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 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
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
NiMH cells are OK as a direct replacement for Nicad, the charging requirements
are close enough not to matter too much, especially at 50mA charge rate, which
is probably just a fixed resistor job.
AA Nicads are still very easily purchased, RS have tagged Nicads for £2.88 each
plus VAT, and other makes.
Dean at Multicell (01455 283443) will make you up a proper pack if you ask him,
we use them a lot for our 25-cell 7AH Nicad packs.
Peter & Rita Forbes
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.
The standard continuous charge rate is C/10 or 50mA.
However, cell water/moisture loss can occur due to heating if they are enclosed,
so normally 14 hours is the longest you should leave them, assuming an unknown
state of charge at the beginning.
Note that AA cells are 600mAH these days, so 50mA is less that C/10 charge rate.
Dry AA cells are 1500mAH with less self-discharge, worth remembering.
Peter & Rita Forbes
The manual specifies a charge time of 16 hours, though I don't know
what the original battery capacity was. Those currently fitted are
I did try some time ago running the instrument from a 9V dry battery,
it didn't seem to like the higher voltage much!
Not for Nicads, that's NiMH you're referring to? I should have made that
Our bulk purchases are for 7AH Nicads and 4AH Nicads in 25-cell packs, we don't
regularly use the small stuff any longer.
Peter & Rita Forbes
_I_ am certainly thinking of NiCd's/NiMH's, but possibly the reference
from which I got the info was not. I'll try to look it out as it was
(apparently) directly relevant to my application (RC aircraft range)
and the curves shown were certainly nowhere as similar as your
reference - indeed the dissimilarity was the thrust of the article as
I can't remember at present where it was, but expect I've kept it
somewhere. It was what I perceived to be a reputable source, probably
the BMFA, but I'm not sure.
Thanks for the additional info.