At 0.85v it was just weaker than the other batteries and depleted first. If it measured close to 0 or 0, or negative volts that would be a worrisome sign of reverse charging which can damage LSD nimh permanently.......
I think many do not realize just how easy it is to reverse charge a cell. The voltage reading you obtain from a cell after being used in series really doesn't tell you very much, however it can provide a guideline as to whether reverse charging might have occurred.
An example. Within the last week I discharged 4x 10Ah NiMh D cells that I always use in a series application. I discharged the cells in series on my CBA II. Since it is not possible see what the voltage of the individual cells is during discharge, when the discharge is nearing the end, I check to see if the cells are discharging evenly with a DMM.
The discharge was done at 1A (0.1C) and the cutoff set at 4.00 Volts (1.00 Volt/cell). When the "pack" had discharged to about 4.6-4.7 Volts, at which point the plot was well into the "knee", I began checking the voltages of the individual cells under discharge. I immediately noticed that cell #1 was reading lower than the other three, about 0.940V vs. 1.20-1.25V for the other three. Just before the discharge terminated, Cell #1 was at 0.234 Volt, the other three ranged from 1.12-1.18 Volt. The voltage of cell #1 during the final minutes of discharge, obviously, fell much more rapidly than the other three cells. I've noted this before, when discharging NiMh and NiCd cells (or any cell, really). When the voltage of a NiMh, or NiCd cell drops to ~1.00 Volt under discharge, the bottom literally "falls out". In this case cell # 1 came very, very close, to being reverse charged.
What I want to mention, that I think is important, is that the voltage of all 4 cells, within a couple minutes after the discharge, all read 1.20-1.25 Volts OC. Granted my discharge test was at a much higher discharge rate than Mr H's scale likely draws, I still think that it is entirely possible that the one cell that read 0.85 Volt was likely reverse charged. That is especially true with the amount of OC voltage spread observed between the four cells.
My main point being again, that the voltage observed after a discharge really doesn't tell you very much. A cell that has been reverse charged will almost never read a negative voltage after reverse charging has occurred. In fact, the only cells that I have ever seen that measured a negative voltage, were cells that had been left in a device for weeks, or months, that had a parasitic drain. Normally a cell that has been reverse charged will nearly always recover to a positive voltage when the discharge has stopped. I suspect this happened in Mr H's situation, as well.
Dave
