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Bit of an odd lipo reading


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I recently bought a couple of what I would class as budget 6S 5000mAh lipos that are wired in series into a 12S set up. I have two flights with them after balance charging them to full capacity.

IMG_0948.thumb.jpg.374fe0a151e49090e0d6e837c27261a8.jpg

 

Now the discharge rates for the batteries are mismatched on both flights.

 

IMG_0950.thumb.jpg.b3408421ad94bd8f7346743b458efc19.jpg

 

 

IMG_0949.thumb.jpg.5cf113d29722a4425777a6daf97c69ab.jpg

 

Never had that before with any of other matched lipos in 12S operations. I grant you the lipos were of the cheaper price range, but can't say I understand why this is happening.  Any ideas?

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Batteries could just be from different batches with slightly different chemical properties.

Each particular battery looks to be using matched cells (from the same batch?).

 

That is the explanation of what is happening.

Whether or not this means that the 22.07v battery is inferior to the 22.94v or is duff is another matter.

Maybe they will even out after a few cycles?

 

Edited by Gary Manuel
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Based on the voltage readings, the lower voltage indicates the battery is circa 95% discharged whereas the other is only 55%. I assume when you recharge them you put back the same mah into each battery.

 

Might be worth doing a discharge on your charger as at low rate to see what the measured capacity of each battery is.

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Looks likely that they have different IR per pack and the lower IR pack is being discharged first because having lower IR it is able to offer up the current needed by the ESC easier.

 

That's why you always MUST use the same C rated packs in a series connected power pack or the higher C rated pack with lower IR gets ued first.

 

Effectively it is likely that you are using different C rated packs, albeit inadvertently.

Edited by Philip Lewis 3
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Frank & Shaun, I am going to put the lipos on my charger today and use the IR mode to see what I find.

 

As to the C rating, Philip. The labels on both packs (in the photo above)reflect the same C rating if that's what you mean so in theory they should be identical (although I appreciate there may be chemical differences).

The fact of the matter is my 4-Max, Overlander and even my Turnigy NanoTech 5000 mAh lipos have not ever displayed this trait while each make paired together and used in a 12s setup. I just came to the conclusion these lipos are just too budget a range. 

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That's how it looks Adrian, both packs are reasonably well matched, but the cell matching between the two packs is off -likely due to IR differences between the packs. They would probably be fine used individually and they only have a few cycles on them so the "poorer" pack might conceivable come up to spec a few cycles further on. 

Do you have a model that needs 6s1p packs of that size, so that you could use them for a few cycles and see whether they get any closer?

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47 minutes ago, Adrian Smith 1 said:

The labels on both packs (in the photo above)reflect the same C rating if that's what you mean so in theory they should be identical (although I appreciate there may be chemical differences). 

 

Totally agree with the theory bit but the practical will be different, C rating (the true one not the label) and IR go hand in hand, a higher C rated pack MUST by definition have a lower IR value. I'm sure you will see a diffenece between the packs when you measure them.

 

Used individually they will likely be fine but one will have more "punch" that the other, used together looks like a no go or very short flight time and you will kill the better battery first if you overun. 

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The higher discharge battery (lower closing voltage) IR values are 6.6.6.6.5.6.

the lower discharge battery (higher closing voltage) IR values are 6.8.5.5.5.7. 

 

Make of that what you will. I am not sure it takes me much further other than to say this pair of lipos have gone through the short so far lifespan on a 12s setup with the same plane, timed duration, balanced charge before flight from lows of the previous flying session. The one thing I haven't kept a record of is the precise mAh that has gone into each lipo at the time of balanced charging. The 2-3 flights usage have displayed the same disparity in finishing flight reading on the lipos in question. 

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I fly a couple of models that use 2 x 5s 5000mAh, to be brutally honest I haven't bothered to check their respective cell voltages after flying, I just balance charge them for the next flying session. I have 4 packs, nothing special, Zippy ones which I've had for 3 - 4 years and they still give me 10+ mins flight times with the Rascall and 8 mins with the Xtra Wot. The other pack are from CNHL with slightly better performance but they are only 1 year old. 2 of the Zippy packs were also used 4 years ago in my F3A Vanquish so we’re certainly thoroughly flight tested!

Edited by Ron Gray
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What is important though is to check that you are putting roughly the same mah back into each pack, if you haven't had any trouble then the chances are that you are but if one pack needs considerably more mah put back in than the other (which is what will be happening here) then something is wrong.

 

IR is very hard to get any meaningfull figures on and depends on so many variables, best used as an indication (i.e. when new it was X now it is Y) rather than an absolute measure.

 

If a new model also it could also be worth checking the solder joints for a high resistance joint.

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Andy48. Both batteries are charged at the same time. Plus I measure the after flight cell voltage 30 minutes or so after the flight rather than straight away.

Philip, I will check the solder joints to see what I can find although I will say one model is some years old while the other is new. One flight from each so far with  these lipos.

 

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I would run a couple of discharge cycles in your charger to get a measurement of each packs capacity. 


Also measure the cell voltage immediately after flight finishes.

 

Find out what you get from that then move on to other checks.

 

Let us know what you find. 

 

 

Edited by Nigel R
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Adrian Smith 1

As pointed out batteries in series pass the same current so for one to end up with a lower voltage means it had less capacity to start with.

This raises the question did both batteries receive the same charge input and where they both at the same cell voltage to start with?

 

As you surmise with a series set up you will always be limited to the performance of the weakest cell or cells.

It does certainly look like one battery is 'weaker' than the other.  

  

 

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