Different types of batteries have different internal resistances. The same type of battery has different internal resistance due to the inconsistency of internal chemical characteristics. The internal resistance of the battery is very small, we generally use the unit of milliohm to mean it. Internal resistance is an important technical indicator to measure battery performance. Under normal circumstances, a battery with a small internal resistance has a strong high-current discharge capacity, and a battery with a large internal resistance has a weak discharge capacity. In terms of the schematic diagram of the discharge circuit, we can consider the battery and the internal resistance separately, and divide it into a power supply with no internal resistance at all and connect it with a small resistance in series. At this time, if the external load is light, then the voltage distributed to this small resistor is small. On the contrary, if the external load is very heavy, then the voltage distributed to this small resistor is relatively large, and part of the power will be consumed in this Internal resistance (may be converted into heating, or some complex reverse electrochemical reaction). The internal resistance of a rechargeable battery when it leaves the factory is relatively small, but after long-term use, due to the exhaustion of the battery’s internal electrolyte and the decrease in the activity of the internal chemical substances in the battery, this internal resistance will gradually increase until the internal resistance is large enough. The power inside the battery cannot be released normally, and the battery is dead at this time. The vast majority of aging batteries are useless due to excessive internal resistance and have to be scrapped. Therefore, we should pay more attention to the discharged capacity of the battery rather than the charged capacity.
1. The internal resistance is not a fixed value
The
troublesome thing is that when the battery is in different power
states, its internal resistance is different; when the battery is in a
different service life state, its internal resistance is also different.
From a technical point of view, we generally consider the resistance of
the battery into two states: internal resistance in the charged state
and internal resistance in the discharged state.
a).
The internal resistance of the charged state refers to the measured
internal resistance of the battery when the battery is fully charged.
b).
The internal resistance of the discharged state refers to the internal
resistance of the battery measured after the battery is fully discharged
(when discharged to the standard cut-off voltage).
In
general, the internal resistance in the discharged state is unstable,
and the measured result is much higher than the normal value, while the
internal resistance in the charged state is relatively stable. The
measurement of this value has practical comparative significance.
Therefore, in the battery measurement process, we all use the internal
resistance of the charged state as the measurement standard.
2. The internal resistance cannot be accurately measured by general methods
The
internal resistance of the battery is very small, we generally use the
unit of microohm or milliohm to mean it. In general measurement
occasions, we require that the internal resistance measurement accuracy
error of the battery must be controlled within plus or minus 5%. Such
small resistance and such precise requirements must be measured with
special instruments.
3. the battery internal resistance measurement method currently used in the industry
In
industrial applications, the accurate measurement of the battery's
internal resistance is carried out by special equipment. Let us talk
about the battery internal resistance measurement method used in the
industry. At present, the battery internal resistance measurement
methods used in the industry mainly include the following two:
1). DC discharge internal resistance measurement method
According
to the physical formula R=U/I, the test equipment forces the battery to
pass a large constant DC current in a short period of time (usually 2
to 3 seconds) (currently a large current of 40A to 80A is generally
used), and the battery is measured at this time The voltage at both ends
and the current internal resistance of the battery are calculated
according to the formula.
The
accuracy of this measurement method is relatively high. With proper
control, the measurement accuracy error can be controlled within 0.1%.
But this method has obvious shortcomings:
(1) Only large-capacity batteries or accumulators can be measured, and small-capacity batteries cannot load a large current of 40A-80A within 2 to 3 seconds;
(2)
When the battery passes a large current, the electrodes inside the
battery will be polarized and polarized internal resistance will appear.
Therefore, the measurement time must be very short, otherwise the
measured internal resistance value will have a large error;
(3) The large current flowing through the battery will damage the internal electrodes of the battery to a certain extent.
2). AC pressure drop internal resistance measurement method
Because
the battery is actually equivalent to an active resistance, we apply a
fixed frequency and a fixed current to the battery (currently 1kHz
frequency and 50mA small current are generally used), and then sample
its voltage, and go through a series of processing such as rectification
and filtering. Then calculate the internal resistance of the battery
through the operational amplifier circuit. The battery measurement time
of the AC voltage drop internal resistance measurement method is
extremely short, generally about 100 milliseconds.
The
accuracy of this measurement method is also good, and the measurement
accuracy error is generally between 1% and 2%. The advantages and
disadvantages of this method:
(1)
Almost all batteries can be measured using the AC voltage drop internal
resistance measurement method, including small-capacity batteries. This
method is generally used to measure the internal resistance of laptop
battery cells.
(2) The
measurement accuracy of the AC voltage drop measurement method is likely
to be affected by the ripple current, and there is also the possibility
of harmonic current interference. This is a test for the
anti-interference ability in the measuring instrument circuit.
(3) This method will not cause much damage to the battery itself.
(4)
The measurement accuracy of the AC voltage drop measurement method is
not as good as that of the DC discharge internal resistance measurement
method.
The component error of the test instrument and the problem of the battery connection line used for the test
Regardless
of the above method, there are some problems that are easy to be
ignored by us, that is, the component error of the test instrument
itself and the problem of the test cable used to connect the battery.
Because the internal resistance of the battery to be measured is very
small, the resistance of the circuit must be taken into consideration. A
short connection line from the instrument to the battery also has
resistance (about micro-ohm level), and there is also contact resistance
on the contact surface between the battery and the connection line.
These factors must be adjusted in advance within the instrument. .
Therefore,
a regular battery internal resistance tester is generally equipped with
a dedicated connection line and a battery fixing shelf.
4. summary
In
fact, many aging batteries still have a lot of internal power, but the
internal resistance is too large to be able to discharge electricity,
which is really a pity. However, once the internal resistance of the
battery is added, it is even more difficult to artificially reduce the
internal resistance. Therefore, with regard to the aging battery, even
if we think of many ways to activate it, such as high current impact,
low current floating charge, and putting in the refrigerator, most of
them are of no avail. After understanding the above knowledge, we can
basically understand that when choosing a battery, we should choose a
battery with a smaller internal resistance as much as possible. Another
very important point is that if the battery is left unused for a long
time, its internal resistance will continue to increase. It is
recommended that you always use the battery to keep the battery's
internal chemical substances active.
BENZO Energy Technology Co.,Ltd
UFine Technology Co., Ltd
Site: www.bz-battery.com / www.benzoenergy.com
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