Understanding battery ratting from C to mAh - Page 1
Forum section: electronics / beginners

author: mathk
author_rank: Contributor
timestamp: August 06, 2015, 01:16:27 pm
content: According to wikipedia Coulomb is the unit of A time sec.Some batteries that have a rating of 25C and 3700mAh and I am a bit confused with that. C and mAh is the same unit but supposedly with different meaning.If I do the math 3700mAh correspond to 3.7 x 3600 C = 13320 C nothing closed to 25C.So why do we have those rating? Can someone try to give me a way to interpret those rating and what is it important for?Note that if the rating was 25A I would understand that the battery can deliver at most 25 amps.Thanks

author: amyk
author_rank: Super Contributor
timestamp: August 06, 2015, 01:28:06 pm
content: It's an "abbreviation collision". C in battery discharge/charge ratings does not mean coulombs, it is a multiplier for the capacity to determine the amperage.If anything, this "C" has units of inverse time.

author: mathk
author_rank: Contributor
timestamp: August 06, 2015, 01:50:36 pm
content: Thanks Amyk.So back to the wikipedia page at the secion "C" rate of the battery page.If I understood correctly 25C for 3.7Ah battery mean that the battery can deliver a maximum current of 92.5 A ?

author: PTR_1275
author_rank: Frequent Contributor
timestamp: August 06, 2015, 01:52:32 pm
content: In this context C is a discharge (or charge ability) You can discharge the battery at a rate of 25C (25 x capacity which is 3.7 amps x 25 = 92.5 amps) At that rate due to peukerts laws you will not get the full 3700mAh out of the battery at that current as LiPo's are normally rated at 1C. Batteries with higher C ratings allow higher discharge rates from higher power motors (in electric quadcoptors for example).Hope this helps-Chris

author: macboy
author_rank: Super Contributor
timestamp: August 06, 2015, 01:58:51 pm
content: In battery talk, "C" is not degrees Celcius or Coulombs. It is meant to refer to capacity of the battery, but generally has a units of A (not Ah as with true capacity).Battery manufacturers often give recommended or maximum fast and trickle charge rates in units of "C". You then need to work out what the current really is. e.g. If you charge at a rate of 2C, then for a battery of 1800 mAh, this means charging with a current of 3600 mA or 3.6 A.Similarly, if a battery is rated for "25C" discharge, this means that you can safely pull up to 25 times the capacity as a load current. So for a "25C" 1800 mAh battery, this is 25*1800mA= 45 A. That is the maximum discharge current for that battery. If you exceed that for a short time, the voltage will likely sag too much (voltage drop due to internal resistance, V=IR). If you exceed it for a long time, the battery will likely overheat (power dissipated in internal resistance P=I2R). An overheated lithium battery can be extremely dangerous, so buy appropriately rated batteries and stay within their specs for charging and discharging.

author: apis
author_rank: Super Contributor
timestamp: August 06, 2015, 04:33:41 pm
content: I find the "C" rating convention of batteries weird as well, doesn't seem to make much sense from a physics point of view, but I suppose such specifications are targeted towards more practical everyday use.For some reason batteries are usually rated in Ah, which is current * time which doesn't really mean anything unless you also specify an electric potential (voltage) which then gives you a power == current * electric potential and if you multiply power (ie work) and time and you get energy (watt hours or watt seconds == joules) which to me would make more sense for a battery capacity rating.As I understand it, the Ah rating means the battery can deliver a certain current for an hour at some nominal voltage, so the energy in a battery would be approximately the nominal voltage * battery capacity (in Ah) which then gives you the energy in watt hours and if you multiply by 60*60 you get energy in joule (Ws) or divide by 1000 to get kWh. But only approximately since the available voltages and amps will vary due to all kinds of complex reasons (depending on the internal battery chemistry and physics).As long as one compare batteries of the same type, the voltage will vary in a similar way and the amp-hour capacity rating is comparable. I'm sure there are some kind of standard manufacturers use for calculating the capacity in Ah.As the others have explained better: "C" apparently means capacity and is equal to the amp-hour capacity but the quantity now refer only to current not current * time, and is used when talking about recommended charging and discharging currents and such.

