In ,
ransley wrote:

On Apr 14, 2:51*pm, Fleemo wrote:
My string trimmer runs on a rechargeable NiCd battery. It's only two
years old but doesn't hold much of a charge anymore. The trimmer
manufacturer's tech support suggested that I may be able to bring it
back to life by discharging the battery completely, charging it for
six hours, and repeating this cycle three times, only charging it for
one hour the final time. After each of the charges, when I attached a
clamp to the trimmer's switch to let it run so it would discharge, it
certainly did seem to have plenty of power. *But after doing this
process, when I went to actually use it, the trimmer barely had enough
juice to spin even though I had left the battery on the charger for
several days after doing the cycling process.

Does this technique of bringing a NiCd battery back to life work?
Once I charge a NiCd, can I leave it in the charger until its next use
or must I take it off the charger after a certain time? *Do I have to
discharge the battery completely before recharging it every time I use
the trimmer?

What they told you is a way to just about guarntee ruining the
battery.

They say to discharge NiCd batteries before recharging to avoid the
"Memory Effect".

However, there are 2 effects of NiCd - "memory" and "voltage
depression".

The latter is avoided with faster charge time "smart chargers" that
detect "full charge". Overcharging a NiCd reduces its voltage by 4% or
so, but that is often repaired by full recharge after full discharge.

The former is actually less of a problem, mainly cropping up if partial
discharge to same extent as before followed by recharge is repeated to
great extent.

Meanwhile, overdischarge of NiCd batteries appears to me to be the main
cause of damage.

If you have NiCd batteries, I find best so far for a "maintenance
proceedure" is to discharge them until they have voltage "in an earlier
stage of going downhill" due to approach to full discharge, and then fully
recharge them.

They should also be recharged if not having any recharging in the past
year, maybe 6 months. I think more like 6 months for NiCd if recharged by
a "smart charger".

Rechargeable batteries do not last forever no matter what you do. If
you get them to be useful for 7 or 10 years, you are doing well.

One more thing - chargers with instructions to charge for 14 hours or
anything more than 8 hours appear to me to be mostly "dumb chargers" that
have a high rate of achieving significant overcharging. That "should
cause at most minimal damage", though can cause the above "voltage
depression" in NiCd and, if repeated frequently, can shorten life of NiCd
and especially NiMH.
If you want to guard against this, get a voltmeter connected to the
battery pack while charging it - if it takes a rise from a stable
level around 1.26-1.3 volts per cell to something higher and then
stabilizes or "peaks out", then the battery has been fully charged and is
being overcharged. Watch out for a series of alternating voltage
stabilizations and rises as each cell in the battery achieves full charge
at a different time - I find better to get all cells fully charged at
expense of mildly overcharging some cells.

NiMH has no "true memory" and much less than NiCd of "voltage
depression", but appears to me to be aged more by overcharging than NiCd
is. However, damage from overcharging of NiMH appears to me to be small
enough to be better worth charging a "series pack" until all cells are
fully charged as opposed to charging until one cell achieves full charge.

I actually like "smart chargers" of kind that achieves small extent of
overcharge due to delay of detection of full charge - by detecting
directly (with a temperature sensor) or detecting indirectly (voltage
peaking out and then decreasing, apparently-to-me as result of temperature
rise) that results from overcharging. I like such "smart chargers" for
actually minimizing overcharge in comparison to "dumb chargers". Such
"smart chargers" tend to have charge time 4 hours or less.

Bottom line - it appears to me that if rechargeable batteries are
"optimally babied", they last maybe 7-10 years and occaisionally more, and
if "used reasonably well" they last 5-7 years.

I would not fret about minor to moderate deviation from "whatever is
optimum treatment" for rechargeable batteries, since they will die and
need to be replaced anyway - they will degrade at some rate or another no
matter what you do! I think better to have them have a well-utilized good
life and not fret too much! If you get both at least 4 years and 200
discharge-recharge cycles with a rechargeable battery 1/3 as good as new
(in terms of time supplying "sufficient voltage" to the load, at least 1
or 1.1 volts per cell) afterwards, I think you are doing well!

- Don Klipstein )