Is it possible to say that operating time of a portable equipment with battery directly depends on battery capacity?
In most cases, yes. However, in portable medical equipment, for example, the dependence of operating time on the battery energy may be nonlinear.
The reasons why portable devices (such as battery-powered defibrillators and cardiographs) often run less than the device operation time specified in the instructions are discussed below. This is especially actual for used batteries.

⦁ Reasons for reducing battery capacity and actions required to restore it.
⦁ Battery internal resistance and its effect on operating time.
⦁ Self-discharge and reasons for turning off portable equipment though the battery is charged.
⦁ Battery voltage and its change during battery operation.

1. Reduction of battery capacity during operation.

Battery energy (capacity) gradually decreases due to the usage, aging, and improper maintenance of the batteries. A new battery with 100% capacity gradually loses its original capacity, and when it is about 70-60% of rated capacity, it is advisable to replace such a battery. As a rule, a typical battery life threshold has capacity 80%.
Note: in practice, the remaining capacity of NiCd batteries used in such portable medical equipment as defibrillators and cardiographs is even smaller – about 20-30%. This is due to lack of money for proper testing, recovering and replacing batteries.

The total battery capacity can be divided into three imaginary zones:
⦁ A zone that contains energy.
⦁ An empty zone that can be recharged.
⦁ “A stone” – an unusable zone.
In nickel-cadmium and metal-hydride batteries, “stone” refers to large-crystal formations (this is where the term itself came from) that do not accept charging. Otherwise, this phenomenon is called "memory effect".
The capacity of nickel-cadmium and metal-hydride batteries can usually be restored by cycling, that is, by making several charge/discharge cycles using a special method on battery analyzers.
Now there are methods that are more effective than simply conducting charge /discharge cycles.
One of them is to discharge battery in two stages: firstly, to discharge battery with rated current up to 1V per cell, and then to perform slow discharge to almost zero (usually up to 0.4 V per cell). This method is used to destroy crystal formations and restore original cell structure.
Using this recovery method, you can return 60% - 70% of reused NiCd batteries to their original capacity. However, it should be noted that an old battery might have a high self-discharge current due to damage to the separator of individual cells. Such damage occurs when the battery has been operated without periodic servicing for a long time.

2. Internal resistance and its effect on battery performance.

Internal resistance is an essential part of battery characteristics.
It determines the battery performance mostly. If a battery with high internal resistance requires large current, the output voltage of the battery will drop due to a large drop in internal resistance.
As power consumption of defibrillators and cardiographs is pulsed, at peak load the battery voltage may reduce to its lower limit and the device will report (if, of course, such function is available) that the battery is discharged, though in the fact it is still far from full discharge. Moreover, sometimes you may notice that the device indicates full battery discharge though the battery has 100% of capacity. Battery with high internal resistance can function normally with small DC load, similar to the load used in a pocket flashlight or a portable CD player. The main part of the battery charge is spent here with such a load, since high internal resistance does not play a special role in this case.
There are different reasons why internal resistance of batteries with different types of chemistry increase. As for nickel-cadmium and metal hydride batteries, this happens only because of memory effect. It is recommended to cycle batteries once a month or at least completely discharge them. If not cycling for three or four months, the battery capacity may drop by a third or more, and to recover such a battery will be very difficult. Regular battery cycling reduce its internal resistance.
However, it is not recommended to discharge battery completely before each charge. Some battery chargers do that. This cause premature wear of cells and reduction of battery life. It is not recommended to leave the battery in the charger after recharging it either.

2.1. The internal resistance measurement methods.

There are several ways to measure battery internal resistance.
One of the most common is method of constant load, when discharging the battery the voltage drop is measured relatively to the battery rated voltage. The voltage drop divided by the current gives the internal resistance.
The alternating current method (AC method), also known as conductivity test, measures the electrochemical characteristics of a battery when it is exposed to AC current. Battery defects that cause loss of capacity affect the battery conductivity and the battery analyzer fixes that.

