Thomas Anderson
New Member
I have been reading some books on NiMH batteries. Some authors state that for optimum performance and longevity NiMH batteries need to be "exercised" or "refreshed" or "reconditioned", that is subject to a complete discharge, followed by a complete charge.
For instance, from "Ni-MH Technical Handbook (Panasonic, 2017)":
"As with Ni-Cd batteries, repeated charge and discharge of these batteries under high discharge cut-off voltage conditions (more than 1.1V per cell) causes a drop in the discharge voltage (which is sometimes accompanied by a simultaneous drop in capacity). The discharge characteristics can be restored by charge and discharge to a discharge end voltage of down to 1.0V per cell."
This is form the "Handbook of Batteries (Linden, 2002)":
"The voltage drop occurs because only a portion of the active materials is discharged and recharged during shallow or partial discharging. The active materials that have not been cycled change in physical characteristics and increase in resistance. The active materials are restored to their original state by the subsequent full discharge-charge cycling."
One author states that such an "exercise" should be done for NiMH batteries at least once every three months.
There is some disagreement as to what "end of discharge voltage" should the cells be discharged to. Some authors suggest 1.0V per cell, others 0.9V per cell. Most authors agree that overdischarge is detrimental to NiMH batteries, but the rationale that they provide is only applicable for cells connected in series: if you discharge several cells connected in series, then because of the capacity mismatch one or more of the cells might get their polarity reversed, if the cut-off voltage is too low. To prevent that from happening people suggest setting the cut-off voltage to 1.0V per cell, or even higher when more cells are connected in series.
Polarity reversal is no doubt detrimental, but it could not happen if cells are discharged individually in a "constant resistance" mode (that is being connected to a load of a "constant" resistance). It is established that for NiCd batteries, discharging to a voltage below 0.9V per cell frequently works better for restoring the battery's performance.
In addition to this I have not found info on how this end of discharge voltage should be measured. By logic it should be the open circuit voltage, but if you look at the discharge plots, it seems as if they measure it with the load connected to the cell.
If you connect a load to a cell, the voltage will keep decreasing, but as soon as you disconnect the load the voltage will start increasing back, not to the original value, but considerably.
Say, I had been discharging a NiMH cell (an AA battery) in a "constant" resistance mode for about 30 hours and the voltage has dropped to 4.8 mV with the load still connected. After I disconnected the load, the cell's voltage started to increase rapidly and in an hour the cell's voltage has risen to 1.1V. Does this mean that the cell was not fully discharged?
So I have these questions:
1) What is the best end of discharge voltage for "refreshing" of a single NiMH cell in a "constant resistance" mode?
2) What is safe end of discharge voltage for discharging of a single NiMH cell in a "constant resistance" mode?
3) Is this an open circuit voltage, or a voltage with the load connected to the cell?
4) If this is an open circuit voltage then how one can practically measure it while discharging a single NiMH cell?
For instance, from "Ni-MH Technical Handbook (Panasonic, 2017)":
"As with Ni-Cd batteries, repeated charge and discharge of these batteries under high discharge cut-off voltage conditions (more than 1.1V per cell) causes a drop in the discharge voltage (which is sometimes accompanied by a simultaneous drop in capacity). The discharge characteristics can be restored by charge and discharge to a discharge end voltage of down to 1.0V per cell."
This is form the "Handbook of Batteries (Linden, 2002)":
"The voltage drop occurs because only a portion of the active materials is discharged and recharged during shallow or partial discharging. The active materials that have not been cycled change in physical characteristics and increase in resistance. The active materials are restored to their original state by the subsequent full discharge-charge cycling."
One author states that such an "exercise" should be done for NiMH batteries at least once every three months.
There is some disagreement as to what "end of discharge voltage" should the cells be discharged to. Some authors suggest 1.0V per cell, others 0.9V per cell. Most authors agree that overdischarge is detrimental to NiMH batteries, but the rationale that they provide is only applicable for cells connected in series: if you discharge several cells connected in series, then because of the capacity mismatch one or more of the cells might get their polarity reversed, if the cut-off voltage is too low. To prevent that from happening people suggest setting the cut-off voltage to 1.0V per cell, or even higher when more cells are connected in series.
Polarity reversal is no doubt detrimental, but it could not happen if cells are discharged individually in a "constant resistance" mode (that is being connected to a load of a "constant" resistance). It is established that for NiCd batteries, discharging to a voltage below 0.9V per cell frequently works better for restoring the battery's performance.
In addition to this I have not found info on how this end of discharge voltage should be measured. By logic it should be the open circuit voltage, but if you look at the discharge plots, it seems as if they measure it with the load connected to the cell.
If you connect a load to a cell, the voltage will keep decreasing, but as soon as you disconnect the load the voltage will start increasing back, not to the original value, but considerably.
Say, I had been discharging a NiMH cell (an AA battery) in a "constant" resistance mode for about 30 hours and the voltage has dropped to 4.8 mV with the load still connected. After I disconnected the load, the cell's voltage started to increase rapidly and in an hour the cell's voltage has risen to 1.1V. Does this mean that the cell was not fully discharged?
So I have these questions:
1) What is the best end of discharge voltage for "refreshing" of a single NiMH cell in a "constant resistance" mode?
2) What is safe end of discharge voltage for discharging of a single NiMH cell in a "constant resistance" mode?
3) Is this an open circuit voltage, or a voltage with the load connected to the cell?
4) If this is an open circuit voltage then how one can practically measure it while discharging a single NiMH cell?