self-discharge

[sam0set]

I keep reading that NiMH have worse self-discharge than NiCd. That hasn't been my experience.

I started using AA NiCds in 1981. Before many cycles, a cell was likely to lose much of its charge after a week on the shelf. The self-discharge would get worse until it couldn't be charged at all.

I started using NiMH in 1998. Contrary to what "they all say," I found self-discharge to be a much smaller problem. In a mouse or a keyboard, they might go two months between charges. When I didn't have AAA alkalizes on hand, I put a pair of old NiMH cells in a thermostat. They didn't need attention for 5 months.

NiCds still worked better in high-current devices such as krypton flashlights. Using the fast charger I'd bought for my NiMH cells greatly reduced the self-discharge problem.

Both kinds were likely to deteriorate if unused for months. Repeated cycling by charging and draining through a resistor, could improve performance.

Eneloops are supposed to hold most of their charge for years. Will they deteriorate on the shelf? Will self-discharge remain that low after years of use? Nowadays, what causes the self-discharge of a NiMH cell to increase?

 

[inetdog]

I have never heard of NiCd cells described as low self discharge. But a significant loss in one week does not seem reasonable either unless extreme temperatures were involved or the cells had problems.

Sent from my XT1080 using Tapatalk

 

[sam0set]

Wikipedia says 10% a month for NiCd and 4% a day for conventional NiMH.

I've certainly done better than 4% a day with NiMH and worse than 10% a month with NiCd.

In the 1980s, articles in electronics magazines indicated that "everybody" had trouble with high self-discharge in NiCds. Eventually, a cell wouldn't take a charge at all. Then you might get it working by zapping it with a capacitor.

The problem was the recommended charging method: C/10 for 16 hours. All those hours on a charger caused deposits to form. Current would leak through the deposits from anode to cathode.

 

[SilverFox]

Hello Sam0set,

Actually the C/10 charge for 16 hours was not the problem. It was the C/10 charge for 24 hours a day 7 days a week. The separators that keep the + and - electrodes from touching inside the cell would break down and with NiCd chemistry dendrites would form under the constant charge conditions. These could be removed by "zapping" but it did nothing to repair the hole in the separator that allowed the dendrite to form in the first place.

With NiMh chemistry it seems that the electrolyte within the cell drys out and this leads to high self discharge rates.

I believe Eneloop cells were introduced somewhere around 2003. In 10 years of use my cells are still performing very well with minimal self discharge.

Tom

 

[sam0set]

Hello SilverFox,

My first experience with NiCd was a battery pack for a military walkie talkie in 1975. The battery was supposed to be kept on a trickle charge. The battery pack failed in a matter of weeks. I've avoided leaving batteries on a charger ever since.

My first experience with AA NiCd was in 1981. I'd wait until they ran down, then put my GE batteries in my GE AA charger modules 16 hours. In a year, self-discharge was so bad that they were useless.

One reason it was so bad was that the batteries said 45ma, but the charger GE sold for them delivered 75ma. I bought another charger and modified it with resistors so I could set it to whatever current a particular pair of batteries required. High self-discharge was still what rendered nicads useless, but now the deterioration was slower.

I guess the rechargeable dust buster appeared in the 1980s. They were supposed to be left on the charger. Naturally, they didn't hold up long. There were also drills and screwdrivers that ran on 3-5 sub-C nicads. They were supposed to be charged 3 hours when they got weak. I had one and a relative had two. We charged them only as needed, but I had to replace several shorted cells in them after little use.

I've had much better service from AA cells since 1998, when I bought my first charger that would shut off each cell when it was charged.