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Old Feb 04, 2013, 02:17 AM
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Rugby, UK
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Originally Posted by prodriverex View Post
I need it for up to 23.88Ah per cell. Will it work?
Yes, it will work on any cell with a cell voltage up to 4.8V provided the whole pack is between 7V and 28V..
The 16A pulse is large enough to be a significant load on the battery.

Wayne
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Old Feb 05, 2013, 08:40 PM
The Engineer
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What is the principal behind measuring resistance? How does it work?
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Old Feb 05, 2013, 09:09 PM
ancora imparo
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A LiPo battery is a complex electrochemical reaction but to a reasonable order of accuracy it can be treated as an "equivalent circuit" of simple parts. This is a common technique in electrical design in order to analyse circuit behaviour.

There are a number of such equivalent circuit models for batteries but the very simplest is a perfect voltage source in series with an internal resistance. It works reasonably well for us since we are using LiPos in DC circuits.

When you draw current from the pack, that current passes through the internal resistance and the load and so there is some voltage drop internally and some across the external load.

By measuring the voltage with no load and then with a known current you can calculate the internal resistance in Ohms.

Note; although the value of IR is in Ohms (since it is calculated from a voltage drop divided by the current) this is not a "real" resistance and does not have the properties of resistance.
Some people prefer the term "ESR" for Equivalent Series Resistance to emphasise this. For example the value of ESR for an individual pack varies with temperature (and probably with other variables like current load).

However if we measure IR/ESR under the same conditions, it is a very good indicator of the capability of the pack to deliver current.

The LiPo tool is a simple calculator that will derive a suggested safe maximum discharge for any pack based on cell IR and the pack capacity. That suggestion is an empirical one based on a great deal of experience in practice and the observation that most packs have a practical limit based on energy dissipation, above which heating will significantly degrade cell life.

http://www.jj604.com/LiPoTool/

A low IR cell will heat up less for the same current and will also have a lower internal voltage drop thus providing more volts at the terminals. A number of chargers now provide a reasonably accurate IR value for each cell and it is simple to measure with Wayne Giles specialist ESR meter. Beware that some cheap meters that claim to measure IR are worthless as they use only two wires and include all the resistance in the connectors and leads. This is important since a large good modern LiPo can have cell IR of 1mOhm or less whereas connector and lead resistances can be many times that if the meter uses only the balance wires to measure IR.

The advantage of the IR value is that it can be clearly specified - how it was measured and at what temperature - whereas the "C rating" figures quoted by manufactures may be of unknown origin or pure marketing fluff.

Hope this makes sense.

John
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Old Feb 05, 2013, 09:39 PM
The Engineer
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Thank you John for taking time to explain in such detail!
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Old Jan 19, 2014, 04:00 PM
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United States, CA, Long Beach
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hey guys, I had great day at the field today. those who flew, hope yours was great too!

icharger 106b 2 months old and LOVE IT. upgraded from accucell 6.
turnigy blue lipo 2200 3s 20c 2 years old. ~60 cycles. cells never reached below 3.71/cell. always stored at 3.85. never drew more than 30amps. application draws 28amps wot.
voltages per cell when ir was measured 3.85/3.85/3.86
ir measurement 8/8/8. pack total 24@ 73degrees f

im trying to find out what the real c rating is. I found the lipoly objective performance tool which is great!!. what charge state should the battery be in for the ir to be measured? is there any other factors I should know about to get the real c rating besides temp, capacity, and ir? thanks in advance.
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Old Jan 19, 2014, 05:16 PM
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Rugby, UK
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Measure the IR at fully charged for consistency.

You don't need any other information to get the real C value. If you put 8 milliohms into the Lipotool it gives you a max current of 41A which is well over your WOT value and a C value of about 18.2.

Wayne
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Old Jan 19, 2014, 08:22 PM
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Quote:
Originally Posted by Wayne Giles View Post
Measure the IR at fully charged for consistency.

You don't need any other information to get the real C value. If you put 8 milliohms into the Lipotool it gives you a max current of 41A which is well over your WOT value and a C value of about 18.2.

Wayne
ok sounds good! thanx for your help. wish I had this charger b4 I had bought all my batteries to see what the numbers would have been "fresh" outta the factory.

one more question... awhile back, either mark m. or hoppy said, from testing ir, they found a faulty main charge lead that had a cold solder joint. how can you tell? total pack ir didn't add up correctly from individual cell ir?

-kevin-
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