[Elecraft] SLA Battery charge level

[Steven Pituch]

John,

I did exactly the same thing many years ago.  I looked up the Powersonic
information on their web site.

Lets assume the battery is to be charged with the 1.6 amps when the voltage
is down to 10.5 Volts, also that the current must be 80 milliAmps when the
charge gets to 14.2 Volts like you said.  Using V/A=R  we can figure out the
voltage and resistance to do this.

For the initail charge rate and V/A=R, where R is the dropping resistor, and
V is the regulated voltage:
(V-10.5)/1.6  = R
For the final state:
(V-14.2)/.080 = R

Since R is the same

(V-10.5)/1.6 = (V-14.2)/.080
Therefore V = 14.49 Volts
Substitute V back into one of the original equations and you find the
ballast resistor is 2.43 Ohms.

Therefore using a voltage regulator at 14.39 V with the resistor of 2.43
Ohms will charge the battery at 1.6 Amps when the battery is at 10.5 Volts,
and the charge will taper to 80 milliamps when the voltage reaches 14.2
Volts.  This is exactly what Powersonic advises.

Next you use a 25 cent op amp to make a voltage comparator and turn off the
circuit when the voltage reaches 14.2 volts.

Next you use a little relay to switch to a 13.8 volt regulated supply.  The
battery will stay on this supply indefinitely jusy taking enough current to
keep it charged.  When the battery is exactly at 13.8 volts, there would be
no current.

I wrote an article for QRP Quarterly years ago describing this.  My original
charger is still working without any adjustments needed.

It does everything needed without using a CPU or an IC.  You can get all the
parts at Radio Shack.

Steve, W2MY
 

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of John R. Lonigro
Sent: Saturday, December 11, 2004 6:32 PM
Cc: Elecraft
Subject: Re: [Elecraft] SLA Battery charge level

Dan:

According to the PowerSonic instruction sheet, after "bulk" charging (<
1.6 Amps for my 8 Amp Hr SLA), you are supposed to switch to about 14.2
volts for "absorption" charging.  Once the current gets down to 80 ma, you
can switch to a 13.8 volt "float" charge indefinitely if you wish.  
Since you are drawing 80 ma at just 13.8 volts, my guess is the battery is
still charging.  It should slowly drop to quite a bit less than that, maybe
30 ma or less when totally charged.  Leaving the charger on longer shouldn't
harm anything.  It just might take longer to charge if you are only using
13.8 volts for all 3 types of charging.

At least that's my understanding of the matter.  Perhaps others can
enlighten me if I'm way off base here.

73's,

John AA0VE

Dan Barker wrote:

>When is a 12v SLA full?
>
>I'm charging at 13.8v and the current is slowly falling. Currently (har 
>har) at 0.080 A.
>
>Dan / WG4S / K2 #2456
>
>
>  
>
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