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Capacity checker accuracy?

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Keith Miles 222/01/2020 19:33:29
303 forum posts
6 photos

Traditionally, I have been an IC flyer.

I bought my first electric model about four or five years ago, a palm sized quadcopter, since when I have added a number of similar small helicopters, a couple of small fixed wing park flyers and a Mini Wot4. I recently bought a Max Thrust Riot. Despite this limited collection, I have a 540w power supply and an I-Charger 208B.

From the outset, I have noted the wide use of pocket sized battery checkers but given the fairly flat discharge curves of the battery types commonly used, I have always doubted their accuracy and, therefore, how useful they really are especially when compared with the re-charge information provided by most, if not all modern and popular chargers.

Someone used a very popular model of battery checker today, post flight, on one of my Riot LiPo batteries and it, apparently, read 93% discharged after a five minute flight with a slightly higher pitch prop.

When I later recharged the 3000mah battery concerned, the charger put back, in fact, 2086 mah.

So that would make the checker out by 25%, albeit on the safe side!

So, I would be interested to hear if anyone else has had the same experience with these “convenient” and “pocket sized” devices?

 

Edited By Keith Miles 2 on 22/01/2020 19:34:14

Bruce Collinson22/01/2020 19:59:50
511 forum posts

Keith,

I’m about 70% electric, 1 to 6S, but no electrician. These generic multi battery checkers are so useful that I have about 6 of them, one permanently in the cargo pocket when flying. I have heard stories of inaccuracy but have not knowingly seen it. Generally, when checking for charge, they agree with the charger, both depleted and fully charged.

iI have just checked a storage charged new 6S LiPo against a digital multimeter and they were within 5% of each other.

One of my meters is a considerably dearer version ... Hyperion? ... but I suspect it runs the identical circuit/chip to the £5 Ebay versions as it has an identical screen and gives identical readings.

Where does that leave us?

BTC

Allan Bennett22/01/2020 20:19:47
1654 forum posts
49 photos

As you mention, some batteries have quite a flat discharge curve (as measured by voltage), so it's difficult for any meter to give an accurate measure of capacity other than by discharging to, say 3.5v and then measuring what's put back when charging it. The only checker I use in the field is one that gives me per-cell voltage readings.

To decide what my flight duration should be I keep a record of how much is put back into the battery when charged, and then maybe adjust the duration of my next flight based on that info.

Keith Miles 222/01/2020 20:45:01
303 forum posts
6 photos

Bruce,

Where, indeed?

At least one would seem to be wildly inaccurate, unless it was just having a bad day!

I can see how they might be fairly accurate at the top and bottom end, given the wider voltage differential but it’s the bit in the middle (the majority of the working capacity) where the voltage differences are very small, that intrigues me, especially for a cheap device.

Unless I am very much mistaken, capacity cannot be measured directly so “ capacity checker” is also a bit of a misnomer, in my opinion.

As for top voltage denoting full charge, that ain’t necessarily true either. I have had a number of small LiPos that charge to 4.2v only to last, in some cases, just a few seconds in my little helis! So, a voltage measurement, especially off load, is not a definitive piece of information concerning capacity or battery condition.

Keith Miles 222/01/2020 20:46:30
303 forum posts
6 photos

Allan,

Ditto!

smiley

john davidson 122/01/2020 20:48:04
54 forum posts

If one cell has packed in the checker can be tricked into seeing a three cell as a two and give a totally false reading

Frank Skilbeck22/01/2020 21:02:42
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4681 forum posts
101 photos

Also bear in mind that Lipos lose capacity over use, especially if you run them right down, so what was a 3,000 mah battery when new might now only be a 2200 mah battery, also wait 15 mins or so before using your checker you may see the voltage rise and the capacity come up.

I use the capacity checkers as a rough check, but use a telemetry ammeter to alarm after xxxx mah rather than rely on a timer.

Andy4822/01/2020 21:09:14
1495 forum posts
8 photos

I've used a couple of such battery checkers and frankly they have ended up in the bin as next to worthless. All they can do is measure the voltage. Now I use a current sensor and this give a very accurate reading of the capacity, and invariably matches the reading on the battery charger.

