How do I find the maximum "discharge C rating" of a battery?

Question

I've been experimenting running my 3A LED strips off battery power and the results are disappointing. I'm only getting a few minutes (< 10) out of a YSD-168 1800mAh lithium ion. How can I find the "C rating" of this battery? Is it something mainly dependent on battery chemistry (as this Dan's Data article I just remembered implies, search for "[space]C[space]" on the page) or something mainly dependent on manufacturing quality/technique? Macho RC battery packs always quote C-ratings, other battery types not so much. And it's difficult to google.

Also, what are typical maximum "discharge C ratings" of AA and AAA NiMH batteries?

Answer 1

You're not really looking for the C rating (maximum discharge current in multiples of nominal capacity), you're looking for the adjusted capacity at your nominated discharge current.

A 1.8Ah lithium battery can theoretically give 1.8A for 1hour, or 3A for 1.8/3h = 36 minutes. HOWEVER the capacity for a battery is traditionally quoted for a 20 hour discharge. That is, a capacity rating of 1.8Ah means the battery delivered 90mA for 20 hours in testing.

The relationship between continuous current and time-to-full-discharge is NOT linear. You have discovered that when discharged at 3A, your 1.8Ah battery is delivering much less capacity (only about a quarter!) than a linear interpolation of the amp-hour rating would suggest. This is not unusual.

The better batteries will give rated capacity at several discharge time samples, or even a graph of current vs capacity. A battery intended for remote control uses will probably give better performance, as quick discharge is the intended application of these batteries.

(I use a 1500mAh 3-cell (11.1v) 25C lithium pack to power a 3A LED bike light, and I get around an hour, which is close to rated capacity given a reasonably efficient buck regulator).

Answer 2

This battery pack contains a protection device to protect the battery from overcharging, over-discharging and over-current. You're load seems to trip the over-current protection build into the battery pack. I suggest you do not try to do something about it as it might result in big explosion. NEVER use this battery without it's protection circuit.

(the circuit contains some MOSFETS and things like that and they might not be rated for your demand.)

Finding the capacity of batteries at different discharge rates is best done with Peukert's law that you can find here:

http://en.wikipedia.org/wiki/Peukert%27s_law

The wiki page mentions lead acid batteries. But it can also be used for Li-Ion batteries. However, getting the right numbers from a battery manufacturer can be a problem.

A more practical explanation can be found here:

http://www.bdbatteries.com/peukert.php

Regards, Hendrik

Answer 3

The best bet is to check datasheet and/or run an experiment :-) And yes, it depends on chimestry AND quality.

But for Lithium & NiMh batteries you usually can get more than 1C reliably. The more current you get - the more looses & heat generated.

I wouldn't got over 3C in any case.

Answer 4

Acoording to Energizer for NiMH batteries, the normal discharge rate is 0.2C. The battery nominal capacity is measured at this rate. At higher discharge rate the actual capacity of the battery will be much less than the nominal.

Answer 5

Many RC aircraft batteries can be run at 20 or 30C continuously, but those are specifically designed to do so. I wouldn't imagine a C rating exceeding 1C for a consumer battery, like the CCTV camera.

Answer 6

You'll find the "C" rating of most standard alkaline and NiMH AA batteries falls between 0.25C and 0.5C. Poorly designed or cheap batteries may allow you to exceed 0.5C, but the battery may become dangerously hot and (both temporarily and permanently in the case of NiMH) lose capacity (MaH). I hope this helps.

Answer 7

In the past, most "power" batteries (UPS Systems) used a nominal 8-hour discharge rate for battery amp hour capacity. If one had a 160 AH battery, then only at the 8 hour discharge rate would the exact 160 AH be realized, or 20 amps. if 10 amps was the load, then more than 160 AHs would be available, and if 40 amps was the load, then less than 160 AH would be available.

When I was designing my home battery back-up system, I could find nothing about a nominal 8-hour discharge rate, as used in the utility industry, but instead I found a "C" rating. The "C" was similar to my 8-hour rate, but it seemed to change from battery to battery and from manufacturer to manufacturer.

It appears that NiMH battery manufacturers have consistently used a "C" factor of 1C for full capacity, but the actual discharge time will be a fraction or multiple of 1C. e.g take the energizer datasheet:

http://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf

All the charts refer to a typical C/3 or C/5, indicating that 1C is AH capacity but the AH rating is still a factor of time. Energizer uses a C/5 for its full capacity rating, or 1/5 the mAh rating for 5 hours. Any load above the 1/5 mAh rating will reduce capacity and below the 1/5 mAh rating will increase capacity.