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Rechargeable batteries.

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Posted by harry on October 8, 2009, 6:18 pm

The "AA" sort of size I mean.   My house is full of gizmos that need
batteries. TV remotes, electric clocks, smoke alarms, computer bits
sort of thing.
So, I thought why not get rechargeable batteries. Be green!  I'm now
wondering if this is not some sort of con.
First you need to buy a charger.  Then the batteries cost five or six
times more than the non-rechargeable ones. Then there is the question
of capacity. The rechargeables are marked X mAh but the nons are not.
But the rechargeables don't seem to last as the long as the nons. Also
they don't seem to be able to be recharged as many times as
advertised.  And I suppose there's the cost of electriciy to recharge.
And they're always flat when you need one.
The latest thing I've found out after a lot of head scratching is that
some things won't work with them because they are a slightly lower
voltage than the nons. (I'm thinking of my wireless computer
keyboard.)  The range is cut down by a factor of five or six.
I think I'll go back to the cheap old nons.

Posted by Richard on October 8, 2009, 7:04 pm


Posted by Daniel Who Wants to Know on October 8, 2009, 9:03 pm

Normal NiMh cells are NOT designed to operate any of the items you listed
because the cells self discharge faster than those devices they are powering
can run them down thus most of the stored power is wasted.  A normal fully
charged NiMh will lose 10% of that charge in the first 24 hours just due to
internal losses.

Not a con per se but they do need to better label them as far as what the
designed purpose is, which is high drain devices like portable CD players
and such.

And it had better be a good one.

A normal alkaline "AA" cell seems to be about 2850 mAh

Once again you NEED a good charger.

Which is less than 5 watthours to fully charge one cell so you would need to
charge over 200 of them to use 1 KWh which the price of can be checked by
looking at your electric bill.

Self discharge (see above).

Alkaline cells don't have a fixed voltage, the voltage falls off gradually
as they are used.  They start at about 1.6 when new and are "dead" at .8-1.0
depending on the device.  NiMh start at 1.4, fall gradually to just above
1.2, slowly pass through 1.2 at about 50% used and have a voltage "cliff" to
1.0 when discharged.

For the mentioned devices a good set of alkalines are your best bet.  A set
of the "pre-charged" rechargeable NiMh cells would be almost as good as they
claim to have less self discharge.

Consumer devices and most chargers don't have a BMS (battery management
system) built into them so the life of the cells is very short.  Toyota
hybrids by comparison have an excellent BMS that keeps the NiMh pack between
40% and 80% charge and applies equalization charges as needed so the pack:
never sees damage due to overdischarge, never risks wasting 10% by charging
to 100% (except during the rare equalization charges), has active thermal
control using a fan, is never fully cycled, and is charged in short high
current pulses all of which make it last a very long time.

If you could find a way to keep the cells between 40 and 80 percent at all
times in your devices your cycles per cell would be in the thousands instead
of the hundreds.

Posted by harry on October 9, 2009, 6:26 pm

On Oct 8, 10:03 pm, "Daniel Who Wants to Know"

Found out most of that the hard way....... Still, one remembers it
OK for my camera then????
The charger I have is allegedly "intelligent". Whatever that's
supposed to mean.

Posted by Ralph Mowery on October 9, 2009, 9:36 pm


Some cameras only specify the rechargables.  They require a very short high
current pulse.  The rechargable batteries will do that many times.  The
alkiline batteries will not deliver the high current surge for as many pics.
The same alkiline will power a flash light much longer.

Intelligent chargers can mean many things.  I hope the one you have is set
so it will cut off when the battery is charged.

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