RC Plane Flyer22/01/2020 21:14:21
711 forum posts
22 photos

I was led to believe that the best way to check batteries capacity is to have a little bit of load on them using a suitable size resistor. I have recently purchased a Fusion product that puts a load across my IC reciever batteries and amazed that a green good to go light drops to an amber and be cautious

Maybe this could be set up for those lipo checkers units

Keith Miles 222/01/2020 21:19:50
303 forum posts
6 photos
Posted by john davidson 1 on 22/01/2020 20:48:04:

If one cell has packed in the checker can be tricked into seeing a three cell as a two and give a totally false reading

Not sure about that, unless two of the three positive outputs became one, thereby connecting two cells in parallel, in which case, assuming no internal damage was caused, you would still get three voltage/capacity readings, I would have thought, two being identical.

Alternatively, I would also have thought that an open circuit on a cell would provide a zero reading on that cell.

Do not all “capacity” checkers give a voltage/capacity reading for each cell or just some of them?

Happy to be educated!

Not that I’m inclined to buy one!

Frank Skilbeck22/01/2020 21:19:57
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4681 forum posts
101 photos

With lipos the offload voltage is a good measure of the remaining capacity, what is misleading is to think that your 3,000 mah battery is still 3,000 mah many cycles down the line, especially with the load we put on them.

With Nimh and Life batteries voltage is not an accurate measure of remaining capacity.

Keith Miles 222/01/2020 21:47:55
303 forum posts
6 photos

Yep! Seems that my doubts are well founded!

My electrical mind suggests that the only accurate way to see how much charge is inside a battery is by measuring discharge current over time (the very definition of ampere hours!) until minimum voltage is reached and by, similarly, measuring current over time to maximum charge voltage to check maximum capacity.

For that, I prefer my charger although, admittedly, the process takes much longer and my charger doesn’t fit into a jacket pocket!

And further to my earlier point about full voltage not necessarily indicating full capacity, many in the hobby will also recall the legendary NiCd “memory effect”!

Frank,

Totally agree with you there but many modern, and not too expensive, chargers are as good as it gets, in practical terms, for monitoring and testing the condition (or deterioration) of one’s packs.

Edited By Keith Miles 2 on 22/01/2020 22:04:25

Martin Harris22/01/2020 21:48:30
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9262 forum posts
245 photos

I just charge after or before flying - as long as it's not several weeks since the last charge when I give the battery a top up. With a new model, I check the voltage when I land and in the rare event that it's lower than 3.7v per cell just readjust the model timer. My batteries don't seem to suffer and the timer setting seems to last the life of the model/battery - any significant change indicates a battery past its best.

Quite frankly, I'm not bothered about checking "percentage" with a tester - if the battery I'm considering flying with isn't fully charged, it gets charged! I have a simple 3 digit LED voltmeter which I pop onto the balance lead - if it says 4.16V or more, the battery is good to go...

Keith Miles 222/01/2020 22:19:15
303 forum posts
6 photos

Here’s another thing.

So, you land your model, put your checker on the battery and the checker says, for example, 30% remaining.

So, what do you do with that information?

Not take-off again with the same battery, one hopes!

Also, I don’t see a need to check batteries before flight when they should already be charged for use.

I will accept that it might be a handy device, perhaps, in some circumstances, but certainly not a necessity nor something to place too much faith in.

 

 

Edited By Keith Miles 2 on 22/01/2020 22:24:12

Jason Channing23/01/2020 07:56:54
148 forum posts

Most of my checkers have ended up in the bin as there so inaccurate using both Nimh and lipo. The same battery on different checkers can vary quite wildly from between 25 and 35 % difference. if the checker reads anything like 50% then I just check it using my charger which is far more accurate.

Trevor Crook23/01/2020 08:04:35
940 forum posts
67 photos

I have a little checker in the field box, and would agree that it is pessimistic. However, it is more accurate if the battery is given a few minutes to "recover" after a flight. I'm sure I've read that the discharge curve for LiPos is quite linear, and the remaining capacity can be derived quite accurately from an off-load voltage reading.