3. High self-discharge current and its effect on battery life.

Self-discharge current is a parameter inherent in any type of battery. Every time battery tends to return to its discharged state as a compressed spring. NiCd and NiMH batteries have the highest self-discharge current compared to other types of batteries. A NiCd battery loses an average 10% of its charge within the first 24 hours after the charge ends. After one day, the capacity loss is about 10% per month.
Self-discharge may happen because of separator damage, when large-crystal formations of clumped crystals break through it. A separator is a thin plate dividing positive and negative electrodes.
To avoid it you need to apply less active material to the electrode plates during production. In this case, the capacity is degraded, but the expansion/compression characteristics of the plates during charge /discharge and load specification are improved, and battery service life, a number of charge /discharge cycles, is increased.
A damaged separator cannot be repaired by performing charge /discharge cycles, even using battery analyzers. The reasons for this are incorrect maintenance, lack of servicing or use of poor-quality battery chargers. Another reason of battery discharge is depletion of battery life. In a worn-out battery, the electrode plates swell, sticking together, that increases the self-discharge current.
Another reason of battery discharge is depletion of battery life. In a worn-out battery, the electrode plates swell, sticking together, that increases the self-discharge current.
The self-discharge current graph is not linear and reaches its maximum immediately after the end of charge. High-performance batteries with an increased electrode area and a highly conductive electrolyte are subject to self-discharge largely than less productive models.
As the temperature rises, the self-discharge current increases for all types of batteries.
It is believed that when the temperature rises by 10°C, the self-discharge current is doubled. For example, the energy loss is very high for a battery left in a closed car during hot summer. It looks impressive when the battery loses more energy during the day due to self-discharge than from direct use.
The self-discharge current gradually increases depending on battery’s age and number of charge/discharge cycles. For example, an increasing self-discharge current renders a NiMH battery unusable after 300 - 400 charge/discharge cycles, and a NiCd battery - after 1000 cycles.
To cycle the battery or to recover it will not help in this case, the only alternative is to replace damaged cells or the whole battery pack itself.

Self-discharge can be calculated using battery analyzer according to the following algorithm:
1 . The battery is fully charged (with nominal current) and after that its capacity is measured (discharge time is measured too).
2. The battery is recharged and left for 24 hours, after that its capacity is measured.
It is possible to obtain more precise data of self-discharge if leave the battery alone for 72 hours or more. A longer rest period compensates for a relatively high self-discharge in the first day after the battery is fully charged. After 72 hours, the capacity loss should not exceed 15-20%. The most accurate results of self-discharge can be obtained after seven days of battery rest.
3. Power supply voltage and its change.

Any well-designed portable equipment should be developed to use the full volume of battery energy. This means that if the battery provides voltage in the range of 6.0V-7.5V during its discharge, then the device powered by it should also be developed for this voltage range. However, the voltage threshold in the equipment, made by some well-known manufacturers in the market, is significantly higher than the minimum battery voltage.
We can say that the problem of an overestimated power cut-off threshold is more concerned with the equipment developer than with battery usage problems. However, as long as the manufacturer solves this problem, such equipment will appear in the market and purchasers will buy it accordingly. It is necessary to draw attention of potential users to the fact that portable equipment with an overestimated power cut-off threshold is even more demanding on the battery. Because equipment operate succeefully only when it has a high-quality battery with high maximum voltage.
There are the following reasons of reducing battery voltage:
⦁ Short circuit in the battery cells.
⦁ Memory effect (only for nickel-plated batteries when lack of servicing).
⦁ Increased temperature may also cause the battery voltage drop. In this case, reduction of battery voltage is temporary. When the battery cools down its voltage will return to normal.


1. You need to evaluate the battery state, considering at least three parameters: internal resistance, capacity and self-discharge current.

2. You need to have battery service regularly, using specialized equipment - battery analyzers.

3. If such maintenance is not possible, unfortunately, it is necessary to replace 100% of batteries that are in operation every 2-3 years.