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Battery Internal Resistance

The batteries also have an internal resistance that in case of large series connection could be significant and could be necessary to take into account. The internal resistant is a function of several factors among them are the design of the battery and thickness of cells plates, materials, electrolytic fluid, and terminals design and materials. Normally the internal resistance of a battery is just few ohms to fraction of an ohm.

How to Calculate the Internal Resistance of a Battery

Lest start by analyzing the battery internal and the load resistance circuit on the figures

A and B below:

1- Measure the open circuit voltage of the battery with a high impedance meter, let assume that it is 12.2 Volts, see figure A.

2- The equation that defines the close circuit voltage including the internal battery resistance Rint and the voltage at the load Rload in figure B is:

Vtotal= VRint + VRload;

3-Choose a current for the which the internal resistance will be calculated, always select a current that is safe for the battery current handling capability, lets say 10 amperes for this example.

VRload = Rload X IB ; where IB is battery current flowing in the resistors Rload

Then; Rload = VRload / IB = 12.2 volts / 10 amperes = 1.2 ohms

4- Now connect this Rload into the circuit of figure B

5- Now measure the voltage between across Rload and write the result (it is the same if measured at the battery terminals)

The new total voltage is =12 volts

6- Now calculate the voltage drop, voltage drop = Voltage in open circuit - voltage with the resistor load Rload connected

= 12.2 - 12 = 0.2 volts

The 0.2 volts difference is the voltage drop caused by the current flowing in the internal resistance of the battery Rint

Vtotal= VRint + VRload

Vtotal = 0.2 v + 12 v = 12.2 volts, this equation completes the battery voltage circuit with the resistor load.

7- Calculate the internal resistance:

Rint = VRint / IB = 0.2 v / 10 amperes = 0.02 ohms at the given current

How to Calculate the Internal Resistance of a Battery

Operating Temperature Range

Example:

For Charge = -10 C to + 45 oC

For Discharge = -20 to + 55 oC at a given current

Charge Retention Also Known as Shelf Life, given at room temperature 20 oC or any other specified

97% At one month

90% At three months

82% At six months

Deep Charge Discharge Capacity Cycles

This concept refers to the number of deep discharge than a battery can tolerate before

+

_

Rint

Rload

Battery

Figure A, Open Circuit Voltage

Figure B, Closed Circuit Voltage

S1 Open

S1 Closed

Open Circuit Voltage

12.2 V

0.2 V

VRint = Voltage Drop in Rint

VRload

12 V

IB

VRload

0 V