Knowledge Base

Understanding Battery Capacity and Rating

Understanding Battery Capacity and Rating
Understanding Battery Capacity and Rating

When a fully charged battery with maximum terminal voltage of 12.5 V is discharged by a constant load current to its lowest terminal voltage of 10.5 V in a certain time period, then this time period is called as Rating of the battery and is usually 5 hr, 10 hr, 20 hr or 100 hr depending upon application.
When a fully charged battery with maximum terminal voltage of 12.5 V is discharged by a constant load current to its lowest terminal voltage of 10.5 V in a certain time period, then this time period is called as Rating of the battery and is usually 5 hr, 10 hr, 20 hr or 100 hr depending upon application. The product of Discharge Current and this Time Period is called as Battery Capacity at that Rating.

To simplify this, if you have a battery which drains fully:
When discharged by 30 ampere in 5 hr then its capacity is 150 AH (30×5) at 5 hr and abbreviated as 150 AH C5
When discharged by 15 ampere in 10 hr then its capacity is 150 AH (15×10) at 10 hr and abbreviated as 150 AH C10
When discharged by 7.5 ampere in 20 hr then its capacity is 150 AH (7.5×20) at 20 hr and abbreviated as 150 AH C20

This Rating of batteries is application dependent.
5 Hr Rating – Industrial Batteries e.g. traction batteries or fork lift batteries IS-5154
10 Hr Rating – Stationary Tubular batteries e.g. UPS and Invertor batteries IS-13369-1992
20 Hr Rating – Engine Starter batteries e.g. automobile batteries IS-7372
100 Hr Rating – Long Term back-up batteries e.g. Solar batteries
Significance of Rating with respect to Battery Capacity

As you have seen above a same Capacity battery can have different Rating but if you notice carefully then you will find as the Discharge Time increases the Discharge Current ability of the battery decreases or vice versa i.e. as the Discharge Current increases the Discharge Time decreases. This is a typical battery phenomenon first discovered in 1897 by German Scientist called Peukert and is the result of loss of battery energy due to internal resistance of the battery.
This effect is reflected in Invertor and UPS Backup Time which means that for a given load, a battery rated with lower Hrs will give more backup time i.e. to say that 150 AH C10 battery will give more backup than 150 AH C20 battery for same load or 150 AH C5 battery will give more backup than 150 AH C10 battery. This difference varies as per the amount of discharge current and is usually between 35 to 40 %.
Inter Relationship between Battery Ratings

A battery can have any rating like C10, C20 etc. but then its capacity will be different according to that rating. There exist a relationship between C10 and C20 rating and is given as:
Capacity @C10 Rating = 0.8 X Capacity @C20 Rating
e.g. 100 AH battery at C10 rating will have capacity of 100/.8 = 125 AH at C20 rating. This implies that C20 rating just inflates the capacity but the discharge current ability reduces as compared to C10 rating.

IMPORTANT OBSERVATIONS
Battery capacity (AH) mentioned without any discharge rating (Hrs) as per battery chemistry has no meaning at all since battery capacity is a function of rating.
Lower the discharge rating of the battery higher the discharge capacity of the battery.
Therefore for same capacity the C10 rated battery give more backup than C20 rate.
Battery Capacity Selection as per Required Battery Backup

Back-up Calculation Formula
DC Current drawn from Battery = (UPS Capacity in kVA x 1000 x Load Power Factor) / (Invertor Efficiency x End Voltage of Battery Bank)
Battery Capacity = (DC Current drawn from Battery x Time required in hrs) / (% Capacity Utilisation/100)
Battery Capacity Utilisation Chart

Duration Hrs Capacity Utilisation %
1 50.0
2 63.3
3 71.7
4 78.2
5 83.3
6 87.9
7 91.7
8 95.0
9 97.9
10 100.00

Example:
3 KVA UPS 96 V DC system i.e. with 8 x 12 V battery bank with invertor efficiency 90%, load power factor 0.8 and backup time required 3 hrs.
DC Current drawn from Battery = (3 x 1000 x 0.8) / (0.9 x 8 x 10.5) = 31.75 Amp
Battery Capacity = (31.75 x 3) / (71.7/100) = 133 AH = 130 AH (say)
Therefore you will need 8 nos of 130 AH batteries and for all practical purposes it is better to select next capacity and hence 150 AH battery is recommended.

