nimh-full vs. partial cycles

[julesrules]

ive been reading about eneloops and their cycle lifespan, 1800 cyles and a certain charge and discharge rate to a certain voltage and was wondering what findings (if any) people had on real world benefits to either practice?

 

[Wrend]

In theory partial charge and partial discharge would probably help them to last longer (cumulative lifetime capacity total) if you could balance them and cut the charge/discharge off precisely. That isn't going to happen in normal real world use though. Just don't leave them charging on a "dumb" charger for too long or charge them at too high of a rate or use a charger that overcharges them and gets them more than just a little warm. Also avoid discharging them less than 1V per cell when used in series, and don't use cells that aren't at the same charged level as each other.

I don't have any of the "1800" cycle ones, just the "1000" (first gen.) and "1500" (second gen.) ones. The "1000" ones are still performing like new after 3 years of use. The "1500" ones too, of course, but I haven't had them as long.

They should last you quite a while.

I may pick up some of the "1800" ones when I need some more to run more things and have more sets charged and ready to use (or maybe "2000" by then). I just haven't needed to buy any AAs or AAAs, which is nice.

 

[mattheww50]

Actually NiMh (and NiCd) cells should tolerate full discharge and recharge quite well as long you avoid the two cell 'killers'.
1). Reverse charging (discharging the cell below 0 volts, such as happens in a battery pack when one cell goes flat before the others)
2). Avoid overcharging.
These are the two cell killers. I am unsure of the exactly what happens with reverse charging that kills cells, but overcharging does so by using the excess charging energy to break down the electrolyte,
and vent the resulting gas. The cell capacity declines as the amount of electolyte goes down (you end up with cathode/anaode surfaces that are no longer covered in electrolyte, reducing the usable electrode surface area), until it finally dies. Fortunately it takes a lot of mAH to do this, but if you overcharge in a dumb charger long enough, you do indeed pump a lot more mAH into the cell than it has capacity for. Most dumb chargers have fairly low charging currents, so it takes a long time to 'boil' away substantial amounts of electrolyte but it does happen. IIRC it takes about 55Ah to convert 20 grams of water into hydrogren and oxygen, so a large battery like a D sized NiCd/NiMH can sustain a great deal of excess charging before you electrolyse enough electrolyte to make much difference.


Overcharging also tends to happen in series battery packs if the cells are not all of identical capacity, and initially all charged to the same level. If as much effort went into balancing charge and discharge levels in NiMh and NiCd battery packs as goes into Li-Ion battery packs, the NiCd and NiMh packs would last a long longer.

If you avoid the two cell 'killers', NiMh and NiCd batteries should last a VERY long time.

 

[Wrend]

1V to 0V goes by very quickly. You're not going to be able to catch a cell at 0V before it risks becoming reverse charged. This is the main reason I recommend not using them below 1V per cell (1.1V per cell might be even better), as I said, when using them in series. Keep in mind that this is a low load voltage level. Also, the Eneloop FAQ recommends not keeping them discharged because of reduced performance characteristics, and that cycling them several times could be needed to help increase their performance again.

Should I store my batteries charged or uncharged?
If stored with charge, it is possible that when you return to use the battery there may still be some charge left in the battery, therefore it will enable you to use it right then and there. Also, by keeping some charge in the battery, it will require you to "cycle" the battery fewer times until it reaches its peak charge. If you store them with no charge, you will have to "cycle" the battery multiple times until it reaches its peak charge.

-http://us.sanyo.com/eneloop/FAQs


Here is a graph of the voltage over time of an 8 cell Eneloop pack being discharged at a 170mA rate (At the end of the graph it hasn't even reached 1V per cell yet and is starting to drop off very quickly.):

...

All that aside, I'm not sure that they would, but I wouldn't be surprised if they had a higher cumulative lifetime capacity if they were charged to maybe 80% and discharged to maybe 20% at ideal rates, temps, and intervals. It's inconsequential either way since there's no practical way to do that and actually use the cells.

Just charging them more frequently (so that they don't get as low) probably isn't a good way to go since the normal charging process likely adds more wear to them.

 

[StorminMatt]

These are the two cell killers. I am unsure of the exactly what happens with reverse charging that kills cells, but overcharging does so by using the excess charging energy to break down the electrolyte,
and vent the resulting gas. The cell capacity declines as the amount of electolyte goes down (you end up with cathode/anaode surfaces that are no longer covered in electrolyte, reducing the usable electrode surface area), until it finally dies. Fortunately it takes a lot of mAH to do this, but if you overcharge in a dumb charger long enough, you do indeed pump a lot more mAH into the cell than it has capacity for. Most dumb chargers have fairly low charging currents, so it takes a long time to 'boil' away substantial amounts of electrolyte but it does happen. IIRC it takes about 55Ah to convert 20 grams of water into hydrogren and oxygen, so a large battery like a D sized NiCd/NiMH can sustain a great deal of excess charging before you electrolyse enough electrolyte to make much difference.

NiMH batteries actually have SOME capacity to recombine the generated hydrogen and oxygen back into water. The key word here is SOME. Specifically, they can't recombine hydrogen and oxygen quickly enough if the hydrogen and oxygen are generated too quickly. And this happens if the charging current is too high (typically, greater than .1C). This is probably the biggest reason why a typical 'dumb charger' charges as slowly as it does. By keeping the charging current low enough, venting of hydrogen and oxygen can be prevented, even if the battery is overcharged somewhat.