TESLA BLATNÁ has many years of experience in the development and production of chargers for all types of batteries and electronic battery protection devices (BMS).

We have many years of experience in developing battery chargers according to customer requirements. We produce chargers in power ranges up to 35, 50, 90, 150 and 300W. Currently we are working on development of new charger design with a power of 550W. When designing products, we follow the latest trends in the development of specialized circuits and advanced technologies, and if necessary, we consult with manufacturers about the possibilities of their application.

We have been designing and manufacturing protection electronics for Li-Ion batteries for more than ten years. Today, these batteries are among the most widespread, they can be found practically everywhere - from use in mobile phones, to powering hand tools, to large battery energy storage for photovoltaic power plants, or the drive of electric cars.

We manufacture chargers and electronic battery protections from first-class quality components so that their operation meets both strict safety requirements and requirements for energy consumption in idle mode.

The safe operation of chargers produced by us is ensured by multiple protection and safety functions.

Li-Ion battery charger and protective electronics complement each other.

Li-Ion cell construction

A lithium-ion battery is a type of electrochemical cell that was first introduced in the 1970s. In these batteries, lithium ions move from the anode to the cathode during discharge and back again as the battery charges. These cells have high energy density, no memory effect and low self-discharge current. However, they also have their negative properties, which mainly include the possibility of explosion or fire, and the development of chargers needs to be subordinated to them.

When charging lithium batteries, voltage charging is used, or a method referred to in foreign literature as CCCV (Constant Current followed by Constant Voltage). It is constant current charging until the battery reaches a predetermined voltage and then constant voltage charging.

Charging is terminated when the charging current drops to a predetermined value, in most cases a current value corresponding to 1.5 ÷ 2% of the nominal capacity is recommended, of course expressed in current units.


You can find Li-ion accumulators either in the form of individual cells or so-called "battery packs" for mobile devices. Battery packs can be found in consumer electronics and portable tools. If the maximum current or permitted voltage range is exceeded, the circuit disconnects the cell.

Battery packs are usually equipped with a protective circuit that prevents the destruction or explosion of the cell in the event of improper handling or a malfunction of the powered device or charger. Protection electronics usually monitor the minimum and maximum voltage of the cell, as well as the maximum discharge and charge current.

The terminals of the accumulator are not connected directly to the cell. The protection circuit permanently draws a current of the order of units to tens of microamperes from the cell. Battery packs are also equipped with a thermistor that informs the charger about the temperature of the cell.

General characteristics of Tesla chargers

We have many years of experience in developing battery chargers according to customer requirements. Tesla produces chargers in power ranges up to 35, 50, 90, 150 and 300W. New charger design with a power of 550W is under development. When designing products, we follow the latest trends in the development of specialized circuits and advanced technologies and, if necessary, we consult with manufacturers about the possibilities of their application.

The charging process is controlled by a microprocessor, for which our team of programmers creates software. This solution provides great variability both in the types of charged batteries (Li-Ion, NiMH, LiFePO, LeadAcid, Gel...) and in the battery connection options, from the simplest two-wire connection, only plus and minus contacts of the battery to various types of digital communication protocols, typically: SMBUS, I2C, HDQ, UART, RS232, etc.

The chargers are designed as high-frequency switching sources (90-150kHz, possibly up to 500kHz). It is a modern solution with both MOSFET and GaN transistors, which enables a design with small dimensions in relation to the resulting performance.

Charging is finished when the charging current drops to a predetermined value. In most cases, a current value corresponding to 1.5 ÷ 2% of the nominal capacity, expressed in current units, is recommended.

Functional safety of the chargers manufactured by us is ensured thanks to multiple safeguards for the termination or interruption of battery charging (monitoring of the battery temperature, monitoring of the charging time, detection of a drop in the charging current, measurement of the internal resistance of the battery). The chargers also have a number of protective functions (monitoring the function of the switch-off transistor, protection against polarity reversal, protection against cycling, interruption of charging in case of faulty operation of the control circuits).

We meet the standards for low power mode.

Electromagnetic compatibility standards.

Simple but well-thought-out construction.

Possibility of charging different number of cells.

We charge safely

Cell safety

With every charger for Li-Ion batteries, an important safety parameter is the way to end charging after the battery is fully charged (4.1 - 4.15 V / cell). We pay a lot of attention to the charging termination function and it is therefore secured multiple times. At the same time as monitoring the parameters for the termination of charging, it is necessary to ensure charging even if the temperature of the battery or the surrounding environment causes interruption of charging. In such case, the so-called temperature restart function is used, i.e. that if the battery is not yet fully charged, the charger "waits" for its temperature or the ambient temperature to drop to the permitted value (i.e. less than 42-45°C) and continues charging again. The function of temperature restarts therefore allows the battery to be charged even in adverse temperature conditions.

When charging the battery, we monitor the PARAMETERS, ensuring the termination or interruption of charging. Closely related to parameter monitoring are PROTECTIVE FUNCTIONS.


