How to extend the life of Ni-MH cell phone batteries. Scheme, description

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How to extend the life of Ni-MH cell phone batteries. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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In the article brought to the attention of readers, a simple device is described, which, according to the author, will extend the life of a Ni-MH cell phone battery.

One of the once popular cell phone models "Siemens C25" is equipped with a Ni-MH battery with a nominal voltage of 3,6 V from three 700 mAh batteries. During operation, it turned out that the telephone set turns off when the voltage on the battery (AB) drops to approximately 3,5 V. However, it is known that the final discharge voltage of a Ni-MH battery is about 1 V, respectively, a battery of three batteries is 3 B. Thus, it turns out that each time you have to put an incompletely discharged battery into charging. As a result, over time, the so-called "memory effect" [1] begins to manifest itself in Ni-MH batteries, which consists in an increase in their internal resistance and, as a result, less energy output to the load. And although the status indicator shows the presence of a small charge, i.e., the battery is working, an attempt to connect or receive an incoming call leads to the fact that the telephone is turned off.

The internal resistance Rin NiMH AB should be within 0,2...0,3 Ohm [1]. To determine it, it is enough to measure the voltage of a fully charged battery without load (E), and then under load (U) of a known resistance R: Rin = R(E/U-1).

If the calculated value exceeds 0,2 ... 0,3 ohms, this is most likely the result of the "memory effect".

More advanced telephones, for example, "Siemens C35", are equipped with a "built-in" function called "Battery Care" [2], which consists in the forced discharge of the battery before charging it (to prevent the "memory effect", it is recommended to periodically "care"). Similar maintenance should be performed for cell phone batteries that do not have this function. To do this, it is enough to assemble a simple discharge device and periodically discharge the battery to the required level before charging.

Readers may have a question: will there be any negative consequences after such an additional discharge? Indeed, due to a deeper discharge at the first moment, the charging current can be very large. On fig. 1 shows a diagram of a "regular" charger for telephone sets "Siemens C25", from which it can be seen that the maximum charging current cannot exceed the values Imax = 1,25 / R1 = 1,25 V / 8,2 Ohm = 150 mA, where 1,25 V - voltage between the output of the stabilizer (pin 2) and control pin 1 [3] Such a current for a battery of the specified capacity is completely safe.

How to Extend the Life of Ni-MH Cell Phone Batteries

A schematic diagram of a possible variant of the discharge device is shown in fig. 2. Its feature is the absence of an additional power source, in contrast, for example, to that described in [4]. On the TL431AC (DA1) chip, a comparator with a threshold of 3 V is assembled, set by a resistive divider R1R2. Transistor VT1 is a key that disconnects the load - resistor R5 - from the battery when this threshold is reached. The HL1 LED performs the functions of a load element and a discharge indicator (when it ends, it goes out). Resistor R3 limits the current of the microcircuit, and R4 is the base current of the transistor.

The device is assembled on a board (Fig. 3) from one-sided foil fiberglass. Its outlines repeat the shape of the end part, on which the AB leads are located. To connect with them, contacts (Fig. 4) made of springy brass or bronze are soldered to the printed conductors in the places outlined by dashed lines (relay contacts can be used).

How to Extend the Life of Ni-MH Cell Phone Batteries

A properly assembled device does not require adjustment. A battery with increased internal resistance is discharged to 3 V before the next charge. To do this, wait until the telephone turns off, remove the battery and connect it to the discharge device: its contacts are pressed against its terminals, and a rubber ring is used to fix their mutual position, as shown in fig. 5.

How to Extend the Life of Ni-MH Cell Phone Batteries

In this case, the LED should light up, signaling the start of discharge with a current of about 90 mA. When the LED goes out, the device is turned off, and the batteries are installed in the telephone and put on charge. After charging, the telephone is used as usual. Experiments have shown that an AB with an internal resistance of approximately 0,75 ohms, after several additional discharges, loses the "memory effect" and its normal performance is fully restored.

Instead of those indicated on the diagram in the discharge device, you can use the KR142EN19 microcircuit, any p-n-p transistor (for example, the KT502, KT814 series), any LED with an additional current-limiting resistor R6. In the author's version, a Dialight LED with a built-in resistor is used, so a jumper is soldered on the board instead of R6.

Literature

Khrustalev D.A. Accumulators. - M.: Izumrud. 2003.
Biryukov S. Microcircuit voltage stabilizers of wide application. - Radio. 1999. No. 2. p. 69-71.
Vinogradov Yu. Automatic for additional discharge of Ni-Cd battery. - Radio, 2003, No. 1, p. 33.

Author: I.Podushkin, Moscow

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