If the manufacturer provides detailed discharge curves for individual cells or for a general design covering various cell capacities, the cell internal resistance may be determined graphically. The internal cell resistance is the slope of the “initial volts” line in the discharge curve (this is not the same resistance that would be measured with an impedance tester): R Cell =ΔV /ΔI where R Cell = cell internal resistance (Ω) ΔV = voltage change (V1 – V2) corresponding to ΔI on initial volts curve (V) ΔI = current change (I2 – I1) corresponding to ΔV on initial volts curve (A) Also the short-circuit current available from a battery is: ISCB= VB / RB where ISCB = short-circuit current at battery terminals (A) VB = battery open-circuit voltage (V OC × N c) (V) RB = total battery equivalence resistance including cell interconnection resistances (Ω) Figure below shows typical short-circuit current magnitudes available at the terminals of telecommunications batteries. Variations can be expected for different brands and for models within a brand.
