Re: KISS 126 by Andre Jute: Using batteries for negative grid bias -- now online
- From: "pfjw@xxxxxxx" <pfjw@xxxxxxx>
- Date: 13 Mar 2006 13:26:38 -0800
Ha! It is a resistor I'm trying to avoid.
I think you missed the point. A NiMH battery is not a rapid-discharge
device on the same order as a NiCd. It is actually vastly superior in
everything except current, and very close there.. and you have stated
repeatedly that your need is very low-current. What Mr. Broens is
suggesting is likely that if you *want* to cycle them in a very low
drain system, then you will have to install a bleed-resistor. But by
nature, NiMH batteries do not require cycling. So it is not that you
*must* install such a resistor due to the high-current capabilities or
nature of the battery. The very real advantages of the NiMH system is
that it does not suffer from memory problems, something that low-drain
devices can cause in NiCd batteries. And a VOM will not tell you that
your battery has a memory problem, nor will a 'blind' cycle charger.
Keep in mind that blind (most consumer-grade LED-is-all-you-see)
chargers measure output voltage during the discharge cycle and look for
the 'cliff'. NiCd batteries are very prone to 'voltage depression',
which fools many, many blind chargers into thinking that is the cliff,
so they start the charge cycle.
If you want to get really-really dry, NiCd batteries actually do not
have a true 'memory'. But as the system is chemical by nature, when a
discharge cycle passes beyond the prior average cycles, the chemical
reaction slows down slightly... by something on the order of 12%-22% of
rated voltage or so... and will remain at this level until it jumps the
'cliff' and becomes fully discharged. Many cheap cycle chargers will be
tripped by this drop, and each time this happens, the 'voltage
depression' happens a bit sooner in the cycle. So, it is always best to
discharge your NiCds on either metered cycle chargers or devices that
give a visual clue of the state... lamp or motor or similar.
But even at that, if you need a precise and accurate 2.4V, a pair of
batteries prone to voltage depression may not be the ideal choice.
Yours may be just fine as measured by a VOM, but unless you have cycled
them regularly, you might be quite surprised at their actual state.
Just for giggles, get an AAA-type Maglite flashlight. Use a fresh AAA
NiCd, properly charged and one of yours charged to your satisfaction.
Run-'til-dead. If there is a significant time difference between them,
or if one visibly dims, you now understand the issue.
Peter Wieck
Wyncote, PA
.
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