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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 06-26-2014, 01:38 PM   #1
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Default lipo cell resistance

I just moved up to the i-charger 308 DUO and started looking at the internal resistance of some of my Lipo cell's.


I have looked around this site wanting to know what is a good level of resistance for a lips cell. I see a lot of info , but nothing about levels that are good or bad?


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Old 06-26-2014, 02:41 PM   #2
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I have a meter for it separate to charger ... I used it a few times and then put it aside.

My view in the end was - I was doing something I didn't really have much idea about .. so decided I was looking for CHANGE in value ...

Last time I used the meter ? months !!

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Old 06-26-2014, 06:06 PM   #3
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Cell resistance is inversely proportional to the mAh size of the battery i.e. all things being equal a 2200mAh battery will have half the cell resistance of an 1100mAh battery, so it's impossible to give a a single 'good' value.

Also high c rating batteries should be lower cell resistance than low c rating.

There is no standard way of measuring c rate, which leads to wide discrepancies between different makes of batteries, but there is a calculation kicking around which attempts to calculate 'true c rate' based on an input of cell resistance and mAh capacity. I've put the formula into an Excel speadsheet, find attached.


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File Type: zip LiPo_C_calc.zip (7.6 KB, 7 views)
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Old 06-26-2014, 06:20 PM   #4
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If I had a small - moderate number of LiPo's - I would probably take more interest in recording IR numbers and watching trend of the packs - that's where I think any real benefit is --- watching to see if a pack is failing.

But with so many boxes of them now - It would be a real chore to cover them all ...

I write an ID on each .. clear tape over so it doesn't rub off ... when I first get them ...
I have a 3 strikes red mark system .. if I find a LiPo doesn't perform as well as prev. - I make a red mark on it. That then says to me - not for high demand applications like my speed machines or EDF. If it drops more - it gets a second mark and is relegated to low demand such as Tx pack or my SE5 - etc. If it really goes down - it gets a 3rd red and it's purely for bench test work and then in the bin.

TBH - use of pack has served me better than the IR meter ..

But that's me.

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Old 06-26-2014, 06:53 PM   #5
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IR isn't something i check regularly either, only if i think there is an issue with a battery.
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Old 06-27-2014, 03:13 PM   #6
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Default good info

Hi,

thanks for all the input, its something new to me and I wanted to know how to use this feature of the charger.

guess I could start with new packs and record them and see where this takes me.

If I see any thing down the road, I will get back. some one may know more by then or say it useless

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Old 06-27-2014, 03:58 PM   #7
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Default new battery

I see what you mean about smaller battery's, I just put a parkzone 1800 on the new charger. Its only been charged a couple of times at 1c and its giving a 11.4 to 12.6 IR.

I thought my Vernon 3 cell 2100 with a 14 to 15 IR was going bad, but its two years old and now I don't think so.

I noticed that the IR goes down as it charges?

thanks

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Old 06-27-2014, 04:47 PM   #8
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Originally Posted by sticks View Post

I noticed that the IR goes down as it charges?
Yes, that's purely due to the battery heating internally during the charge. If you leave it for an hour to cool after charging you will find that the IR goes back up again. This is why it's important to let the battery settle to room temperature before checking IR.
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Old 06-28-2014, 05:08 AM   #9
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Originally Posted by sticks View Post
I just moved up to the i-charger 308 DUO and started looking at the internal resistance of some of my Lipo cell's.


I have looked around this site wanting to know what is a good level of resistance for a lips cell. I see a lot of info , but nothing about levels that are good or bad?


sticks

I've sometimes found that the indicated cell resistance of a battery pack as calculated by a charger doesn't always come close to the actual under real load conditions.

FYI, the IR value can be calculated per this procedure. You need two load resistors that pull about 10 Amps, and about 20 Amps. With a decent wattmeter, you have enough info to calculate IR.

Formula is as follows:
IR = (Volts at 20 Amps minus volts at 10 Amps)/(20 Amps-10Amps)

For a real test, assume we measure as follows:
Volts at 26.4 Amps equals 10.71 Volts DC
Volts at 12.5 Amps equals 10.08 Volts DC.

Plugging it in you get
IR=(V2-V1)/(I2-I1)
IR=(10.71-10.08)/(26.4-12.5)
IR=0.045 ohms for three cells
IR=0.015 ohms for one cell.

That is a lot of work. If you calculate the IR value with different load currents on the same battery pack, you're likely to get different IR readings. It's much simpler to take your brand new battery pack, plug it into your ESC and motor, measure the Amps/Volts/Watts, and RPM with a tach.

Then, after a number of flights, just repeat the above Amps/Volts/Watts and RPM on the same motor and propeller and see how much it dropped off. At the very least, a simple RPM check will tell a lot on your batteries performance.

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