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From: ray@cs.rochester.edu (Ray Frank)
Newsgroups: sci.electronics
Subject: Re: NICADs
Message-ID: <1619@sol.ARPA>
Date: Mon, 24-Aug-87 15:52:42 EDT
Article-I.D.: sol.1619
Posted: Mon Aug 24 15:52:42 1987
Date-Received: Tue, 25-Aug-87 04:26:48 EDT
References: <8708040222.AA14785@ucbvax.Berkeley.EDU> <646@gec-mi-at.co.uk>
Reply-To: ray@cs.rochester.edu (Ray Frank)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 43


Some questions about nicads to you nicad wizards and some observations of
my own.

First, I would like to begin by admitting to what some might consider a
cardinal sin:  fast charging(3C) of slow charge nicads.  The nicads
used in my radio control plane are the 'charge for 14 hour' types at 45ma.
They have a 500ma capacity.  I charge them at about 1.5 amps.  You have to
be very careful not to let them get hot.  They stay cool just until they
are at full charge and then they'll heat up rapidly and would probably explode
shortly after that.  I usually don't charge them to full capacity.  How do
I know when the batteries are fully charged?  With an amp meter in series
with the charging source I watch the current gradually go down in increments
of a few hundred mills/minute until at some point at which the current will not
continue to go down and in fact will actually reverse slightly and it is at
this point that the nicads are fully charged.

I've asked several people what the difference is between slow charge nicads
and fast charge nicades.  Mostly the answer they give is that the fast charge
nicads are vented to release the heat.  Does any one know if this is true and
if there are other major differences?  I've been fast charging my nicads all
summer long and flying with out a battery failure.  I realize that the nicads
will not last as long by fast charging them but if this is the only thing I
can expect from doing this to them I'll be satisfied.

Finally, does any one know of a reliable way to determine how much charge is
left in a nicad?  For example, I can safely fly for about 120 minutes on one
charge before bringing it in and recharging it.  As the battery life decreases
this 120 minute figure will decrease also.  I have measured my fully charged
nicad pack at 5.2 volts (4 AA) and after about 2 hours they are down to around
4.8 volts.  But they will still read over 5 volts even after 90 minutes of 
flying so they begin dying all at once and this can be dangerous when 500 feet
in the air.  I bring the plane in every 45 minutes or so to check the voltage.
By the way, I fly slope gliders and if my neck and batteries held out I could
keep one up quite easily the entire day so you can see it is important to
monitor the nicads and it is also nice to be able to charge the nicads from
your car battery rather than leaving the hill for the day and waiting 14 hours
for them to charge.

Any suggestions and help would be appreciated as I LOVE my glider and wouldn't
want it to crash because of a nicad failure.  

ray