NiMH - memory or not?

[powernoodle]

I've read a few places that NiMH batteries do not experience the memory effect of NiCads. But a recent issue of Popular Science, which had an nice article on batteries, said that the NiMH needed to be fully discharged once a month "to maintain full power". Is Popular Science full of horse hockey, or are they referring to something other than memory?

best regards

 

[BatteryCharger]

Horse pucky to the fullest extent.

 

[PhotonBoy]

On the contrary, I've heard that deep discharging NiMH is not good for them. I've got 8 of them and have had no problems or obvious indications of memory effects (yet). YMMV.

 

[powernoodle]

Ok, thx.

best regards

 

[JanCPF]

As far as I know deep discharge destroys NiMH (and Li-Ion for that matter), but can benefit NiCd cells.

Jan

 

[Blikbok]

Memory is too general of a term.

Will NiMH mysteriously loose capacity before they should? Yes. This isn't memory.
Repeated shallow discharge will not reduce the capacity of a NiMH. Repeated overcharging (as little as 10%) will depress the voltage of a fully-charged cell to the point where many devices will report the battery is dead.
NiMH don't need to be cycled like NiCd (and I doubt NiCd needs to be cycled as often as many users do), but a brand new NiMH will not have it's full rated capacity unless it's been exercised before shipment.

Get a NiMH-specifific charger, preferably a "smart" pulse charger with delta-V cutoff and a tempature cutoff, and don't cycle them and you'll be fine. Use them and top them off when done and they will be very happy.

 

[Phaserburn]

How much charging is overcharging of a NIMH? Would daily charging for 1/2 hour to top off cells be too much?

 

[illumiGeek]

Both NiCad and NiMh batteries can have the very miss-named and misunderstood "memory" effect, but with modern cells and a good charger you really don't have to worry. You really have to abuse the batteries to have problems. Cell "memory' has been a non-issue for a long time, but that stupid old wives tale just won't go away.

Proper charging is very important in NiMh cells, as they are very sensitive to charge rate and temperature. NiCads can take quite a lot of abuse in stride, but typically offer about 1/2 to 2/3 the capacity of similarly sized and priced NiMh cells.

Aloha, iG

 

[asdalton]

Yes, "memory" is too often cited as a general (and incorrect) cause of battery failure. Permanent damage to rechargeable batteries is usually caused by overcharging or cell reversal.

 

[kitelights]

Overcharging is charging past when the cell has been charged to its full capacity.

Your question is too general to answer. It depends on your cell's capacity, their present state of discharge, the output of your charger, and mainly the ability of your charger to sense when the cells reach their capacity and then either shut down or go into a low enough trickle charge.

The easy answer is, if you're using a good smart charger like a 401, you'll have no problem if you want to top them off daily. They prefer frequent charging from a shallow discharge rather than deep discharge.

If you're not using them at all, and topping them off just to maintain full charge state, I think daily charging is a little overkill, but hey, so is owning 100 flashlights. I used to try and top mine off every one to two weeks, but I've gotten pretty lazy and don't get to do that lately.

Point is, if you're using rechargeables, a high quality smart charger is not an option - it's a must. Without one, not only will you not achieve full use of your cells, you'll ruin them.

 

[SilverFox]

I am of the impression that Nickel chemistry (NiCd and NiMh) involves the formation of crystals. If the cell is shallow charged or left idle, the normal small crystals combine and form larger crystals. The larger crystals have less surface area and you end up with less capacity.

A good charger will keep the cells from loosing the electrolyte due to overcharging and may, through pulse charging, have some effect on breaking the crystals down to a smaller size, but I am not sure if this really works. I do know that discharging and then recharging causes the crystals to be used up and then reformed during the charge process.

I believe most people agree that the discharge cycle is important for maintaining cell capacity, the question is how often?

Battery University states that NiCd's should be cycled down to 1.0 volts once a month and NiMh's once every three months. The RC community suggests doing both types once a month to get maintain the maximum capacity. The flashlight community is less competitive (at least on battery run time) and tends towards longer intervals.

I think in normal flashlight use, cells will get discharged to a low level every now and then. This may not be optimum, but may be good enough to get reasonably good life from the batteries.

I do a deep discharge of my Nickel based cells at least every six months.

It would be interesting to do a study of this, but I am afraid that it would be quite involved.

Tom