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Old Feb 19, 2011, 07:59 AM
Wayne Giles is offline
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Rugby, UK
Joined Feb 2007
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Lipo "C" Ratings - A Replacement Overdue?

I believe it is widely accepted that “C” rating of lipos is discredited as a measure of performance and is being used as a sales gimmick by less scrupulous merchants.

The “C” rating is the multiple of the rated capacity of a lipo which the battery is capable of discharging continuously over a full discharge.

A couple of years ago suppliers were claiming “C ratings of 30C when many of them were struggling at 20C and some I tested were only capable of 15C. Gradually development caught up an ratings were starting to look more realistic, but we appear to have now rushed off with another round of wild claiming with 45C, 50C and several claims of 65C.

Apart from being dishonest and misleading the customer, it is unfair on the few makers and merchants who are honest and state real “C” ratings – they miss out on sales whilst the “wild claimers” prosper.

In the IR Meter thread Mark Forsyth said that:-

“My personal internal resistance 'number' for high performance cells is 12000 / cell capacity, when measured after stabilization at room temp (~72F) for 1 hour minimum.. Anything around or below this number I consider to be outstanding performance.”

I very much agree with the principle and although we might debate the 12000 figure, ( he does say “outstanding”) it is certainly close enough as a start. Possibly 15 – 20,000 might be better for a “good” lipo.

On such a basis a number could easily be derived for any lipo cell which could be a “Figure of Merit” or FOM by dividing the 12000 figure by the product of the cell capacity in mAh and the IR of the cell at a standard temperature. This would give an aiming value of 1; lower would be below average and >1 would be good, >>1 outstanding.

Obvious disadvantage is ensuring constant temperature measurements but the advantage is that you don’t need to do a full power discharge to find out whether the rating is genuine or just sales splurge.

I wait to be torn to pieces! – please try to be constructive.

Wayne
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