Battery Discharge Rates Versus Capacity (C10/C100 etc)

Maybe more than you want to know: Battery Performance Characteristics - How to specify and test a battery

Thanks for that link Lyn, lotsa info there but mainly Lithium batteries, - which of course have their (fast growing) place, yet still vastly out numbered by the lead acid battery. So in their info I couldn't find an article on the difference in capacity due to faster or slower discharge rates, and it may well only apply to lead acid batteries. Regards, Geoff.

Geoff,Sorry about that.

It seems pretty straight forward to me. Kinda like a compressed air tank. The Bigger the tank the more air you have. The higher the outflow rate, the sooner you run out of air.

This is from :

Capacity and Battery Ratings - Engineers Edge

Cheers.

Hi Lyn, yes you would think so, but it is not the case, the slower you discharge the batteries the more total power you will get, - as your link article says, "Manufacturer's literature on batteries will normally specify several discharge rates (in amperes) along with the associated discharge time (in hours)." - Hence we specify the size of the battery at a certain discharge rate, and many battery retailers use the 100 hour rate, ie C100, which is discharging 1% of that capacity and should do that for 100 hours. If, however, you use that same battery, - say in a home power situation, where most of your power is consumed from dusk till 9PM, folk often don't realise they are discharging at perhaps the 10 hour rate (C10) per hour for that time, and say with a thick plate Telstra type battery, that same battery rated at the 10 hour rate is approximately 65% of the rating at the 100 hour rate. - This leads to many irate customers who think they are being ripped off, - and they are if the different discharge rates were not made clear to them. - I am hoping for some in depth research material on this phenomena if possible, - Geoff.

Geoff,

I'm going to be no real help to you. I'm sure the research is there, if you can find it.

Some of the really smart guys here may know something, but it's over my head.

Good luck!

Cadex, a manufacturer of battery analyzing/charging equipment, has a pretty good "school" on batteries. I've used them as a good source of battery information a number of times:

Battery University

Lots of good info there. No affiliation, just a user of information.

Tom D.

Lead Acid battery capacity vs current draw is pretty much summed up by Peukert's law.

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

Hi Johnny, I think that is a helpful answer for some who have come into this discussion, - however, for me it is a further description of the symptoms, not the problem, - I sell Inverters and Regulators that use Peukert's Law in their data collection of input and output, but where the information is scarce is on Why Lead Acid batteries behave this way, (and maybe other batteries do also) - why do Tubular batteries have less difference between eg. 10 hour and 100 hour rate of discharge, than Plante (Telecom style) batteries? - what actually happens in the chemical reaction to make it less efficient whilst discharging more, - indeed one wonders does that lead to why does lead itself only suffer approx. 1500 to 1800 total discharges before weakening (how can a chemical reaction weaken?) and how come by simply melting it and making new battery plates out of it the process can begin again? - born again, as it were? This all started from my commissioning a sophisticated MPPT battery charger, - which claimed to use Peukert, - and asking the supplier what discharge rate should I set the ampere hour of the battery at, (they require the battery capacity be entered) - 100 hour rate? 10 hour rate? if it doesn't matter why bother? - With the language difficulty with Taiwonese and English, (I have heard that a Chinese person, to learn English, must be turned inside out) of course the agent doesn't even understand the question, so maybe time to admit the Emperor has no clothes and to get to why this phenomena exists! I know it exists, battery manufacturers know it exists, (although their tables seem mainly based on measuring, - perhaps with a bit of Peukert help?) but why? - Lead acid batteries have been around a hundred years of so I believe, how can we possibly not know such a basic thing? Hopefully someone out there might know. Cheers, Geoff Thomas.

I'm not sure many grasp what you are seeking, but I remember in Clayton and Shelly "Basic Electrical Engineering" [or similar title, circa 1900], having a paragraph or two on this.

I inherited my fathers copy, which unfortunately is not at hand at this juncture, but I'm fairly sure it was relevant to the solid lead plate art and with the invention of sintered plates became largely irrelevant - provided you stayed within the curves folk keep directing you to.

As to why they degrade, the reactions are not 'pure', the physicality generally asymmetric, so each cycle produces a small amount of lead sulphate where the current density is lowest. Better "current symmetry" is why wound cells tend to last longer.

If the charger does not contain the circuitry to largely dislodge this sulphate coating then it further creeps over more area. Meaning the manufactures cycle number curve is what you can best expect.

Geoff, C2 (2 hrs or 50%) is the specified discharge rate for telecoms applications (contrary to what it suggests at the end of this.)

The C8, C10, C20 caper is well explained there.

Hi Wal, that link didn't work, but ta anyway, - although telecom batteries are very rarely cycled, - usually, - only when the power fails, at least in Australia, then they have to carry the exchange through the longest recorded reasonable break in power supply. ( - well that used to be the case, maybe now not considered economic, - sort of what happened with "the mail must get through" - values change). G

Here it is againhttp://www.engineersedge.com/battery/capacity_battery_ratings.htm A typo, my bad...

Thanks for the link Wal, I can see they claim telecom batts to be C10, but they would certainly not be my choice for such a hard work situation. The old telecom batts out of telephone exchanges used to have both their 10 hour and 100 hour rating stated, (very big difference) usually they were 500a/h@ C100, famed for not being able to start a car, although I never met anyone who tried with a brand new set. Cheers, Geoff.