Re: Rechargeable battery question
- From: Richard <sharkface-pilot@xxxxxxxxxxxxxx>
- Date: Sun, 13 Jul 2008 15:31:49 +0100
On Sun, 13 Jul 2008 15:14:38 +0100, Duncan Munro
<postmaster@xxxxxxxxxxxxxx> wrote:
Tim Leech wrote:
The battery pack on my ultrasonic thickness tester is dying.
It consists of six AA NiCd cells strapped together.
I went to Maplins this morning planning to get some new cells, but
virtually all their rechargeables are now Nimh or what they call
'hybrid'. They had a couple of NiCd cells but not enough, and they
seem to be discontinuing most of them. The gormless youths in charge
couldn't tell me anything useful about what these 'hybrids' are, or
how well either type would do as replacements for NiCd cells in kit
with a built-in charger.
Any suggestions how well they would work, especially with reference to
the charging?
The charger is labelled 'NiCd charger for up to 10 cells, 50mA'
Tim, NiCD cells are out of favour now due to the Cadmium content and
associated disposal issues. The two replacements as you've found out
are the NiMH and Hybrid batteries.
NiMH works the same as NiCd but has a higher self discharge. This means
that if you leave it to one side fully charged, it will flatten itself
quicker than the NiCD which is about the only significant disadvantage.
The Hybrid batteries are NiMH batteries that have a different internal
construction which lowers the self discharge rate. They do, however,
have a lower capacity as a result.
Hybrid is better suited to things like clocks, air fresheners, the kind
of things that you want to stay charged for 6-12 months as the normal
NiMH would be flat in that time. RC models, digital cameras, and other
high drain / short turnround applications might benefit from NiMH but to
be honest I'm just using Hybrid for everything now.
The 50mA charger is a tiny trickle charge and is perfectly suitable for
the newer batteries, although it will take some time to charge up the
higher capacity batteries. To get a full charge into 2100mAh Hybrid
batteries from empty would take 3 days, but the bonus is you can leave
the thing connected on charge for as long as you like without damaging
the them.
Finally, if you are after hybrids, Wilkinsons do a bargain price on
Uniross Hybrio batteries, 4 x AA for a fiver, a lot less than Maplins.
Another place I've found helpful (they do tagged packs as well) is
www.vapextech.co.uk
Hope this is some help to you.
As a further comment, NiMH's are pretty immune to the 'memory'
problems associated with NiCd's. ie charge them from any discharge
state with impunity.
I doubt it matters for your application, but NiMH's have a more
gradual loss of voltage with discharge ie they loose more voltage
earlier whereas NiCd's are pretty flat until the last moment when they
go dead in moments. This means you can see the discharge state more
accurately (from cell voltage), but on the other hand, if you have a
very voltage sensitive application you may not get as much use from a
NiMH as a NiCd of nominally the same capacity.
I do a fair bit of RC modelling and consequently bought a 'smart' or
Delta-Peak charger of which there are many available. They're not
hugely expensive (£15~60) depending on bells & whisltes and can charge
LiPo's NiMH's NiCd's from 1~8 cells and up to 6AH or so. I use it for
everything from the cordless drill to the camera as well as the models
as it gives a good indication of the health of the cells as well as
not over charging them. As noted earlier NiCd & NiMH's are quite
tolerant to overcharging, but it's better not to.
Richard
.
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