In article ,
john d hamilton wrote:

Have just bought the latest type of rechargeable *Hybrid* batteries from
Maplin, made by Uniross, called Hybrio. (also Argos are now selling them
made by Sanyo).

They are supposed to have the advantage over Nickel Metal Hydride in that
they don't have the 'memory effect'. Also the advantage of the Alkaline, in
that they dont self discharge at quite a high rate when not being used.

The charger I have is an Energiser designed for Nickel Hydride batteries and
which indicates when a battery is fully charged, by the charging light going
off. But when putting in the Hybrid batteries the light does not go off.

Is there an harm done to these hybrid batteries if they are overcharged?

According to the manufacturer web pages I've read, overcharging *any*
NiMH cell significantly can shorten its life... they're somewhat less
tolerant to this than nickel-cadmium cells. Also, it's somewhat more
difficult to detect the "full charge" state in a NiMH than it is in a
NiCd, especially at low charge rates.

Based on what I've read, there seem to be two charging schemes for
NiMH which the cells will tolerate fairly well:

- Slow charge (0.1 C or so) with a timed cutoff after 12 to 16 hours.
If you touch the cells during charging and find that they are
significantly warm, then they're probably "full". [They do warm up
somewhat during the normal charging process, so judging whether
they're warm enough to indicate full-charge is not always easy.]

- Fast charge (0.5C to 1C, or in some cases even higher) with primary
cutoff based on temperature rise, secondary cutoff based on zero
delta-V (i.e. the cell voltage stops rising when full-charge is
reached) and a timed cutoff as a failsafe.

Intermediate rates (above .1C and below .5C) have some
disadvantages... this amount of current may not result in a rapid
temperature rise at full-charge (thus making full-charge harder to
detect reliably) but is high enough to affect the cell's lifetime if
you do end up overcharging the cell.

It sounds to me as if your Energizer charger has its full-charge
detection circuit tuned properly for this newer type of NiMH cell. If
it's a slow (overnight) charger, you probably won't hurt the cells
significantly using it as long as you shut it down manually at the
proper time. If it's a "quick" or "fast" charger, it may very well be
overheating the cells enough to reduce their lifetime.

If you plan to use a lot of NiMH cells, or to recharge them
frequently, it might very well be a good investment to buy a
high-quality charger specifically designed for reliable fast-charging
of such cells. I like the Powerex MH-C9000 myself, as it's fast and
reliable and has a lot of useful features.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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