Accumulator  (from Latin accumulator — collector), in other words, Battery is an energy storage device whose main job is to accumulate and store energy for later use. An electric accumulator converts electrical energy into chemical energy and provides reverse conversion when necessary; it is used as an autonomous electric power source.
Battery, as an electrical device, is characterized by the following main parameters: electrochemical system, voltage, electrical capacity, and internal resistance, self-discharge current and service life. Its state is assessed by a combination of its three main characteristics: real capacity, internal resistance and self-discharge current. If you underestimate or ignore any of these parameters, or exaggerate the importance of one of them (usually capacity), you may find yourself much disappointed.
According to electrochemical system, lead-acid batteries (SLA), nickel-cadmium (NiCd), nickel-metal hydride (NiMH), lithium-ion (Li-ion) and lithium-polymer (Li-Pol) batteries are currently the most widely used batteries to power portable electronic devices and equipment.
Electric energy is stored directly in the cells during charging. Therefore, the quality of the whole battery depends on the cells’ quality. Control circuit controls the charge and discharge process and in some cases provides identification of the battery additionally. The control circuit of NiMH batteries contains minimum of passive electro radioelements, the control circuit of Li-ion and Li-polymer can contain a microcontroller.

Voltage, V of battery determined by the device it is intended to power. If the required voltage value is not provided by one element, then the battery is assembled from several elements connected in series, parallel or series-parallel.

The nominal capacity of a battery  is the amount of electrical energy that a battery should have theoretically at a fully charged state. The amount of energy is determined when a battery is discharged by constant current during a certain period until the specified threshold voltage is reached. Battery capacity is measured in ampere per hour (A*hour) or milliampere per hour (mA*hour). It is usually indicated on the battery label or encrypted in battery type. Practically, the real battery capacity ranges from 80 to 110% of the nominal capacity and depends on a large number of factors: manufacturer, storage conditions and storage period, commissioning, service technology, the battery chargers used, conditions and terms of exploitation. Theoretically, a battery with a nominal capacity of 600 mAh can deliver a current of 600mA for one hour, 60mA for 10 hours, or 6mA for 100 hours. In practice, the rated capacity is never reached at high discharge currents, and exceeded at low discharge current. 
Battery capacity rating is often indicated by letter “C”, so references like C, 1/10 C, or C/10 are often found here and below.
When we talk about a battery discharge equal to 1/10 C, this means a battery discharge at current equal to one tenth of the nominal battery capacity. So, for example, for a battery with capacity 600 mA*h, the discharge current 1/10 C is 600/10 = 60mA.
Similar to the above information about battery discharge, battery charge at current 1/10 C means charging battery at current equal to one tenth of the declared battery capacity.

The internal resistance of the battery, measured in milliohms (mOm), is the saver of the battery. It to a great extend determines the duration of battery life. The lower internal resistance, the more peak current the battery can deliver, and therefore the more peak power the battery has. A high internal resistance makes the battery ‘soft’ and leads to a sharp decrease in voltage at a sudden increase in the load current. Such a voltage collapse characterizes the ' weakness’ of an outwardly good battery, because the stored energy cannot be fully released into the load (remember Ohm's Law). On the other hand, a ‘strong’ battery with a low internal resistance puts almost all of its energy into the load.
The internal resistance of the battery depends on the cell capacity and the number of cells in the battery connected in series.
The internal resistance of batteries is measured using special battery analyzers, such as analyzer of BA400 series.

Here is the table with approximate internal resistance for batteries for phones of various electrochemical systems at a battery voltage of 3.7V:


Battery type Internal resistance (MΩ)  
  New By the end of the service life
NiCd 50 — 100 300max
NiMH 50 — 200 300max
Li-ion 100 — 250 300max

The phenomenon of self-discharge   is more or less common in all battery types and characterized by the loss of capacity after battery has been fully charged. To quantify self-discharge, it is convenient to use the amount of capacity lost by them over a certain time, expressed as a percentage of the value obtained immediately after charging. As a rule, a time interval is taken equal to one day or one month. So, for example, good NiCd batteries can permit about 10% of self-discharge within 24 hours after the end of the charge, NiMH batteries - a little more. For Li-ion batteries 10% of self-discharge is absolutely nothing and the time interval should be regarded as a month. Note that self-discharge of batteries is maximum in the first 24 hours after charging, and then significantly decreases. So, NiCd batteries can lose up to 20% of their capacity in a month, NiMH — up to 30% and Li-ion-up to 8% of their capacity.  Self-discharge largely depends on the ambient temperature. Thus, when the ambient temperature increases by 10 degrees relative to room temperature, self-discharge might be doubled.

