Need a simple schematic for SLA battery chargers

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Hi there guys,

I'm in search of a schematic (preferably not a finished product, as I am
limited for space in my box), for a basic 12V SLA Battery Trickle Charger,
that is capable of keeping a battery on trickle-charge almost indefinately.
I need it to be as simple as possible. I am not too concerned about the
speed of the charge, even the battery is slightly discharged.

Basically, my intention is to connect a fully charged battery to this
charger for the first time (so the charger won't have to do much work I
guess), and the charger is to keep maintaining the voltage for me until the
battery power is "diverted" to a connected PCB via a relay or switch.
It's for an emergency backup lighting system I am designing, so the charger
will be built into the box, similar to that of an alarm system. I am aware
you can purchase plug-in SLA battery chargers and the like, but that's not
what I'm after, because I don't want to have to manually charge and monitor
it.

I intend to use the following equipment :

-  12V SLA battery, 1.3A/Hr (Specs:  Initial Charge=  <0.39A;  Cycle V=
14.4-15;  Standby V 13.5-13.8), purchased from Jaycar Electronics, CAT
SB-2480.
-  16V AC 1.25A Alarm Power Supply as the main power source.

Thanks in advance =)

Jason.
Electronics hobbyist, but no expert! lol




Re: Need a simple schematic for SLA battery chargers



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Hello Jason,
have a look here at the data sheet for an LM317 which you
can pick up cheap from anywhere like Dick Smith and Jaycar
http://wlan.sdvanime.com/wireless-leeuwarden/img/zelfbouw/LM317_datasheet.pdf

Have a look  on page 5,  figure 7, The current regulator.
R1 sets the maximum current.

If you feed the output of the current regulator (figure 7) into
another circuit, the voltage regulator, figure 5 on page 5
you can then set the voltage at which you wish your battey
to charge up to.

Two LM317s flatpack types, screwed to a small piece
of aluminum with insulating pads/washers and several
components is very cheap.  

Regards,
John Crighton
Hornsby


Re: Need a simple schematic for SLA battery chargers



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Why not use figure 10?

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Just use figure 10 and add the protection diodes. Choose 'Rs' (current
limiting) to be fairly high, 10 ohms or so 5W wire wound.  You don't really
need a heatsink as the LM317 has thermal limiting, but it will get quite hot
when the battery is being 'charged'!! But if you can mount it on some metal
all the better.

The resistor values in the circuit will give 13.75 volts. If you can't get
240/2.4k you could use 220/2.2k.



Re: Need a simple schematic for SLA battery chargers



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It depends on how simple (aka pushed for time/money/space) Jason wants it to be.

Ideally, a charger based on the Unitrode/TI UC3906 which provided the proper
care and feeding of SLA's.  Kits available still AFAIK (Jaycar etc)

Next scheme I would use is that described by John.

The fig 10 approach has the charge current taper off as voltage rises,
progressively to a poofteenth of what it starts at.  This asymptotic charge
curve is not real clever if he wants decent recovery time after his emergency
lighting load has been connected.  The two-in-series approach gives you constant
current up to the point where the second reg chokes to death.

I have test gear costing >$20K that came with the "Fig 10" arrangement, and it
roots batteries very effectively.  In the end I reconfigured it to save my $$.

Your mileage may vary, as may Jason's.

Re: Need a simple schematic for SLA battery chargers



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I agree, depends on how 'simple' simple needs to be. Typically emergency
lighting is only used 'once in a blue moon', but that depends on your local
electricity supply.

Figure 10 has limitations, but it is 'simple'. Battery life can be extended
by dropping the voltage though. There is also no temperature compensation
which the UC3906 does have - something else to consider.

The simplest scheme I have seen comes in a rechargeable torch made in China.
A series resistor! Contains a 6V SLA battery charged from 12V. Instructions
warn 'do not charge the torch for more than one day'....! I can see these
being 'buggered' by the 1000's in very short notice. So they sell more
torches - more turnover.

Here's a good battery resource....

http://www.batteryuniversity.com



Re: Need a simple schematic for SLA battery chargers



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I don't like the sounds of the fugure 10 approach!
What did you mean by the two-in-series approach?  Were you referring to the
Figure 5 and 7 approach recommended by John?  If the second reg would choke
to death, is there a way to somehow overcome this hurdle?

Jason.




Re: Need a simple schematic for SLA battery chargers




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Figure of speech.  Maybe I should have said "drops out of regulation".

The current source will limit the current through the voltage reg.  A constant
current source is really a variable impedance that drops excess voltage so that
the load will see a constant current.  If you place a dead short on a current
source, you see ...  the constant current into that short.

So the voltage regulator will determine the maximum voltage that the load
(battery) can get from the duo, while the current limiter will do the same for
the current under recovery after discharge.

There's also a circuit there in the data sheet for a current-limited voltage
reg, but it's not really any getter or simpler to configure than the tandem
arrangement John suggested.

