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
alt.engineering.electrical
sci.electronics.components
sci.electronics.design

 

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.

 

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.

 

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

 

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.[/QUOTE]

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.

 

[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

 

Thank you for the info in the posting and for the PDF reference.

 

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!

 

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