[Ecky]

There's a lot of debate over on InsightCentral, but this post caught my attention. Bear in mind that this is theoretical, but I think consensus is that these deep discharges DO damage cells, but people still do them anyway because they get rid of voltage depression and can have an overall net positive.

Originally Posted by sser2

I think there are few misconceptions here re. grid charging and discharging a NiMH pack.

Looking at discharge plot, a NiMH cell is fully charged at about 1.5V. Upon discharge, the voltage quickly drops to about 1.2V and stays around there during the most of discharge. At about 0.9V, the cell is almost fully discharged. If discharge continues, voltage drops precipitously.

A NiMH CELL (but NOT battery, as you will see further) can be safely discharged to zero volt, and kept for a long time in this fully discharged state without degradation of capacity. However, if a current in the discharging direction continues through the cell after its voltage has dropped to zero, cell's polarity is reversed, and this condition damages the cell by screwing its electrolyte. This is exactly what happens when a BATTERY of dissimilar cells (e.g. old tired IMA pack) is discharged to zero. Weaker cells drop to zero faster than stronger ones, and stronger cells then push the reversing current through the weaker ones, damaging them. So, the result is further damage to those cells that are already weak.

As a rag-tag battery with zero volts on its terminals is charged, stronger cells reach their full charge much faster than those whose polarity has been reversed. So, as the weaker cells are catching up, the stronger ones are being punished by pushing current through them for a long time after they have been fully charged.

There is absolutely no point in discharging a cell below 0.9V. If the IMA battery is not discharged below 0.9x120=108V, this will pretty much guarantee that no cell in the pack is re-polarized.

The safest regimen for reconditioning an old pack is to charge it at low current (200-300 mA), and stop charging as soon as voltage reaches the plateau at 170-180 V. If there is no voltage sensor that automatically stops charging at this voltage, it is necessary to monitor charging to stop it manually. Discharging should be also monitored and stopped at the safe voltage.

To Natalya: instead of galvanizing the corpse, pull out all the sticks that you have, measure their capacity, assemble the pack with 20 best sticks, and cycle it several times using your grid charger.