Internal resistance of the battery / accumulator - an important parameter of the power source

What is the internal resistance of a battery / accumulator?

As we have already explained in our glossary of terms, the internal resistance in batteries and accumulators defines the inherent resistance of the cell. Each accumulator and battery operates within a certain range like a standard resistor, where the current flowing through them causes a specific voltage drop. The greater the resistance a battery presents, the greater the voltage drop it causes, and as a result, the voltage that physically reaches our receiver is always lower than the voltage measured on the accumulator outside the device (without load).

This lower voltage during operation will have significant implications for us. A typical device refuses to operate when the voltage drops to a certain, appropriately low level, so in the case of batteries with high resistance, it may prematurely refuse to work. Additionally, in many devices, the quality of operation of a given device depends on the voltage level (at higher voltage, devices often operate with greater power - motors run faster, flashlights shine brighter, toys move faster, etc.). 

Batteries with higher resistance, even when showing the same mAh capacity, generate a higher voltage drop at the terminals during discharge - according to Ohm's basic law. Even new batteries or accumulators differ from each other in this parameter. Additionally, the increase in internal resistance is a natural phenomenon of battery and accumulator operation as they are used and discharged. What is very important, this parameter deteriorates even in unused accumulators - it is one of the symptoms of cell aging. This is a very negative phenomenon, as it often leads to premature and unexpected shutdown of the device in use. Energy losses associated with the internal resistance of the accumulator are released in the form of heat directly on the accumulator and its contacts. High internal resistance can lead to overheating of batteries and accumulators in demanding devices - e.g., flashlights. In general, the conclusion is that the lower the resistance, the better. A low-resistance accumulator or battery can effectively and safely deliver higher current values.

Alongside capacity, this is one of the most important and significant parameters determining the performance of a battery or accumulator, of which not everyone is aware.

How to check the internal resistance of our batteries or accumulators?

The measurement procedure used by different manufacturers is not consistent, and usually, the resistance values provided in technical data sheets are not comparable with each other. Therefore, it is worth conducting such measurements on your own.

A typical measurement procedure involves (A) measuring the voltage of a cell that is at least half charged, under stable, repeatable conditions (with a small load or without load), and then loading it (B) with an additional, specified current value and quickly (usually within a few seconds) re-reading the voltage value (C). The difference between the measured voltages A-C divided by the value of the additional load - current B gives us the value of the internal resistance of the cell.

The results obtained even on the same battery will vary depending on the state of charge of the given cell (the highest, worst results are achieved by discharged cells). They will also usually be higher (worse) the higher the load/current B we use for measurement or later measure the voltage drop (C).

Therefore, when comparing these values, it is very important that the procedure is the same each time.

Self-conducted, "manual" measurement of internal resistance can be cumbersome, which is why chargers that can perform such measurements automatically and repeatedly come in handy.

There are only a few chargers on the market that measure these values correctly. Among the few exceptions are everActive chargers (e.g., model NC-3000) and Xtar chargers (e.g., VP4+ Dragon).

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