I find the field checker most useful for checking a new setup or model. I set the timer very conservatively for the first flight, and check the capacity afterwards. If the checker says there is 30% remaining, I can judge how much I can safely increase the motor on flight time by for the next flight. Of course, this can be further refined back at base when recharging.

Most of my batteries that have expired have done so by having one cell go faulty. The capacity of my packs stays good for scores of charge cycles, I must do a quantitative measurement to check how good!

Frank Skilbeck23/01/2020 08:17:02
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4681 forum posts
101 photos
Posted by Keith Miles 2 on 22/01/2020 21:47:55:

Frank,

Totally agree with you there but many modern, and not too expensive, chargers are as good as it gets, in practical terms, for monitoring and testing the condition (or deterioration) of one’s packs.

Edited By Keith Miles 2 on 22/01/2020 22:04:25

Yep, but just measuring mah put back in doesn't tell you what the actual capacity of the battery is, you'd need to do a controlled discharge at set discharge rate until the battery reached it's minimum volts and then recharge it. Yes Lipo checkers are only measuring voltage (and the ones I've used are fairly accurate), and converting it to % based on the "actual" capacity of the battery. The original post was noting that the checker was saying 7% when based on the name plate capacity it should have been 25%, but was that the actual capacity of that battery I know I have some which are at less than 60% of their original capacity.

If you discharge a Lipo to too low a voltage you do lasting damage. If after landing you find the resting cell voltage is very near the fully discharged state (0% capacity) then either reduce your timer or if using telemetry the mah used alarm.

I

Dad_flyer23/01/2020 08:50:14
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282 forum posts
309 photos

The capacity given by a checker is vague, but I find the rest of the information useful. Unlike Keith I am not good at keeping track of which battery I just took out of the model, and which is the fresh one. I have not been flying long enough for a battery to really die, but as it is likely that one cell will go before the others I check for balance between the cells after flight as well as when charged.

One factor which has not been mentioned is temperature. The discharge curve changes with temperature, which gives another difficulty with predicting capacity from voltage at the field.

Trevor23/01/2020 09:07:26
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425 forum posts
57 photos

My experience is similar to Keith's - battery checker says 25% left, charger then only puts in about half of the battery's nominal capacity. As others have said, it's hard to be sure how much this is the checker erring on the safe side and how much may be a discrepancy between the label on the battery and its actual usable capacity.

Even so, I do find the checker useful, as others have said, for adjusting the flight timer of a new model.

As for keeping track of which batteries have been used and which are charged, I have for years operated the simple regime of putting a rubber band round the battery when you take it off the charger. Remove the rubber band when you put the battery in the model. This system has a built-in failsafe and has so far proven to be proof against this particular fool for over 30 years!

Keith Miles 223/01/2020 09:42:21
303 forum posts
6 photos

In my case this was one of four almost new 3000 mah batteries, each of which have had only two or three flights.

Also, the model has been tested for current draw at full power with several props for the purpose of determining a safe initial timer setting on the transmitter.

All four batteries have also been monitored on the charger, at the flying field, after flight and all four have shown an expected amount of recharge and no signs of any discrepancies or lack of capacity.

The fact is that the checker used was showing 93% discharge when, in fact, the charger was showing just under 70% discharge. That’s a BIG discrepancy irrespective of temperature issues and other suggested factors.

As voltage is being used to provide capacity information for a battery with a fairly constant working voltage, it seems to me that unless it can accurately and consistently measure to at least two decimal points, it can’t be trusted.

Furthermore, as Frank acknowledges, it will not tell you anything about a reduction in maximum capacity of the battery over time but, if anything, will only provide a not necessarily accurate percentage reading of the existing capacity.

Yes, they can be handy for a quick check if you get your used and unused batteries mixed up at the field but for that job you only need a simple voltage checker not a supposed “capacity” checker.

Again, however, full voltage also does not guarantee that the capacity matches what is printed on the label, only a monitored discharge and charge will do that.

Edited By Keith Miles 2 on 23/01/2020 09:49:02

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