# Internal resistance of cells.

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Introduction

Internal resistance of cells. Plan: Introduction: A toy manufacturing company has received complaints about a battery driven toy car. Apparently after only a few hours use the toy car's batteries need replacing. The customers are unsatisfied with the toy car and want their money back, the toy company believes that the short life span of the battery is not down to the toy car being faulty but that modern batteries have a high internal resistance, which causes poor performance. It is our job to see if this in fact the case. The internal resistance of a battery can be related to the current, resistance of an external load and emf (electromotive force) by Ohms law: Where IR is the terminal pd and Ir is the pd across the internal resistance, by also considering the equation V = IR we can substitute this in to produce: With this equation we can arrange it in The form y = mx + c, In this form the internal resistance (r) becomes the gradient and the emf (E) is the y axis intercept. So by producing a simple circuit and measuring values for x and y, e.g. y = the voltage across the external load and x = the current we can produce a graph that will give an accurate value for the emf, and more importantly, the internal resistance of the battery. ...read more.

Middle

readings that are too complicated to work with and feel that due to time I would work better with only 2d.p readings. The variable that I will keep constant is the exact battery I use for my results, so if I have to repeat any anomalous results the battery will still have the same internal resistance. To check the accuracy of my readings I can calculate the current and voltage using the equation V = IR. Therefore by considering the sensitivity of the equipment I can work out maximums and minimums if I need to. This may help if the readings for the current become very small and will help improve my accuracy when plotting a graph. Results: By collecting this data and using the equation V = rI - E I can plot a graph where the gradient will be the internal resistance and the y intercept will be the emf, and with these results I will be able to tell if the battery obeys Ohms law and prove if modern batteries have a high internal resistance which could lead to poor performance or not. Alkali Battery: Resistance (?) Current (I) Voltage (V) Current (I) Voltage (V) 1 0.94 1.07 1.070 0.94 3.3 0.38 1.36 0.412 1.254 10 0.13 1.50 0.150 1.30 12 0.13 1.51 0.126 1.56 15 0.10 1.53 0.102 1.50 22 0.07 1.55 0.070 1.54 33 0.05 1.56 0.047 1.65 47 0.03 1.57 0.033 1.41 56 0.03 1.58 0.028 1.68 ...read more.

Conclusion

Using what I found out and comparing to others in the group the results show me that the internal resistance of a modern (alkali) battery is smaller than that of the other. This leads me to the conclusion that the modern batteries don't have a higher internal resistance and that the performance of the toy car cannot be blamed on this. Internal resistance is due to the material that the battery is made from, the internal resistance of any power supply cannot be removed but it can be measured. When there is a current flowing through both r and R, energy is transferred in the internal resistance (r) of the battery as it gets warm, and also in the external load (R). This means that the terminal p.d. (the p.d. across R, the external load) is less than the full emf of the battery. So if the battery has a high internal resistance then less power is transferred to the circuit. In this case the resistance of the external load is fixed, i.e. the toy car, and the current depends on the internal resistance of the battery. So if the battery has a high internal resistance then a smaller voltage will flow, if the toy car has a small resistance the power of the battery will be drained very quickly and this may be the problem in this case. Rachel Jeffreys ...read more.

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