Best for battery to let DECT handset run down?

Am in the UK and have a DECT handset containing two AAA NiMH cells.
The manufacturer says it is alright to indefinitely leave the handset
in the charging cradle. Maybe the charging is done so gently that it is ok.
However the cells last for several hours talktime so they will never properly run down before being put back on charge.
This means the recommended first few charge-discharge cycles will never occur
Is a charging regime where the handsets are always replaced back on charge likely to noticeably reduce available battery capacity?
How about this? -- Is it worth leaving off the charging cradles the DECT handsets for a several days in order to have them run down on their own and then putting them into their charging cradles? ISTR that NiCds tended to like this sort of regime.
--

xposted (4 relevant gps)
uk.telecom
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Run till they're flat, but don't leave them that way, charge as soon as they are. You only need let them go all the way once a month or so. Similar advice is given for electric toothbrushes which also use NiMH.
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Alex Coleman wrote:

NiMH cells do not have the memory effect that you are thinking of.
The worst thing to do to most batteries is to let them run down and then leave them in that state, eg by forgetting to put them back on charge.
The charger is charging them at a very low rate that can be sustained indefinately. That's why the things take so long to charge initially..
Leave them on the charger.
--
Sue

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snipped-for-privacy@palindr0me.plus.com.invalid wrote:

My advice on constant charging, if the battery gets very warm, your in for trouble.
Cycling is often recommended for NiMH, but don't really know if it helps.
greg
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There's some interesting and useful information available at Harding Energy's web site - see http://www.hardingenergy.com/pdfs/NiMH.pdf for one thing.
What I take away from this document (and from an old GE book on NiCd battery chemistry) is:
- Over-discharging either type of battery is Bad. Don't run them down to below 1.0 volts per cell. Doing so can result in permanent loss of capacity in NiMH, and to internal shorts in NiCd. By the time voltage drops to 1.0 volts per cell you've gotten out about 99% of the charge. - Excessive overcharging is also bad. NiCd is usually able to tolerate long-term overcharging at a rate of 0.1C indefinitely. NiMH is less tolerant to overcharge, and any long-term maintenance charge should be limited to C/128 or so (less than 0.01C). Higher overcharge rates can result in temporary or permanent loss of capacity. - NiCd cells subjected to long-term overcharging may suffer from "voltage depression" - higher internal resistance, which causes the output voltage to start dropping under load when there's still quite a bit of charge left in the battery. This can mimic the true "memory effect", which actually has a different cause and is rather uncommon in practice. Both the true memory effect, and voltage depression, can be reversed by discharging the cell to 1.0 volts at a low discharge rate, and then recharging at 0.1C for 20 hours. - Because NiMH is less tolerant of overcharge, the charging circuitry needs to be somewhat more sophisticated than is true of NiCd chargers, if best results are to be achieved. - Both battery chemistries heat up during overcharge... much more than they do when they're actually accepting useful charge. Good chargers detect the heat (either directly, or by a resulting decrease in the cell voltage) and shut down the charging cycle. Bad chargers cook the batteries.
- Both battery chemistries self-discharge over time... NiMH usually quite a bit faster. - Either type can be stored at any level of charge without affecting long-term capacity or reliability... it doesn't matter if they self-discharge all the way to zero. - However, if you let a NiMH self-discharge all the way to zero, it will temporarily lose some of its storage capacity - it'll only give you about 95% on the first recharge/use cycle. Subsequent recharge cycles will restore it to full capacity. To go back to the original poster's question... I'd agree with the advice to simply leave the phone on the charger at all times, if that is what the manufacturer has recommended. A modern device designed for NiMH batteries will have a proper charge-control chip built in, and this should be bringing the battery up to a state of full charge (but not overcharging it) and then applying a proper maintenance charge to keep it "topped up".
For a NiNH battery, this seems to be the best strategy for maintaining maximum useful charge in the battery.
Cycling NiMH batteries (discharging and recharging) seems to be beneficial mostly for batteries which are _not_ being recharged frequently, but which are being allowed to self-discharge in storage.
--
Dave Platt < snipped-for-privacy@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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Thank you for the info in the posting and for the PDF reference.
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[snip]
: : NiMH cells do not have the memory effect that you are : : thinking of.
Actually they do, but to a lesser extent than NiCd.
http://www.batteryuniversity.com/parttwo-33.htm
Ivor
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Neither do NiCd cells! :-)
The 'memory effect' only occurs in NiCd cells which are discharged and recharged on a *very* regular cycle. It occurred (and was discovered on) a satellite where the discharge/recharge was as regular as the satellite going round the earth and getting recharged by solar cells.
What kills NiCd cells (and then 'memory effect' gets blamed) is overcharging them and/or reverse charging them.

I'm not sure that either NiCd or NiMh actually suffer all that badly from being left discharged but I'm not totally sure about that.

Now that bit is OK enough!
--
Chris Green

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Alex Coleman wrote:

If you discharge a battery composed of several cells until it is very very flat, then there is a good chance that some of the cells have slightly more capacity than one weaker cell, and the weaker cell will get reverse charged, which tends to damage it and make it weaker still. If you do it repeatedly then it could presumably kill that cell.
I would recommend against completely flattening a multi-cell battery unless you know that you can stop the discharging before any cell gets reverse charged. If there are N cells, then one method would be to stop when the voltage gets down to N-1 times the voltage of one fairly discharged cell.
Anyway in the long term in most applications it is probably better to have a battery that has memory effect which is curable, rather than a battery with some cells permanently damaged by reverse charging.
Chris
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