I personally use the UC3906 on 6V SLA's and wouldn't change.

Re: Need a simple schematic for SLA battery chargers


Have a look at these two circuits John - perhaps one of them will do the job for
you

http://web.archive.org/web/20030415185423/www.modelwarshipcombat.com/members/rthibault/GellCell_Battery_Charger.htm

http://www.geocities.com/sunsetstrip/concert/2301/charger.html

Cheers

David

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Re: Need a simple schematic for SLA battery chargers


On Mon, 05 Sep 2005 00:15:57 GMT, quietguy

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Yep, that'll do it.  C##t of a colour scheme he chose.

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No use for leaving connected to the SLA.  Will thoroughly root it.


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Re: Need a simple schematic for SLA battery chargers



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hmmm, everybody has different opinions about chargers. I really don't know
now =(
It says it can be left overnight safely. What about months, or even a few
years later when it needs replacing?  See, that's what I need. The battery
is to be kept "topped-up" until it is required.



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Re: Need a simple schematic for SLA battery chargers



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Hi John,

Yes I know the LM317... handy device!
I see what you're getting at, but let me confirm with you, incase I
misunderstood =)....

We use the 'Voltage Regulator with protection diodes' circuit (figure 5)
with its output connected to the Input of the 'Current Regulator' (figure
7)?
Also the R1 value for figure 7, what value would you recommend (including
appropriate wattage), for the battery assuming it is already "fully charged"
(let's say)?  I don't get the formula for it =(

Jason.





Re: Need a simple schematic for SLA battery chargers



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No. see below

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R1 determines the maximum current.
The maximum current will be a constant current.
Build the circuit and have a play with it.
Connect different value load resistors and even
a dead short and check the current for yourself.


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Hello Jason,
if you did this

transformer/rectifier/big capacitor ----> figure 7----> battery

Your charging current will be a constant current
set by the value of R1.

---------------------------------------------------------------------------------------------
You mentioned 0.39A for a charging current.

Look at the formula in figure 7     current out = 1.25 divided by R1

transposing the subject of the formula

R1 =  1.25 divided by  current out

R1 = 1.25 divided by 0.39

R1 = 3.2 ohms

3.3 ohms is the nearest preferred value.

You can do the wattage rating of the resistor (volts times amps)
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To make your charger a bit nicer I was suggesting this

trfr/rct/cap ----> in (figure 7) out ----> in ( figure 5) out
----->battery

If you just used figure 7 on its own. you have a constant
current continuously  passing 0.39 amps into your battery.  
This is fine if you disconnect it after 12 hours or so.  

By adding the voltage regulator stage after the current regulator
stage your battery charger will still deliver 0.39 amps max, when
the battery is discharged but when the battery voltage rises to
equal the voltage regulator output voltage, very little current
will flow.
The current "throttles"  back. Budgie made a joke
"where the second reg chokes to death"
Get it?  throttling... choking ...squeezing the life out of ...
The current reduces to near zero current at the specified
output voltage, set by you..

Lets say you  put a heavy load on the battery of a couple
of amps for a long time. Your charger would deliver 0.39 amps
to the load, helping the battery.  This is OK. This is good.
When the 2 amp load current was removed the battery will
receive a maximum current of  0.39 A from the charger.
This is also good.
As the battery voltage rises and nears the output voltage of
the voltage regulator stage the charging current will reduce.
The charger changes from a constant current mode, 0.39A
maximum, to constant voltage mode at less than 0.39A,
the current eventually reducing  to nearly zero amps.  
While in this  float charging mode, any little load on the
battery,  below 0.39A, the charger will take care of it.
A big load on the battery and the charger will help
up to its 0.39 A limit.

I hope that is a little clearer for you Jason.
Make up the two circuits and play with them
individualy and combined.

Have Fun,
John Crighton
Hornsby



Re: Need a simple schematic for SLA battery chargers



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Hi John,

Ok, makes better sense now.
So you recommend the following connectivity for such application:
    Transformer --> Rectifier/Smoothing ---> Current Regulator (fig7) -->
Voltage Regulator (fig5) --> Battery.

I will build the test circuit on a breadboard soon, but I dont have a couple
of the parts right now.
I really appreciate your time to assist me with this in detail. =)

Regards,
Jason.






Re: Need a simple schematic for SLA battery chargers


put finger to keyboard and composed:

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I have a hand drawn schematic of the SLA charger circuit for WES's
VBV860 camcorder battery pack. It was sold as a 6V system but actually
contains four series 2.0V Cyclon SLA cells. <shrug>

The charging circuit uses an LM723 regulator set to ~9.2V (4 x 2.3V,
IIRC). A 1 ohm resistor limits the current to 0.66A. The pass
transistor is a TIP41C.

I have modified it for 6V (7.2V) operation by changing a few
resistors. You should have no trouble doing the same for 12V (13.8V).

Contact me via email if you would like me to scan the drawing for you.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

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