On Thu, 16 Nov 2006 09:14:10 -0800, Jeff Liebermann
<jeffl@comix.santa-cruz.ca.us> wrote in
<656pl25hfnecs0n71lmj167b8ldhjcdiud@4ax.com>:

>"Thomas T. Veldhouse" <veldy71@yahoo.com> hath wroth:
>
>>In alt.cellular.cingular John Navas <spamfilter0@navasgroup.com> wrote:
>>> On Thu, 16 Nov 2006 06:51:52 -0800, SMS <scharf.steven@geemail.com>
>>> wrote in <455c7b04$0$88640$742ec2ed@news.sonic.net>:
>>>
>>>>Toshiba has Li-Ion batteries coming that can be fully charged in 10
>>>>minutes. The company claims that they will be out in 2008.
>>>
>>> [yawn] We've had 15 minute NiMH recharging for quite some time.
>
>>Yes, and it significantly decreases the life of the battery ... they simply
>>get too hot!
>
>Bad assumption. I've done some experiments in very fast charging NiCd
>batteries at rates up to about 200 times the rated current to see what
>happens (Learn By Destroying). The battery does not get warm until
>overcharged.

NiCd experience doesn't directly apply to NiMH, since NiCd is
sufficiently different from NiMH that different fast charging techniques
must be used. That's primarily because charging of:

* NiCd is endothermic (absorbs heat) until the point of overcharge

* NiMH is exothermic (gives off heat)

Thus NiMH gives off more heat than NiCd, especially during fast charge.

>The trick is to determine when to stop charging.

True, and the sudden rise in temperature of NiCd when they go into
overcharge is a common means of charge termination. NiMH is
sufficiently different that more subtle means of charge termination are
normally employed.

>I
>think I scribbled something on my tinkering in the past....
>| http://groups.google.com/group/ba.in...a0088af9dd770d
>As far as I can determine, I've seen no accelerated ageing problems
>with such fast charging as long as I don't go over about 95% of full
>charge.

NiMH batteries will readily tolerate operating temperatures of 55° C
(130° F), with safe charging up to cutoff at 75° C (167° F). This is of
course quite hot to the touch, which leads some people to think the
temperature is excessive.

>Those where I have exceeded full charge and have started
>heating have died prematurely (or exploded). My guess is that the
>only reason this is not being done in production units is the safety
>issue. One mistake in such a fast charger and we have an explosion of
>attorneys.

The primary issue with NiMH fast charge is overpressure and venting,
which results in damage to the cell.

For more info, see
<http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf>
<http://data.energizer.com/PDFs/nickelmetalhydride_psds.pdf>

--
Best regards, FAQ FOR CINGULAR WIRELESS:
John Navas <http://en.wikibooks.org/wiki/Cingular_Wireless_FAQ>