Rechargeable battery question

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'
Thanks Tim
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to
NiMh needs a different charging regime. This place will re-cell your exiting pack:
http://www.multicell.co.uk
AWEM
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O

Not according to the technical people at G S Saft who advised me on replacements for Nicads on Mazak memory boards.Mazak wanted 92 for a single cell.
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Tim Leech wrote:

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.
--
Duncan Munro
http://www.m0kgk.co.uk /
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On Sun, 13 Jul 2008 15:14:38 +0100, Duncan Munro

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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On Sun, 13 Jul 2008 15:31:49 +0100, Richard wrote:

No.
http://shdesigns.org/batts/battcyc.html
You may be thinking of primary cells, which do show a steady voltage loss with usage.
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On Sun, 20 Jul 2008 10:52:00 GMT, _

_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 recall.
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.
Richard
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On Sun, 13 Jul 2008 15:14:38 +0100, Duncan Munro

Many thanks to all for suggestions. Is it OK to leave that charger on indefinitely with Nicads?
Tim
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wrote:

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 -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk http://www.oldengine.org/members/diesel http://www.stationary-engine.co.uk http://www.oldengine.co.uk
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On Sun, 13 Jul 2008 17:56:35 +0100, Peter A 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 800mAh. I did try some time ago running the instrument from a 9V dry battery, it didn't seem to like the higher voltage much!
Thanks Tim
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mmmmmmm try 2700mAH... you're a bit out of date with your info.....
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Not for Nicads, that's NiMH you're referring to? I should have made that clearer.
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 -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk http://www.oldengine.org/members/diesel http://www.stationary-engine.co.uk http://www.oldengine.co.uk
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wrote:

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 -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk http://www.oldengine.org/members/diesel http://www.stationary-engine.co.uk http://www.oldengine.co.uk
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