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From: bill@thd.tv.tek.com (William K. McFadden)
Newsgroups: sci.electronics
Subject: Re: When is the battery fully charged?
Message-ID: <1991May7.175155.19138@tvnews.tv.tek.com>
Date: 7 May 91 17:51:55 GMT
References: <1900@eastman.UUCP>
Sender: news@tvnews.tv.tek.com (news user)
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Organization: Tektronix TV Products
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In article <1900@eastman.UUCP> levi@kodak.com (Ilia Levi) writes:
>How can you tell when a lead-acid (car) battery is fully charged?

When it reaches the right voltage, typically 2.35 volts per cell
(consult manufacturer's spec. for exact value).  For a 12V battery,
this would be 14.1 volts.  The best way to charge is to build a voltage
regulator for 14.1 V and leave it connected across the battery
terminals at all times (float charge).  I like to use the LM317
adjustable regulator for this.  You will also want to add current
limiting to prevent the battery from charging too fast.  A car battery
can be charged at a very high rate (tens of amps), so your current
limit would be chosen to prevent the charger from overheating.
Finally, don't let a 12V battery discharge below 10 volts or you may
damage it.

>While we are at it, what about the Ni-Cads?

This is more difficult.  The most reliable way is to detect the
temperature rise of the cell as it reaches full charge.  A simpler way
that works well is to use a timer.  Nicads are best charged at a
constant current, C/10 for 15 hours being a safe value, where C is the
cell's capacity.  The LM317 can be used to make a C/10 charger.  Use
the temperature sense method if charging at a higher than C/10 rate,
otherwise you may damage the cell(s).  Also, don't let a nicad pack
discharge below 1V/cell, or some of the cells may short out.
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
bill@tv.tv.tek.com,           {hplabs,uw-beaver,decvax}!tektronix!videovax!bill
Phone: (503) 627-6920                 "SCUD: Shoots Crooked, Usually Destroyed"