Continous verses mAh???

[Tom.C]

Hi guys I have found these two batteries.One has a slightly higher mAh rating whilst the other has a higher continuous discharge rate....

http://cgi.ebay.co.uk/2x-11-1V-900mA...item3ef87fcfbf

http://cgi.ebay.co.uk/2x-11-1v-850mA...item3efb9daab9

I know these are to good to be true price's. I have Friends though who have bought these cheepo lipo's and have been doing quite well.

As always thanks in advance.

[TJinGuy]

What is your question exactly?

[Tom.C]

Which of the two provides the better performance.Or at least which of the two ratings define a battery that performs better...

[RocketSled]

The battery with the higher discharge rating will deliver more current with less internal heating and parasitic loss than the battery with the lower rating. What makes a battery capable of higher currents is a lower internal resistance. Depending on how much juice your heli will be pulling, this may or may not make much of a difference to flight times. If you're pulling high currents, the battery with the higher discharge rating may actually deliver longer flights than the battery with the higher capacity (because more of the power from the lower-current higher-capacity pack will be given up as heat inside the pack). And if you're pulling current levels that approach the limits of the lower-rated pack, the higher rated pack will provide more available power in flight.

[TJinGuy]

It is simple math.

For the 900mAh 15C pack
15C = 15 * .9A = 13.5A

For the 850mAh 25C pack
25C = 25 * .85A = 21.25A

That is all there is to it.

[RocketSled]

There's more to it than just that. The OP's question was asking which was better. Unfortunately, we don't have enough information to judge. As far as peak current capability, as you point out it's simple math, the 25C pack is clearly "better". But if his setup only needs 10A, that 900mAh pack is likely to deliver measurably longer flights, so it would be "better" in that context.

[TJinGuy]

Which of the two provides the better performance.

I answered that question, what question are you answering?

[RocketSled]

It's seems likely from the initial posting that the OP understands the significance of "discharge rate", so your answer doesn't really offer any additional insight. That the higher C rate pack is capable of delivering more current seems like it ought to have been fairly apparent, restating the self-evident as "simple math" doesn't contribute much to the discussion.

"Continuous vs. mAh?" is, I hope you would agree, fairly ambiguous. If you take a moment to think about it, there's more than one possible interpretation of the question. While your answer, "peak power," is absolutely valid, it is only one of those possible interpretations. Power-to-weight could be another. Run-time could be a third. Cost (though probably not in this case) could be a fourth. For all I know, physical size could be a factor. Since the OP didn't specify his question in a way that I though clearly defined his objectives, I thought it would be better to try and answer in a way that would help him understand how to make an decision based on his specific (but unstated herein) priorities, instead of assuming I understood his intent and providing an answer based on that single assumption.

>what question are you answering?

Hope this, above, clears that up for you... I am trying to answer them *all*.

[Teej]

Originally Posted by TJinGuy

It is simple math.

For the 900mAh 15C pack
15C = 15 * .9A = 13.5A

For the 850mAh 25C pack
25C = 25 * .85A = 21.25A

That is all there is to it.

The 850 is a 20C, not 25.

So it's 13.5 vs 17.

Closer, but the "higher performance" still goes with the 20C 850.

[RocketSled]

The 850 pack is a bit more "efficient" because it's got a lower internal resistance, less internal heating and associated parasitic losses. But without knowing the actual internal resistance of the specific cells, and the current draw of the application, it's hard to say how much differently the two packs would perform.

If my Heli only draws 10A, the 850@20C doesn't necessarily benefit performance in an appreciable way compared to the 900@15C. And the 900 could deliver slightly longer flights in spite of it's higher "ir" and lower "efficiency".

That all being said, cheap batteries have, in my experience, turned out to be cheap in more ways than just price. My recommendation is to budget in other areas and not to skimp on batteries.

[Tom.C]

Where can you afford to skimp out on a helicopter?One failour usally results in the whole.But these aint exactly the nost prosperous time's at the moment so gonna have to tighten the belt.


Some times this game becomes a life,

[TJinGuy]

One concept you have to understand is that c-rating is only half of it, the size directly effects the output, internal resistance, etc. The best way to look at this is voltage under load. That simplifies things by comparing batteries under stress.

For example take these 2 packs (cell count plays no part here, as long as they are the same).
4000mAh 20C
2000mAh 40C

One is twice the capacity of the other but only half the c-rating. As such both have the same max continuous output of 80A and both should hold the same voltage under load. Also both should also have roughly the same internal resistance.