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From: george@wombat.UUCP (George Scolaro)
Newsgroups: sci.electronics
Subject: Re: When is the battery fully charged?
Message-ID: <5582@wombat.UUCP>
Date: 12 May 91 21:17:59 GMT
References: <1991May9.041604.24787@crash.cts.com> <1079@eplunix.UUCP> <1991May10.005414.11195@tvnews.tv.tek.com>
Organization: Assn. for the prevention of Polar Bears and Kangaroos
Lines: 56

In article <1991May10.005414.11195@tvnews.tv.tek.com> bill@thd.tv.tek.com (William K. McFadden) writes:
>In article <1079@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes:
>->Recharging them depends on the battery type. For lead-acids, a
>->constant-voltage constant-current circuit is best. Basically, run an LM317
>->as a current source (no more than 1/10 the battery capacity), and use a
>->voltage supply no higher than your maximum voltage to supply voltage to that
>->(another LM317 works just fine). 
>
>It seems like it would work better to have the voltage regulator after
>the current regulator.  That way you wouldn't have to concern yourself
>with the voltage drop across the current regulator.

National Semi's 1989 General Purpose Linear Devices databook has even a nicer
scheme using just one lm317 (see page 1-64):

		  |----------------|
9v-60v		  |		   |		optional diode here
	----o-----| Vin  317  Vout |----o----------------
	    |	  |       Adj	   |	|		|
	    |	  |----------------|	|	      _____
	    |		   |           /	       ---   +
            | +		   |		\ 240 ohms	|
	  ----- 1000uF	   |		/		|   6 volt
	  -----		   |------------o		|
	    |		   |		|	      _____
	    |		   |		/	       ---
	    |		   |		\ 1.1kohm	|
	    |		  C \    B	/		|
	    |		      \|________o________________
	    |		      /|	|
	    |		  E <  2N2222	/
	    |		   |		\ 1 ohm
	    |		   |		/
	----o--------------o------------o


The 1 ohm resistor sets the current limit, the 1.1kohm resistor sets the
output voltage. Adjust both for you application. If the circuit is
permanently connected to your battery, put a diode in series with the battery
so that it wont discharge if the charger is off. I have built this up into
a Dolphin style torch with a 6 volt 4 amp/hr lead/acid and it works great,
with a axial connector for a cable to a car cigarette lighter.

Note that for (some) lead/acid you can charge higher than 1/10C, the battery
manufacturer that I used recommends max charge of 0.8 amp (4 amp/hr). In
addition for cyclic charge the charge voltage should be 7.20 - 7.35V,
and at 40mA it should be disconnected or lowered to the float charge voltage,
which is 6.75 - 6.90V. i.e. get the specs from the battery manufacturer that
your are purchasing from, don't try to second guess them, they do print
specs for a good reason and contrary to popular belief you can ask them to
send you a copy. The wonders of printed material :-)

best regards,
-- 
George Scolaro
george@wombat.bungi.com                [37 20 51 N / 122 03 07 W]