I need to vent...

3.7.5 NiMH Matching
Matching refers to the grouping of individual NiMH cells with similar capacities to be used within a battery pack. Typically, the matching of the cells in a battery pack is within 2%. Matching eliminates the potential of reversing the polarity of one or more cells in a battery pack due to the capacity range of the combined cells being too great. Matching becomes more critical as the number of cells in the battery pack increases. This is due to the potential of one cell having a capacity significantly lower than the average capacity of the other remaining cells. As a result, the lowest capacity cell has the potential to reverse polarity while the other cells remain at safe voltage levels before reaching the voltage cutoff. If a battery pack has one or more cells reversed before the Voltage Cutoff is reached, the performance and cycle life will be reduced.

In addition to the above TAKING THE MODULE BELOW 6 VOLTS INCREASES THE REVERSE RISK, IT CAN'T BE REPAIRED FIXED OR VOODOO'D BACK.


The Question:

I’ve repeatedly heard over the years that it could damage a cell to discharge it to less than 1V per cell and are you recommending this as a routine test for modelers to find weak cells?

Red Scholefield replies:
You cannot damage a cell by discharging it to zero volts. Where the problem arises is that when you try to discharge a multicell pack to a low voltage one or more cells will can be driven into reverse due to the differences in individual capacity of the cells. Reversing the cell is essentially charging it backwards and causes venting and if done long enough can damage the cell. When a cell is allowed to discharge through its own individual load and reaches zero volts, current ceases. There is no charge in the cell. Since there is no charge in the cell it is much easier for a short to develop and maintain itself. Because of this, some may associate driving the cell to zero with causing the cell to fail. The weakness was there all the time but did not show up since the current source (the charged material in the cell) would not allow it to manifest itself.

Usually you can decide if a pack is going bad without resorting to this test. Look for higher than normal self discharge (high resistance short is developing). Look for significant drop off in capacity, (High resistance short is shunting charge current and cell is not getting fully charged). Look for lower voltage after full charge (one cell is already shorted). Good news, I have seldom seen a cell short when it’s fully charged so you should get ample warning from the symptoms above. The short down – spring back test only confirms that you do have a short or allows you to get a high degree of confidence that a cell is good (from the separator integrity standpoint).



The risk of the Prius causing this is near zero as it DOES NOT USE THE UPPER AND LOWER portions of the voltage range, usually, mostly, generally.


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I use 4AA in my 4C Maglite (AA>C adapter)
two of the four SANYO 2500mAh now reads NEGATIVE 0.06~0.07V with a multimeter
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Ok, tried to charge in my MAHA 8-Cell charger but the batteries are not detected.
Now charging with a SONY dumb charger...
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ok, a quick update
charged in my SONY dumb charger for ~5 minutes,
the batteries now reads POSITIVE 1.30V..
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The answer to your question is yes, you can charge NiMh cells that have been reverse charged safely. The problem is, especially NiMh cells, suffer some degree of permanent damage to the cell when reverse charged. This applies to traditional, as well as LSD NiMh cells, such as eneloops.

In my experience, cells that have been reverse charged are never quite the same. They exhibit lower voltage under load and reduced capacity. This condition is permanent, and the cells cannot be "restored" to their original condition. Continuing to use cells that have been affected in this way, in series applications, compounds the situation, and accelerates the damaged cells demise. This is because the damaged cells will always be the first cells to run down, and will likely be reverse charged again, farther damaging the cells.

Dave