What is the "reverse polarity" of a battery?

What is the “reverse polarity” of a battery?

The chemical reaction equation of a lead-acid battery says:


It can be seen from the reaction equation that PbSO4 is generated on the positive and negative plates after the discharge is completed. At this time, theoretically speaking, if the same substance is immersed in the electrolyte, there is naturally no potential difference, and there is no difference between positive and negative electrodes. At this time, if there is still current passing through the external circuit along the direction of the original discharge current, a charging reaction will occur inside the battery, as shown in Figure 1.

What is the "reverse polarity" of a battery?
Figure 1 Reverse polarity of battery

This phenomenon occurs frequently in battery packs, but many people do not notice it. This is because it can only be measured if the discharge current is uninterrupted. After the discharge stops, the polarity of the battery returns to the correct state. If a 12V battery, the terminal voltage drops sharply during discharge, at this time, among the 6 single cells of the battery, a single cell may be in a reversed state, and the discharge should be stopped at this time.

The effective use capacity of the battery pack is determined by the single cell with the lowest capacity. If the battery with the lowest capacity has discharged its full capacity during discharge, the capacity of this battery pack will be reduced to “0”, and the discharge should be stopped at this time. If it is discharged again, the single cell will have reverse polarity. Many people think that when the actual capacity of a single cell drops to “0”, it is equivalent to removing the single cell from the battery pack, and there is no other harm. In fact, this is not the case at all.

Over-discharge will cause the lead-acid battery to appear “reverse polarity”, that is, the positive and negative polarities of the battery are reversed. At this time, the voltage provided by each normal battery is usually 1.5~1.7V, and the voltage of the reverse pole single cell is negative. The reverse voltage of the h battery measured by the oscilloscope is up to 3.8V, that is to say, one reverse pole battery needs to offset the supply voltage of 1~2 normal batteries, which leads to a substantial drop in the effective total voltage of the battery pack. For batteries used in series, capacity balance is an inevitable problem. The more cells connected in series, the more prominent this problem becomes.

The damage of the reverse pole to the battery is carried out in the way of acceleration. The following is an analysis of the reverse pole process and the damage mechanism.

As mentioned earlier, PbO2 on the positive plate is not completely converted into PbSO4 at the lower discharge limit, and Pb on the negative plate is not completely converted into PbSO4, but the two substances coexist on the plate. The continuous discharge of the external circuit requires that each single cell in the series battery pack provides discharge and maintains its own terminal voltage above the specified voltage. However, due to the difference in capacity between single cells, when the difference reaches a certain level, the small-capacity battery can no longer provide enough electric energy to be released outward, and the terminal voltage is shown to drop below the specification until it reaches zero. If the discharge stops at this time, due to the diffusion of sulfate ions, the part of the electrode plate that has not become PbSO4 is exposed to a certain concentration of sulfuric acid again. Then the terminal voltage of the battery quickly rises to more than 1V. But the voltage value at this time has become a “virtual voltage”. If the discharge current is continuous, the voltage of the battery terminal will drop to zero and not stop, and continue to drop to a “negative value”, that is, reverse polarity. At this time, the voltage of the negative electrode of the battery can be measured higher than that of the positive electrode with a voltmeter. The larger the discharge current and the longer the duration, the higher the reverse voltage. The process of reverse polarity is the process of reverse charging the battery. When charging, there is a reaction in which PbSO4 is converted into Pb+PbO2. However, the newly generated PbO2 is generated on the original negative electrode, and forms a pair of batteries with the remaining Pb on the negative electrode. The electrons quickly move from the Pb to the new ecological PbO2 through the conductive metal in the electrode plate, so the original negative electrode plate Pb and the new ecological PbO2 are converted into PbSO4 together. Similarly, a similar reaction occurs on the positive electrode. Therefore, under the catalysis of the external circuit, the active materials on the positive and negative plates of this battery are rapidly converted into PbSO4. Because the PbSO4 generated in this way has a compact structure and coarse particles, it is difficult to recharge when it is recharged. We say that the battery is vulcanized.

Is it possible to use high current for a long period of forced charging to convert the positive and negative electrodes in the primary battery? This is impossible. Because the additives in the positive and negative lead pastes are different, the battery after the pole rotation can only have a virtual voltage, and there will be no real and useful ampere-hour capacity.

Lead-acid batteries are relatively “skinny”, and one reverse polarity will not cause permanent damage to the battery. Lithium-ion batteries are different, and a single reversal can cause permanent damage.