DIYbanter - NiCad or NiMH?

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The batteries in my electric razor are wearing out and I would like to
replace them. There are two AA size cells with solder tags solders to a
PCB incorporating a charger. But, nowhere does it say what type of
batteries they are. Is there any easy way to find out what type of
replacements I need. I would rather not order the relevant part from
Braun because they are certain to cost more than generics from Maplins.

Thanks,

Andrew

There are two AA size cells with solder tags solders to a PCB
incorporating a charger. But, nowhere does it say what type of
batteries they are.

I'd just use NiMH. If it originally had NiCd, then it will just take longer
to charge in proportion to how much higher capacity the new cells had to the
old, unless it has a sophisticated charger, which will probably work out
what to do itself.

Christian.

NimH batteries tend to weigh less than NiCd. use a digital scale to
weigh your existing pack. Go to a shop and take the scale with you.
Weigh a couple of AA Nimh. You should be able to tell the difference.

NimH are not neccessarily better or worse than NiCd. They are just
different. Different battery chemistries suit different applictions.

have a look at this link for some answers

http://www.starbatteries.com/battery...imh%20superior

I think NimH can self discharge quicker than NiCd. About 1% or 2% a day.

Thanks to all. I have download the relevent service manual and it
confirms that the batteries are indeed NiMH. I now have some new ones
waiting to be fitted. So here's hoping that everything works OK.

Andrew

Quote:

Originally Posted by

I was going to say use NiMh it won't matter. But someone said they're no better or worse. If you are looking at it from an environmental viewpoint, NiMh are less polluting than NiCd.

Someone else said NiMh don't like overcharging, I was under the impression NiMh could be charged at any time irrespective of their charge status, whereas NiCd should only be charged when empty. And NiMh could be charged for as long as you like. whilst NiCd don't like overcharging. I await confirmation from a higher source.

Can I also hijack this thread, now that it has been answered, and say that Braun used to sell just the foils as replacements, now you have to buy the foil and top plastic curvy thing (connected) along with a new cutting blade, costing about 13 - 16 pounds (which happens to be 1/3rd the price of the new machine!) My shaver is stuck in a draw after the foil lasted 3 - 4 weeks, no way am I going to pay all that money when all I need is a pack of 5 foils like we used to be able to buy!

Has anyone else noticed this and/or know how to obtain just the foils?

Thanks

On Mon, 23 May 2005 18:12:00 +0100, wig
wrote:

Someone else said NiMh don't like overcharging, I was under the
impression NiMh could be charged at any time irrespective of their
charge status, whereas NiCd should only be charged when empty. And NiMh
could be charged for as long as you like. whilst NiCd don't like
overcharging. I await confirmation from a higher source.

There are basically two ways of charging NiCd and NiMH cells. The
first is to use a charge rate of no more than 1 tenth of the cells
capacity. So for a 1500mA hr cell you could use a charge current of
no more than 150mA. The cell will take about 12 hours (150 x 10 = 10
hours plus some extra for energy losses) to charge from empty to
full. The cell can be left on this charge for well over 12 hours
without harm although for standby use (where they are always on
charge for months on end) a rate of one twentieth of capacity (so
75mA in the example given) would usually be used.

This method of charging is reliable, cheap and safe however it takes
a long time.

The second method is to charge the battery at a much higher rate and
detect when it is fully charged by sensing one or both of the small
voltage change which takes place when it becomes fully charged or
cell temperature.

Voltage change sensing is usually called Delta V (change in voltage)
or DV sensing. It can work either by sensing the small voltage fall
which happens as overcharging starts (-dV) or the rate of change of
charge which happens just before full charge (+dV). With NiCd cells
this voltage change is of greater amplitude than with NiMH so a
charger designed for NiCd will usually fail to sense if a NiMH
battery is fully charged and overcharge it. Most chargers relying
upon dV sensing also use temperature sensing as a backup. The
problem wit h this is that the temperature rises only when the cell
is overcharged so thermal sensing is a bit late to avoid some cell
damage. NiMH batteries also heat up throughout the charging process
so again older NiCd chargers with thermal sensing may fail to detect
the rate of change of temperature which occurs on overcharge.

Putting a NiCd cell in an NiMH charger is always safe for the
battery, putting an NiMH cell in a NiCd charger often isn't. Neither
type of cell will tolerate overcharging at high rates.

THe mythical "memory effect" you will see mentioned everywhere in
connection with NiCd is actually nothing of the sort but something
properly called voltage depression and is simply a reaction of the
battery to _overcharging_. It has nothing to do with
under-discharging (which was the cause of the original "memory
effect" only ever seen on one uncommon type of cell construction in
outer space!).

As an aside the "chargers" supplied with cheap electric drills are
the worst of both worlds. They are unregulated and usually charge at
about one quarter of the batteries capacity. This gives a charge
time of about 5 hours, too long to be useful but with a charge
current far too high to be maintained for more than 5 hours without
causing damage. Leaving one of these batteries on charge overnight
once is usually enough to damage it irreparably.

In general NiCd are better if the application means they are going to
be treated roughly and charged often. They also usually have a
capability to survive more recharge cycles than NiMH. NiMH are
better if the best power/weight ratio is required. So things like
cameras, where a lot of energy is needed in a small package, suit
NiMH whereas electric power tools are usually better suited to NiCd.

--
Peter Parry.
http://www.wpp.ltd.uk/

Peter Parry wrote:
snip
As an aside the "chargers" supplied with cheap electric drills are
the worst of both worlds. They are unregulated and usually charge at
about one quarter of the batteries capacity. This gives a charge
time of about 5 hours, too long to be useful but with a charge
current far too high to be maintained for more than 5 hours without
causing damage. Leaving one of these batteries on charge overnight
once is usually enough to damage it irreparably.

If you have a electronic thermometer, with an alarm, this can be a good
use for it.
Simply stick on top of the battery to be charged, with a foam scrubbing
pad (insulation) and a small weight to keep it there, and turn it off
when it beeps (at room temp +5C)