NiCad Memory

[pgwalk]

How about some references on this subject? I’ve been discharging NiCads for years (eg. a Makita 12 v cordless drill used heavily and on a daily basis), and they have lived long and prospered. Second (& third opinions)?

[Duck_Duck_Goose]

References. Which [ahem] seem to contradict The Master…

However, I think it’s just that the column is cough [sub]poorly written[/sub].

One’s eyes single out this phrase–

But fully discharging a true nicad battery…is definitely a bad idea.

–and since in common usage “NiCad battery” generally means “a single battery” as in “a single object”, it leaves one with the impression that Cecil’s saying you should never fully discharge any NiCad battery, when in fact he’s just saying you shouldn’t fully discharge the ones that are wired in series, like for model airplanes.

This guy points out (for radio-controlled hobby model airplanes and cars, etc.):

Battery packs to power models are usually made up of the Sub-C size cells. At one time These were rated at 1200maH in capacity, but with progression in technology, Sub-C’s are commonly found with 1400, 1500 and even 1700maH capacities. These packs are usually in either 6-cell (7.2V) or 7-cell (8.4V) configurations but some models, especially large airplanes may have 28 or more cells wired in series to form a pack.

If you are wiring your own pack, do not wire NiCad cells in parallel to get higher capacity. NiCads have an extremely low internal resistance compared to ordinary, non- rechargable cells. When they are wired in parallel, if one cell is at a different charge level than the other, thus producing a slightly different voltage, a great deal of current can flow from the higher voltage cell to the lower voltage one. This could cause a fire by heating the wire joining the cells or from the heating of the cells themselves. If you need higher capacity, go to a larger cell.

And the first words in the column–“It’s a myth”–also contribute to the impression that the Straight Dope is, “No, you don’t need to fully discharge your NiCad batteries every so often.”

But all of these websites say you should fully discharge your NiCad batteries every so often. Are they all wrong? Or are you saying they’re all talking about digital camera AA batteries, not big camcorder and laptop batteries, which may or may not contain ranks of smaller batteries inside them?

Extreme Tech website:

NiCads are known for another drawback: memory. When some NiCads are partly discharged, then later recharged, they may lose capacity. Chemically, recharging NiCads before they are fully discharged often results in the formation of cadmium crystals on the anodes of the cell. The crystals act like a chemical memory system, marking a second discharge state for the cell. When the cell gets discharged to this secondary discharge state, its output abruptly falls despite further capacity being available within the cell. In subsequent cycles, the cell remembers this second discharge level, which further aggravate the situation by reinforcing the memory of the second discharge state. The full capacity of the cell can only be recovered by nudging the cell past this second discharge state. This will erase the memory and restores full cell capacity.

As a practical matter, the cure for the memory problem is deep discharge–discharging the battery to its minimum working level and then charging the battery again. Deep discharge does not mean totally discharging the battery, however. Draining nearly any storage battery absolutely dry will damage it and shorten its life. If you discharge a NiCad battery so that it produces less than about one volt (its nominal output is 1.2 volts), it may suffer such damage. Notebook computers are designed to switch off before their batteries are drained too far, and deep discharge utilities do not push any farther so you need not worry in using them.

http://www.planetbattery.com/planetbattery/faq.ihtml#E

NiCad batteries, and to a some degree NiMH batteries, suffer from what’s called the “memory effect”. “Memory Effect” means that if a battery is repeatedly only partially discharged before recharging, the battery will “forget” that it can further discharge. The best way to prevent this situation is to fully charge and discharge your battery on a regular basis.

http://www.skcgc.com/main/support/appguides/pub1363.html

Discharge * and recharge the batteries completely
Batteries should be discharged and recharged after ten recharges to prevent “memory effect” that occurs when a battery produces less than expected voltage output. Memory effect is reversible and vanishes when the battery is reconditioned (discharged and recharged).

http://www.camcorder-battery.com/battery_tips.htm

There are several steps you can take to help you get maximum performance from your battery:
Prevent the Memory Effect - Keep the battery healthy by fully charging and then fully discharging it at least once every two to three weeks. Exceptions to the rule are Li-Ion batteries which do not suffer from the memory effect.

More people use camcorders and laptops than use model airplanes, so I would assume that all these websites are talking about big camcorder and laptop batteries. I would think that the manufacturer of the bigger camcorder and laptop batteries would have some instructions as to whether you were supposed to fully discharge them every two weeks or not. The camcorder battery website above certainly thinks you should, as does ExtremeTech on notebook computers.