The service life of  a battery is characterized by the number of charge /discharge cycles that it can withstand during operation without significant deterioration of its parameters: capacity, self-discharge and internal resistance. The service life depends on the charging methods, the depth of discharge, the maintenance procedure or lack of servicing, the temperature and chemical nature of the battery.
In addition, the battery life is determined by the time that has passed since the date of manufacture, especially for Li-ion batteries. The battery is usually considered to be out of order after reducing its capacity to 60-80 % of the nominal. 
For various reasons, individual cells in the battery pack may have different capacity and voltage, and this may affect its operational parameters negatively.

The memory effect  is a reversible loss of capacity caused by the enlargement of the crystal formations of the active substance of the battery and thereby reducing the area of its active surface. NiCd batteries and NiMH batteries (in a less extent) are affected by the memory effect. Often, the memory effect is attributed to the loss of capacity caused by improper operation and/or improper battery maintenance.

Energy density  is another important characteristic of the battery, which is often used to compare batteries of different electrochemical systems. It is measured in Wh/kg of battery mass. Lithium-polymer batteries have the highest energy density (150-200 Wh/kg), lithium-ion batteries are slightly inferior to them (100-150 Wh / kg), and nickel-metal hydride batteries barely reach the energy density of 60-80 Wh/kg. The energy density of nickel-cadmium batteries is from 40 up to 60 Wh/kg, and of lead-acid batteries - about 30 Wh/kg. Due to this, we can make a conclusion that regarding the same capacity of the batteries lithium-polymer and lithium-ion batteries have the smallest size and weight, nickel-metal hydride batteries are slightly larger, nickel-cadmium batteries are even more, and lead - acid batteries are the most bulky.

Recovery of NiCd and NiMH batteries  is the reverse process to the memory effect, from a physical point of view. This is the disaggregation of crystal formations to fine structure by discharging the battery at small currents up to 0.4V on each cell. The discharge is performed due to a special algorithm and controlled by a special battery analyzer, for example, ВА400.
The operating conditions of batteries are determined by the operating conditions of the cells in the battery pack. These conditions are different considering their different types and different manufacturers. Main differences are in the ability of cells to work at low-temperature and in temperature conditions for fast charge. Typical data for NiMH and Li-ion batteries are shown below.

NiMH batteries:

Standard charge: 0°C ... +45°C. Fast charge: 5°C ... +40°C.
Discharge: -20°C ... +60°C
Storage: -20°C ... 35°C (for 1 year).
Storage: -20°C ... 45°C (for 180 days).
Storage: -20°C ... 55°C (for 30 days).
Storage: -20°C ... 65°C (for 7 days).

Li-Ion and Li-Pol batteries:

Fast charge: 5°C ... +40°C.
Discharge: -20°C ... +60°C
Storage: -20°C ... 25°C (for 1 year).
Storage: -20°C ... 45°C (for 90 days).
Storage: -20°C ... 60°C (for 30 days).

Battery chargers
Battery chargers can be classified according to the battery type, charging method, and charger’s design.
Due to three main charging methods, there are three main types of battery chargers:
1. Standard (night) charger - charge with a constant current equal to 1/10 of the nominal battery capacity for about 15 hours.
2. Fast charger - charge with a constant current equal to 1/3 of the rated battery capacity for about 5 hours. Such battery chargers are equipped with a battery discharge device.
3. Accelerated or delta V (D V) charge - charge with initial charge current equal to the nominal battery capacity, when battery voltage is constantly measured and the charge ends after the battery is fully charged. The charging time is approximately 1 hour. Termination of the charge is based on the registration of a negative voltage drop (Negative Delta V — NDV), which appears in sealed NiCd and NiMH batteries when they reach the state of full charge. In NiMH, this drop is smaller in magnitude than in NiCd, and therefore is used in conjunction with other methods to stop the fast charge mode of the NiMH battery.

Battery Analyzers
Unlike battery charger, Battery Analyzer is a device specifically designed for performing maintenance on various types of batteries and provides:
1. Optimal battery charge and discharge in accordance with recommendations of their manufacturers.
2. Quantification of battery capacity and other parameters.
3. Battery recovery - recovery of the rated capacity of NiCd and NiMH batteries lost from operation.
4. Simultaneous independent servicing of various types of batteries.