capacity batteries <1500mah have IRs of well above 20milliohms - some are in the 60's and a few of the really small packs are over 100milliohms/cell. The larger batteries 4000mah-5200mah are single digits when new and over time have gone into the 10s and 20s. Yet, somewhat surprisingly when put in the model and flown they do rather well in the fun:time stakes, so they don't get retired until they are either puffy, or show a drop in performance.

 

I'll add the same caveat as GG above, in that I'm not flying EDFs or helicopters, I'm mainly flying small to funfighter and some  larger scale models up to 65" span with a wide range of batteries from 1s1p40mah up to 6s1p5200mah.  I typically make five-six minutes flights, don't try to stretch it out  and find that's just about perfect for me. My lipos come off the model quite cool, generally no more depleted than down to 3.75v/cell, I don't "do" percentages and don't have more than one flight per charge.

[Allan Bennett]

2 hours ago, leccyflyer said:

You see all sorts of definitive statements about IR values online - such as that batteries are on their way out if their IR is above 20milliohms/cell and need to be retired - and again, I'd revert to that fun:time ratio I referred to earlier. Firstly in my experience the IR has quite a strong relationship to battery size and plenty of my smaller capacity batteries <1500mah have IRs of well above 20milliohms - some are in the 60's and a few of the really small packs are over 100milliohms/cell. The larger batteries 4000mah-5200mah are single digits when new and over time have gone into the 10s and 20s. Yet, somewhat surprisingly when put in the model and flown they do rather well in the fun:time stakes, so they don't get retired until they are either puffy, or show a drop in performance.

 

I must admit I don't pay much attention to IR, depending instead on how the pack performs (subjectively) in flight.  However one formula I did read a while ago was the IR should be somewhere around 12,000 divided by the pack's mAh.  IIRC the article suggested you could go to three times that value before the pack becomes useless.

[Trevor]

Some years ago I created this simple table as a guide to interpreting IR readings. Loads of caveats though:

It's based solely on my own experience and has no scientific underpinnings.

Measurements are taken at around 19 degrees C.

IR is not a true physical property so different devices measure it differently.

 

Use or dismiss as you wish!

 

Trevor

 

[Tim Kearsley]

"IR is not a true physical property" - I'm interested why you say this Trevor.  If you put a dead short across a battery the current that flows will be very large (but not infinite of course) and will be determined by Ohms Law - I=V/R.  The "R" in this case will be the total resistance offered by the wire plus the internal resistance of the battery.  The internal resistance is a very real, physical property.  To measure it may indeed be difficult to do accurately, but to say it isn't a true physical property is simply not correct I think.

[Trevor]

To enable us to do calculations, we generally model the cell of a battery as an ideal voltage source in series with a resistor. In reality, the  cell is a complex electrochemical device and neither the ideal voltage source nor the resistor exist. This is why, in technical literature, you will often find the abbreviation EIR is used - equivalent internal resistance.

 

At least, that’s how I’ve always understood it, but usual disclaimers apply!🙂

[MattyB]

4 hours ago, Tim Kearsley said:

"IR is not a true physical property" - I'm interested why you say this Trevor.  If you put a dead short across a battery the current that flows will be very large (but not infinite of course) and will be determined by Ohms Law - I=V/R.  The "R" in this case will be the total resistance offered by the wire plus the internal resistance of the battery.  The internal resistance is a very real, physical property.  To measure it may indeed be difficult to do accurately, but to say it isn't a true physical property is simply not correct I think.

I suspect what he means is that it is not a constant value - IR can vary significantly based on temperature and SOC of the pack, so measuring and viewing the IR result in isolation won’t necessarily tell you much.

[Tim Kearsley]

33 minutes ago, MattyB said:

I suspect what he means is that it is not a constant value - IR can vary significantly based on temperature and SOC of the pack, so measuring and viewing the IR result in isolation won’t necessarily tell you much.

No argument with that. Also, the DC resistance and AC impedance can be markedly different, not that the latter is particularly relevant in the context of us modellers' usage.  Yes Trevor, I understand what you're saying.  It's even more complicated than the source and resistor model as the cell has reactive components of capacitance and inductance.  These become significant when measuring the AC impedance.  I find it all fascinating, but then I am a confirmed, dyed-in-the-wool nerd!