[AndrewL]

Unless I’m mistaken, NiCad memory happens when the cell media is held in the charged state for a long period of time. Crystals grow in the material, and when the cell is duscharged the crystals resist breaking down into the discharged state, reducing the effective capacity of the battery. Normal discharging and recharging is enough to inhibit crystal growth and prevent the memory effect from occuring.

NiCad battery technology has improved a lot in the last decade, and the modern cells are much less prone to memory effect. Older, lower-capacity cells such as used in camcorders can still exhibit memory effect, but it takes a serious effort of never discharging fully to make it happen. The latest high-power-rate cells such as used in power tools (2000+ mAH Sub-Cs are standard now) will not develop memory.

Note that any NiCad cell will self-discharge if left sitting long enough. High current rate cells self-discharge faster, I’ve had battery packs left on the shelf for a month go completely flat. This will not harm the cell one bit, and in fact if you have to store NiCads for a long time it is best to do so in the discharged state. This is different from lead-acid batteries, where leaving them sitting in the discharged state for a long time will damage them. Fully discharging a NiCad with an external load can damage it through voltage reversal, but only if the cells were mismatched, in which case the pack’s got troubles already. A mismatched pack is also likely to suffer damage during charging, as the weak cells can get overcharged. A matched pack can be fully discharged safely.

Note that NiMH cells do not form memory at all. They can be damaged through reversal and overcharge like NiCad.

So who’s right? Memory used to be a big problem, and is still a worry with older-technology cells. For modern high-capacity cells it’s not an issue.

[Mikemike2]

I have had a lot of practical experience with ni-cads and other rechargable batteries in laptops,and cameras. These batteries are supposed to be good for 1000s of uses. But I find that they do not hold a charge after a year or so, even though I have perhaps used them only dozens of times. This is the current situation with my IBM notebook and Toshiba digital camera. I can charge the battery, but it will be dead within a week. Any suggestions?

[Earl_Snake-Hips_Tucker]

An earlier thread on this subject:

http://boards.straightdope.com/sdmb/showthread.php?threadid=8

[pgwalk]

Thanks All for the info and links!

What consensus there is seems to suggest that you can’t go far wrong by using NiCads regularly, depleting them with out exhausting them, and using a trickle charger.

Have I missed anything?

[scr4]

What consensus there is seems to suggest that you can’t go far wrong by using NiCads regularly, depleting them with out exhausting them, and using a trickle charger.

Let’s be careful with the terminology here. It’s defintely a bad idea to run down the battery completely, e.g. by hooking up a resistor to it. But most electronics devices have a cut-out circuit to prevent this, so it’s safe to keep using the device till it stops working.

It’s also perfectly safe to charge a partially used battery, so you don’t need to try to use up the battery every time. The exception is if the battery charger is incredibly stupid and runs on a timer, instead of detecting the status of the battery. Using a timer charger on a partially used battery results in over-charging, which is harmful. Even some “trickle chargers” run on timers. One example I came across is the NiteRider bicycle light, which comes with a “smart” trickle charger. It actually runs on a timer and switches to trickle mode after 14 hours of standard charge current. This is why its manual tells you to use up the battery before charging.

[Jeremytt]

Like all rumors you hear, this one definitely has a huge nugget of truth in it…allow me to explain.

Until rather recently, Apple Powerbooks had this exact problem–you had to drain the battery all the way down, or the Powerbook battery did indeed develop “memory”.
If you drained the battery, say, half-way down, repeatedly, there came a time when recharging brought the battery only to the halfway point. Apple users were advised to completely drain the battery as a matter of course.
The last year or so, however, Apple has fixed this bug. The Operating System has a Memory Manager that fixed the problem, and you can drain the battery to any old strength before recharging…

Hope this buries any lingering rumors.

[Jeremytt]

Like all rumors you hear, this one definitely has a huge nugget of truth in it…allow me to explain.

Until rather recently, Apple Powerbooks had this exact problem–you had to drain the battery all the way down, or the Powerbook battery did indeed develop “memory”.
If you drained the battery, say, half-way down, repeatedly, there came a time when recharging brought the battery only to the halfway point. Apple users were advised to completely drain the battery as a matter of course.
The last year or so, however, Apple has fixed this bug. The Operating System has a Memory Manager that fixed the problem, and you can drain the battery to any old strength before recharging…

Hope this buries any lingering rumors.