3
\$\begingroup\$

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.

|improve this question
\$\endgroup\$
2
\$\begingroup\$

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.

|improve this answer
\$\endgroup\$
  • \$\begingroup\$ Weak or shorted cells in series configuration are just as much an issue; if you do not monitor/charge each cell but instead just put volts across the "ends" of the pack, well, you're gonna have a bad time. \$\endgroup\$ – John U Apr 28 '14 at 8:52
  • \$\begingroup\$ One thing I've wondered about---if one were to construct a battery pack with electronic switches so it could connect cells individually, could one improve pack lifetime by making a device e.g. run a third of the cells down to 10% capacity, then the next third, then the last third, etc. (varying which cells go first each time) rather than always having to run all cells in series? If one had six cells, and paired the best two, middle two, and worst two, would such usage help equalize the wear? I would think such an approach would greatly enhance longevity, but I've not seen it done. \$\endgroup\$ – supercat Apr 28 '14 at 16:16
  • \$\begingroup\$ @JohnU, but that's how every SLA battery works to begin with. Cell death causes battery death all the same. supercat, if you could make it cheap that would be a game changer. But realistically you'd only have room for maybe one extra cell in most existing footprints. Large systems that have battery needs just have secondary batteries laying around for when the primary has to go offline. Build it and make it reliable, and the world will beat a path to your door, sir. \$\endgroup\$ – Sean Boddy Apr 28 '14 at 17:30
  • \$\begingroup\$ @SeanBoddy - yes, as you say, cell death = pack death. Wiring them differently does not change that. That was all. \$\endgroup\$ – John U Apr 29 '14 at 12:02
  • \$\begingroup\$ @JohnU, I responded to your comment poorly. I totally agree, there currently doesn't exist any way around the cell death issue. \$\endgroup\$ – Sean Boddy Apr 29 '14 at 17:14

Not the answer you're looking for? Browse other questions tagged or ask your own question.