author: daybyter
author_rank: Frequent Contributor
timestamp: August 06, 2015, 09:04:52 pm
content: For LiPos you should usually have at least 2 C Ratings, like 25 C / 45 C. Which means, that the battery can deliver a constant current of 25 * <capacity> , while you can pull up to 45 * <capacity> for a short time (a few seconds usually).A few years ago, all the batteries where charged with 1 C (~ 1 hour charging time), but now there are batteries, which could be charged much faster.

author: krish2487
author_rank: Frequent Contributor
timestamp: August 07, 2015, 05:17:38 am
content: You are right. However battery cells are standard at 2V and batteries themselves are standard at 12 V. It just is a "given" that any lead acid chemistry battery that we talk about are nominally rated at 12V. The only parameter that varies is the Ah rating which ultimately decided the energy available. It is just a industry norm. Just like telecom power supplies are at 48V. It is just a taken-for-granted parameter.Also, Ah rating is really much more important than many realise. Due to very physical construction of the lead acid batteries, they cannot be charged / discharged beyond a certain rate. Generally, a lead acid battery is of any Ah rating should not be charged with a current more than C/10. In certain conditions you can go upto C/5. Where C is the Ah rating. Which effectively means that the batteries take a little more than 10-11 to charge completely (again peukerts law). Likewise with discharge. Ah rating is not just a calculation of energy availability, It also allows us to calculate available energy over time at a certain charge/ discharge current.PS: On a side note, the internal resistance battery is another important parameter to remember. Because this is not constant even for a specific brand/ model/ batch of batteries, the energy availability varies ever so slightly. This is the reason why "generally" high capacity battery banks are not "simply" parallelled. If we need extra energy then increasing the DC is the correct solution. However, they can be parallelled using active current sharing methods which force equal currents between two parallelled batteries of similar age,model, make and rating.Quote from: apis onTodayat 02:33:41 AM

author: Rick Law
author_rank: Super Contributor
timestamp: August 07, 2015, 05:35:56 am
content: Quote from: apis on August 06, 2015, 04:33:41 pmIt is confusing. C is used as a rating for bothEnergyandPowerbut absent in both cases are the voltages.(1)Casstored capacityrating - i.e.Stored EnergyIn this context, a 2AH (2000mAH) 3.7V (ICR) lithium is measured to be able to go from full voltage (4.2Volt-ish)downto a specific cutoff voltage (3.0Volt-ish) at aspecific currentfor a specific time with a resulting total of current*time=2000mAH. It really doesn't link it to the nominal voltage as the voltage will drop as it discharge.So roughly speaking, it can deliverabout1000mA for 2 hour, and 500mA for 4 hours.But, not so fast! Typically,only at that specified currentwould you get current*time = 2000mAH. At other discharge current, the total mAH could be less. Good datasheet would show the different mAH at different discharge rate, whatever typical discharge rate the market for that battery is designed for. As you can see, the cut-off voltage would affect the total ?AH rating. But there is no "standard" cut-off voltage but merely a "typical" one so there is no gold standard here.With lithium, it is more confusing since there are more variations with lithium chemistry. You have ICR, IMR, and other lithium chemistry. So, both the cut-off voltage and nominal voltage are all over the place. Even ICR, some are rated 3.7V whereas some are rated 3.6V. Some 3.7V discharges to 3.0V whereas others would cut it at 3.2V. So, directly comparing the numbers would be misleading. Even with ICR, I know some has xxxx mAH rating only when charged to 4.3V.(2)Cascurrentcapability - i.e.Max PowerIn thecurrentcontext,Cis used as a unit with implied C/1H (slash means divide: 2000mAH/1H= 2000mA). So, a 2C current is 4000mA. 2C delivery for a 2000mAH battery is capable of delivering 4A without damaging the battery. So: a 10C 500mAH LiPo can deliver about 10x the power of a 1C 500mAH LiPo. Poor rating unit really, the same 10C 500mAH LiPo can deliver just 5x the power of a 1C 1000mAH LiPo. SoCis a varying unit.While a 2C 2000mAH can deliver 4A, however, andthis is important - being able to deliver 4A doesn't mean you can/should charge it at 4A. The datasheet should tell you whatCit should be charge at. Typically, 1C charge rate is acceptable to most lithium battery. I draw the line at 1/2 C as I am not very experienced and don't want to take the risk.Rick