If make a 5V battery pack as a series of four 1.2V NiMH batteries, there is some risk of polarity reversal when the pack is discharged too much. I doubt this risk is reduced by plugging such packs in series.
What if I wire up the eight batteries from two such packs as a single pack with connecters between the cells that lie at the same potential within the pack? A priori, I'd expect this provides some protection against polarity reversal by averaging the characteristics of the two cells that stay at each potential. Is this true?
Are there any issues with charging such a "wide" pack arrangement? Are there any advantages to building in the connectors to access the intermediate potentials, like LiPo batteries require?
As an aside, are high draw 10 amp-hour NiMH D cells at more or less risk of polarity reversal? Obviously, the financial risk in destroying a cell is higher.
Best Answer
The biggest problem with this design concept is that cells in parallel automatically have the same potential at their terminals. It doesn't sound like a big deal, until you consider the possibility that one cell can short out with age.
Normally, a shorted cell just loses its voltage and becomes ineffectual. If it were side by side with a similar cell, however, you have essentially guaranteed the loss of the other cell as well.
Cell reversal occurs after the cell is totally discharged, causing it to develop an increasing voltage in the wrong polarity. Having another cell in parallel won't help, since it has no choice but to discharge to the same voltage as the dead cell, which could actually set up a circulating current between the two cells.
Best battery practice is still to stack up cells to get voltage, then stack strings to get current. No one talks about it much, but this has the advantage of putting the cells internal resistances in the path of parallel problems like this.
Side note: technically LiPo and lithium batteries don't require access to intermediate voltages, so much as it is just a very handy way to charge the battery much faster, or get other voltages into a device. Having access to every cell means you can charge and monitor them all indepedently, too.