[sci.electronics] When is the battery fully charged?

levi@kodak.com (Ilia Levi) (05/07/91)

How can you tell when a lead-acid (car) battery is fully charged?
I want to build a charger for the backup sump pump system in the
basement, and don't want to overcharge the battery (heard it was bad).
How much voltage should a charger put across the terminals?

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

Tired of wet basement :-)

levi@kodak.com

mpratap@cue.bc.ca (Matt Pratap) (05/07/91)

I need to build one myself....

-------------------------------------------------------------------------
Matt Pratap, Victoria B.C. -=- mpratap@cue.bc.ca  *  (Insert quote here!)
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bill@thd.tv.tek.com (William K. McFadden) (05/08/91)

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"

ken@hertz.njit.edu (ken ng cccc) (05/08/91)

In article <1900@eastman.UUCP> levi@kodak.com (Ilia Levi) writes:
:How can you tell when a lead-acid (car) battery is fully charged?
:I want to build a charger for the backup sump pump system in the
:basement, and don't want to overcharge the battery (heard it was bad).
:How much voltage should a charger put across the terminals?
:While we are at it, what about the Ni-Cads?

Well, I'm not sure if this is the RIGHT way of seeing if batteries are
charged, but I've noticed over the years that if they get warm they are
either charged or your frying them :-).  I usually try to charge at about
1/10 the rated capability and leave it on for about overnight or until
they get warm.

Speculation on why they may get warm when charged: while the battery is
being charged, the energy goes into moving chemical ions back and forth
to the various plates and electrolyte.  When the battery is charged, the
energy has no where to go, so it is disappiated as heat.

People who know better are free (and encouraged) to post corrections.

Kenneth Ng
"No problem, this is how you make it" -- R. Barclay, ST: TNG

raoul@eplunix.UUCP (Nico Garcia) (05/10/91)

Depends on the battery type. Charged to 12 Volts is fine for a 12 Volt
battery: you can get away with up to 14.5 on a Yuasa lead-acid, for example.
This also gives you a somewhat larger charge.

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). I use 15 Volt supplies, so use an LM317
with a diode on the output to bring the voltage down to 14.4 volts maximum.
Then I leave that on for trickle charging. Works just fine....

-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu

bill@thd.tv.tek.com (William K. McFadden) (05/10/91)

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.

A good idea, though.  I'll have to try it.
-- 
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"

george@wombat.UUCP (George Scolaro) (05/13/91)

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]

benno@crash.cts.com (Benno Eichmann) (05/14/91)

A led in series with the collector of the 2N2222 might make a useful indicator
if the .7 volt trickle charge isn't of prime concern?

strong@tc.fluke.COM (Norm Strong) (05/14/91)

When is a battery fully charged?  When all the energy fed to it is
dissipated in the form of heat.

-- 

Norm Strong  (strong@tc.fluke.com)
2528 31st S.   Seattle WA 98144   USA