NiMH life if depleted routinely?

[ikendu]

I've heard that the life of rechargeables can be severly shortened if you run them down to almost fully discharged.

I used a set of 4 AAs for a digital camera and my typical usage was to run them down to almost nothing. Later, it did seem like they would hold less and less charge. I eventually quit using them (just got some new, higher capacity ones). Although, it happened over a long enough period that it was hard to know just exactly how significant the degradation it was.

Anyone with a link or direct experience on this topic?

 

[star882]

The problem is charge depression.
This is what is often called the memory effect.
Go to http://dansdata.com/gz011.htm ,
http://www.repairfaq.org/ELE/F_NiCd_battery.html , and
http://www.repairfaq.org/ELE/F_NiCd_memory.html for more info.

 

[Steelwolf]

star882: The second and third links don't seem to be working properly. But that is quite a good website there. Just need to browse through a little bit more. DansData is ok, but doesn't really cover the effect. The following is actually related to point one on his site and may help to explain the effect somewhat. His point two is only relevant with a rechargeable battery PACK, sort of like those you find in cordless phones which deliver 3.6V or something, NOT the 1.2V single batteries.

AFAIK, the problem with the NiMH batteries is not a memory effect. Rather, as a battery discharges, the chemical composition changes. Along with the chemical change, there is also a physical change which is slight but unavoidable, since different compounds have different physical properties like size, shape, nuclear attraction, etc.

When one recharges the battery, the physical changes may not be entirely reversible. This may be as simple as not having all the compounds revert to the original form, or it may be as dire as some form of physical damage to the structure of the battery. The former can be dealt with for the most part by a good trickle charger, but the later is usually irreversible. The damage is usually to the matrix of either the anode or cathode. Sort of like how bits of the lead plates in a lead-acid battery tends to disintegrate and fall to the bottom of the battery where they can no longer play a part in the recharge/discharge cycle.

Hence, the battery loses overall capacity as it goes through each charge/discharge cycle. As the battery will undergo more physical change the deeper it is discharged, one will expect the damage to be greater. There is no way to avoid it, which is why batteries are rated for x-number of recharges. But one can minimise the effect by not allowing the battery to discharge too much.

Design of the battery also helps, which is why some batteries are rated for more cycles than others, and is why there is such a thing as a deep discharge lead-acid battery.

 

[Albany Tom]

From the battery manufacturer info I've read, the largest cause of battery damage is from overcharging, typically from leaving the batteries too long in a cheap charger. Taking them all the way down is what you're supposed to do with the NiCads and NiMH. Exactly what you're NOT supposed to do with lead acid batteries. I use AA NiMh in flash equipment and ham radio gear. With the right charger (my favorite is the wal-mart non-fast charge Rayovac model), I get probably hundreds of cycles out of a battery.

If you leave a NiMh or NiCad battery in a charger that makes it warm, and leave it warm for a day or so, you're going to hurt it. Over a weekend, you may kill it completely. (approximate numbers here)

Also - a NiMh will self discharge on its own. Some batteries can go half dead in about 2 weeks.

 

[star882]

There is no memory effect.
Go to http://www.repairfaq.org/ELE/F_NiCd_Memory.html
for more info

 

[Albany Tom]

Respectfully -

I looked at the site, and saw no reason to convince myself that there isn't memory. Either way, stating something doesn't make it so. I understand "generally accepted as true" doesn't make something true, either.

The last time I checked, battery charger manufacturers, and product makers of things like notebook computers claimed that memory did exist, by suggesting that one should cycle a battery before expecting full capacity, or that their brand x charger was great because it did a discharge first. At the same time, battery manufacturers have been claiming their batteries don't have memory for years.

My experience, over the years, has been that my equipment which is taken down and charged properly lasts longer than other peoples equipment when neither is done. Anecdotal? Sure! But I think two things we can agree on:

1. Overcharging, or reverse charging a battery will damage it.

2. Fully discharging a nicad or nimh cell won't hurt it.

So until demonstrated otherwise, I'll assume memory, because it's safer.

Finally - I've had a theory for a long time that many quick chargers cannot determine the charge end point with their DV/DT stuff unless a battery is near dead before charge. This could explain why fully discharging has worked for so many people - you are more likely to get a good charge, rather than a prematurely terminated or overcharge.

Ok one more thing - Back in the old days, when all chargers were stupid, you had to use a time method of charging your batteries. For a nicad, the only safe way to charge was to take down all the way, then leave in the charger for X hours, where X was about 10. Leaving the cell in longer would cause some degradation of the battery, and you had no practical way to tell how long to charge unless it was dead.

YMMV...

 

[vicbin]

Another better reference

Memory: myth or fact ?

Vic