Battery Discharge Rate

It is the amount of discharge current the AH rate was based on. C5 being a 5 amp draw C10 an 10 amp draw. Since the available current from a battery depends upon the surface area of the plates. The AH upon the amount of electrolyte in the battery. It is information as to how the battery AH was tested and what it's design current load should be.

C Rate represents the charging or discharging rate of a cell or battery, expressed in terms of its total storage capacity in Ah or mAh. So a rate of 1C means transfer of all of the stored energy in one hour; 0.1C means 10% transfer in one hour, or full transfer in 10 hours; 5C means full transfer in 12 minutes, and so on.

http://www.jaycar.com/images_uploaded/battglos.pdf

Generally this information is included in the battery data sheets rather than on the battery itself (and is related to the chemistry of the battery), and yes the Ah rating of a battery is given at a specific C discharge rate.

Put simply, if you try to discharge the battery faster than this C rate then you will get less Ah out (like trying to run a high current device like a digital camera off an Alkaline battery compared to a NiMH or Li-Ion battery, all of which having similar Ah ratings).

C5 or C10 implies that the battery will deliver the rated capacity if discharged at this rate. e.g. 100 Ah battery at C5 rate can deliver 100/5= 20 A for 5 hours. At time point it will reach the end cell voltage as specified by the manufacturer for the type of battery. Conversely if battery is discharged at 20 Amps. constant current, then the battery will reach its rated end cell voltage at the end of 5 hours. This, however, does not mean that if the battery is discharged at higher current rate, then the product of time and current will be the Ah capacity of battery. e.g. if this battery is discharged at 50 Amps. then it WILL NOT DELIVER THIS CURRENT FOR 2 HOURS to specified end cell voltage. It will be less than this by a factor ( generally called K factor ) which depends on the type of battery & rate at which it is discharged. Thus there is a K factor curve for different end cell voltages at different rate of discharges. All these parameters are defined at a particular temp. ( generally 27 deg. C ) & therefore also needs another correction factor for the temp. Effectively capacity of battery goes down with reduction in temp. ( Remember the classic question - Difficulty in starting the car in cold winter? )

But if same battery is discharged at lower current, say 2 Amps. then this will last more than 100/2 = 50 hours before it reaches the specified end cell voltage, thereby giving higher Ah capacity than the specified one. Fundamental reason behind this is the internal resistance of battery. Terminal voltage at battery is difference between EMF generated inside the battery cell & internal voltage drop. Internal voltage drop will depend on discharge current, higher the discharge current, higher is the drop & vice versa.

Thanks and regards

Ashok Toshniwal, Bangalore, India

"100 Ah battery at C5 rate can deliver 100/5= 20 A for 5 hours. At time point it will reach the end cell voltage as specified by the manufacturer for the type of battery"............really this sentence cleared my all doubts(if its true!!!).Thank u......

Oshkosh has it right but can be expanded slightly. The C rate is only a way to compare rated cell capacity with actual capacity output. A 100 amp hour rated battery only produces it's maximum output under very ideal conditions which usually means a very low discharge rate ... say C/100. At higher rates of discharge .. say C/1 other factors like internal resistance and heat come into play and the actual capacity output is reduced ... dramatically in some cases. Safety is a big issue when batteries are discharged at high rates. Some batteries are rate limited due to many factors and can not be discharged at a high rate, even when short circuited. Batteries are fun.

Giri, Yes it is TRUE. We are Bangalore, India based manufacturer and have been manufacturing Battery chargers & DCDB for power plants & substations for more than 25 years ( we supply battery as well to give complete DC system ), are on the approved vendor lists of leading engineering consultants and have installations in India and 12 other countries. For any enquiries, pls. feel free to contact me uni_insta@dataone.in or ashoktoshniwal@yahoo.com or uimcoblr@yahoo.com

For any further clarification, pls. do contact.

Thanks and regards

Ashok Toshniwal, Bangalore, India

Hi..dear Ashok.Pls carify me the following:

1)I work as maintenance engineer in petrochemicals plant & ours is 20 year old system.Ther is a tapping given to the battery bank refered as 80% tapping.In latest battery chargers I came across haven't any system like that.On supply failure whole battery will come across the load.Opinions are differing from persons to persons.Need clarification for that.

2)When one can say that the battery won't be able to feed the load,considering cell voltage or specific gravity in floating condition.?

thanks in advance

Giri, I will answer your first question and please further clarify on second one, since it is not clear to me.

80% cell tap is SHOULD & MUST & is a standard feature in all our chargers, including those installed in various refineries in India. This provides continuity to load IF POWER FAILS DURING BOOST CHARGING. During boost charging, float charger feeds load and boost charger charges battery. During this condition, if power fails, then output of float charger will be zero & thus DC supply to load will not be available. To avoid this situation, a connection from 80% cell tap via blocking diodes is provided. There is a contactor which interlocks load bus and battery bus. This remains closed in float mode & AC failure condition AND opens in Boost mode to disconnect high boost charging voltage being reflected at load terminals. Cell tap diodes provide momentary continuity to load through 80% cell tap, till such time this interlocking contactor closes. As soon as AC supply fails, during boost charging, this contactor closes but there is a time duration involved ( of the order of milliseconds ). During this time, DC supply on load is connected through 80% cell tap and diodes. Immediately upon closing of contact, these diodes again get reversed biased and therefore donot connect, effectively disconnecting 80% cell tap from load and bringing the complete battery bank across load.

There is another school of thought, which says that 80% cell tap is to provide 80% voltage to load, during boost charging. I am dead against this philosophy for the simple reason that sum of load current & battery current is pumped into 20% of cells & thus there is a SERIOUSLY UNBALANCED charging of these 20% cells. No wonder these cells fail quickly. Now there is another problem, one cannot replace only these 20% cells because then good percentage of cells will be old and rest new, so the solution is to replace the entire battery bank!! collosal waste.

Trust this clarifies.

I look forward to your reply on point 2.

Thanks and regards

Ashok Toshniwal, Bangalore, India

Dear Ashok....It would have been better if you could provide a picture that would be easier to imagine.....but I would like to point ut that recent battery chargers which have VRLA batteries doesn't have this tapping ....I was told that they have a floating voltage of around 230v which would not harm the load unlike in tapped ordinary LA batteries where if the battery come across the load then the voltage would be around 275V...since the capacity float voltage of that battery is more.!!! if possibe I would get the drawing of both....

Dear Giri,

Tapping is independent of boost charging voltage. Pls. inform, if interlocking contactor as already mentioned in previous posts, is also provided in the system being offered or told to you. Answer to your question will depend on this reply.

Thanks and regards

Ashok Toshniwal, Bangalore, India

hii...ashok.I was bit busy.......pls give me your mail id so that i can send the drawing of both kind of chargers............

Dear Giri

Thanks for your maıl. My mail is ashoktoshniwal at yahoo.com or uni_iınsta at dataone.in

I look forward to your maıl.

Thanks and regards

Ashok Toshniwal

Dear Ashok

Can u please email the logic that was discussed with pankaj regarding the opening and closing of contactor .If possible can u provide me the ciorcuitery.

I agree that it is necessary to have tap cells, but can u please explain the other school of thougt which says 80% voltage is provided to the load.I could not get this point.

Regards

Drig Pandit

(lovepndt@gmail.com)