Battery Care, Installation and Maintenance Tips

New batteries should be given a FULL CHARGE before use.
Install batteries on an INSULATED surface like wooden planks or Rubber sheet or thermo Cole Sheet or in a painted & properly earthed MS Angle Rack.
Select the battery capacity PROPERLY as per your Invertor & UPS design & back-up required.
Take care not to offer much load on battery terminals while tightening also avoid the shorting of terminals.
Keep the top surface of the battery clean, dry & tidy.
Use only proper current carrying cables & cable connectors and check for it’s proper connection to prevent sparking & melting of terminals.
Apply petroleum jelly or Vaseline to terminals to prevent corrosion but DO NOT USE GREASE.
Ensure polarity of the battery i.e. connect positive cable to positive (red) terminal of battery & negative cable to negative (blue) terminal of battery.
Ensure that batteries are kept in well ventilated place with adequate shelter and check that battery is not exposed direct to sunlight.
Keep the vent plug tight & clean.
Top up the battery with DM Water after 6 months intervals.
NEVER USE TAP WATER, Bore water even mineral water as it can reduce your battery life.
The Water used to replenish batteries should be distilled or de-mineralized (DM) Water. Particular Care should be taken to avoid metallic Contamination (Iron).
NEVER ADD ACID in the battery otherwise it will damage your battery.
Fully charged battery have the acid gravity in between 1245 to 1250. If more then reduce it by adding little DM Water as required.
Excessive consumption of water is an indication of OVERCHARGE it can kill the battery. Check up charger of your UPS or Invertor.
Keep away FLAMES cigarettes etc from the battery, as the gases evolved are INFLAMMABLE.
Avoid MIXED BATTERY state i.e. of different capacities, makes and ages.
Protect battery from METALLIC OBJECTS like spanner, watch, etc. coming in contact with battery terminals.
Battery contains lead which is HAZARDOUS so return the old used up battery to the dealer or battery manufacturer.
PERIODIC BATTERY TESTING is an important preventive maintenance procedure.
Always use a matched voltage charger & battery pack systems. An undersized charger will never get the job done, no matter how long you let it run. An oversized charger will cause excess gassing & heat; this situation could cause EXPLOSIONS or other DAMAGE.
The battery storage area must be properly VENTILATED. Battery emits gases like hydrogen & oxygen. Concentration of 4% hydrogen are explosive, and recommended maximum concentration of 2% are required for battery storage area.

Friendly Advice with Useful Technical Information

Investment – The UPS and Invertor battery must be seen as high value investment. Therefore detailed understanding about the battery capacity, rating, warranty etc. is the most practical approach.
Battery Rating – As mentioned earlier the battery capacity without the mention of rating is absolutely meaning less therefore comparison must be made in tandem with capacity mentioned at same rating.
Battery Capacity & Acid Gravity – There exist a relation between battery capacity and specific gravity of acid i.e. capacity increases with increase in acid specific gravity. In Indian tropical climate battery is filled with 1.220 kg/lit Gravity acid and charged till 1.250 kg/lit Gravity. If acid gravity is made to 1.280 kg/lit then due to high ambient temperature battery will deteriorate faster giving short life. Therefore ignore capacity at test Gravity 1.280 kg/lit.
Battery Charging – Battery need to be charged in a certain way. The 3 step regulated charging technique by using microcontroller has been found to be the best technique. The steps are – Bulk Charging, Absorption Charging and Float Charging. We strongly recommend to use invertor or UPS which has this charging technique in order to get maximum life and performance of the battery.
Sulphation – When a battery is left in discharged state or it sits too long between the charges then the lead sulphate layer begins to coat the battery plates and once hardened this layer inhibits charging thereby shortening the battery life.
Premature Failure of Battery – The likely causes of premature failure of a battery are:
Abuse i.e. subjecting the battery to conditions for which it was never designed e.g. using car battery to invertor or UPS.
Drawing more current than the battery was designed for i.e. using low capacity battery to high capacity invertor.
Overcharging due to improper voltage setting occurring either due to shift in charger voltage calibration effected by power supply voltage surges or due to use of wrong chargers.
Allowing electrolyte level to fall below plate level.
Operating or storing the battery in too high or too low ambient temperatures.
Subjecting the battery to excessive vibration or shock.
Over-discharging on regular basis i.e. allowing the battery to become completely discharged to below safe voltage of 10.5 V.
Topping up with tap water instead of distilled or DM water.

Applications

Our Power back-up solutions can be used in a number of places.
Computer Work Stations.
Local Area Networks (LAN)
BPO / Call Centers
Lab Equipments
Medical Equipments
Industrial Equipments
Air Traffic Control Systems
Software Development Centers
Data Processing Centers
Studio and Telecom Centers
Home
and many many more…