Monitored parameters

  • battery temperature reached (> 45°C)
  • the time that has passed since the start of charging - depends on the battery capacity
  • drop in charging current
  • excessive internal resistance of the accumulator

Protective functions

  • monitoring the function of the turn-off transistor
  • reverse polarity protection
  • cycling protection
  • two-level monitoring of the internal resistance (Ri) of the accumulator
  • battery voltage monitoring
  • interruption of charging due to faulty operation of the current regulation circuits

The safe operation of chargers manufactured by TESLA is ensured by multiple protection and safety functions.

Charging safety is our top priority.

Charger principle

As an example, we present a description of a 35W single-purpose charger. The charger is single-purpose, intended for charging a Li-Ion 4-cell battery (4S1P configuration), connected to five contacts – plus, minus, thermistor 6k8 connected to the minus contact, 1-wire communication, power supply. +5V, where information about the type of battery is transmitted after inserting the battery into the charger.


Start charging

After connecting to the network, the charger is in standby mode - this is the state when the charger has a minimum power consumption in the range of 0.3W - 0.5W; there is low voltage on the contacts; The R-LED lights dimly. After inserting the battery into the charger, the charger exits standby mode and starts transferring data from the battery. After its regular completion, charging is started according to the current state and at a temperature within the limit. After 1s of charging, the processor tests the Ri battery. If its value is greater than 3Ω, the charger signals "DEFECTIVE BATTERY" with a permanent red R/G-LED. If Ri battery is within the limit, the charger continues charging. After two minutes of charging, the Ri level evaluation parameter is changed to 1Ω – testing takes place once every 16 seconds. with an identical error signal. Charging is signaled by the green flashing of the status LED.

End charging

The information from the transferred data determines the limit voltage, from which the processor starts to reduce the value of the charging current with regulation pulses so that the limit voltage does not change. When the charging current drops to 1.4A, the charger starts to signal "battery full", but it continues to charge until the current drops to 400mA, when it switches to energy-saving mode, in which no current flows into the battery and it is loaded by a voltage divider with a max. 1mA at 30V. This state is signaled in the same way as the end of charging - by the continuous glow of the status R/G LED (green) lasting for approx. 8 hours, after which the charger wakes up again, recharges the battery by the loss caused by the voltage measurement load and again switches to the energy saving mode with the signal "AKU CHARGED” (R/G LED lights up green) for about 8 hours.

From the data transferred via the 1-wire line, the processor evaluates whether the inserted battery is not already charged - if so, charging is not started, but the "BATTERY CHARGED" signal is triggered with a transition to the energy-saving mode according to the previous point. If a battery is inserted with a temperature out of tolerance, the charger signals this state by red flashing of the status R/G LED ( interval 1/1 - 0.5 sec. ). After reaching the temperature within the tolerance (approx. 6°C - 42°C), the charger starts charging. In the case of an inserted Li-Ion battery, the charger switches to energy-saving mode - however, the "TEMPERATURE OUT OF TOLERANCE" signal remains.

85 - 90%

Efficiency of Tesla chargers.

2S - 14S

Number of charged cells.

90 - 230V

Input voltage range.

35 - 300W

Power range of chargers.

Practical advice for charging

The important thing is that the voltage of both Li-ion and Li-pol batteries must not drop below a certain value. The manufacturers' recommended lowest voltage is usually approx. 2.5 to 2.8 V per cell. If under-discharge does occur, it usually means deterioration of the parameters or even irreversible damage to the cell.
Quick charging
So far, no manufacturer recommends fast charging. The maximum allowed charging current is given as 0.7 to 1C. This corresponds to a charging time of about 3 to 2 hours. It is necessary to realize that for the last hour (when the cell is already charging in constant voltage mode) it is already charged to approx. 90% of maximum capacity. Currently, manufacturers recommend charging only with the CC-CV (constant current, constant voltage) method. Experiments with various impulse charging so far usually lead to a shortening of the life of the cells. The method of charging with current pulses at the end of charging only leads to a slight shortening of the last phase of charging the cell (i.e. the last 10% of capacity).
Connecting cells
Li-ion cells can be connected in series or in parallel without major problems. The combination is marked, for example, 4S3P, which means 4 series-connected triples of parallel-connected cells. It goes without saying that for both parallel and series connection, the parameters of the individual cells are completely identical. With new accumulators from one manufacturer and from one series, this is almost always fulfilled. For older accumulators, it is necessary to select the same voltage, capacity (at max. load current) and internal resistance. It is necessary to realize that if the cells are not completely identical, one defective cell may cause damage to other good ones. In professional devices (laptops, cameras, etc.), the multi-cell battery is always supplemented with special protection circuits (BMS), which ensure the correct voltage distribution between the individual cells, prevent under-discharge and other functions.
If charging takes place correctly, the battery does not heat up. An increased temperature indicates that something is wrong and charging must be stopped immediately while there is still time. When discharging, cells heat up like others. Their internal resistance changes with temperature. The optimum temperature for large currents is about 40 to 50°C. At low temperatures, the internal resistance increases and the maximum current decreases.