charge a 12 vdc sla ftom cars 12 v system ?

 

hi, any one got any ideas on how to charge a 12 v sla from a cars 12 vdc
supply ?
i suppose a boost converter circuit to get the voltage up ?
all i need is to charge the 12 vdc sla for a torch. i want to leave it
pretty well connected up permanetly, so its always charged, ready to go ,.
any schematics , links etc....

thanks.
mark k

 

On Mon, 1 Feb 2010 10:15:33 +0930, "mark krawczuk"


For starters to *properly* manage the charging of SLA/VRLA's you need
something like the old Silicon Chip kit (Altronics K1685).  They use
the Unitrode/TI UC3906 controller chip, which has arguably the best
charging regime around.  Check out the AppNote by googling for
SLUA115.PDF.

The problem then resolves to providing enough regulating headroom for
the series pass element.  Either EA or SC published a design (which
was kitted, don't have the details to hand) to reputedly charge an SLA
from a  vehicle supply.

If at all possible, cut your losses and convert the torch to 6 volt
operation, then you just need the standalone K1685 charger.  I've been
running a couple of Dolphins for about 14 years using that design with
6V4Ah SLA's, and it DOES look after the batteries.  I've only had to
replace ONE in that time.

PS if you do go to 6V, the transistor will require a bit of a
heatsink.

 



You don't need huge current - just a slight voltage boost, I'd try a simple
blocking oscillator inverter..

A good starting point might be a salvaged common mode choke (preferably one
with not very large number of turns), one coil from the transistor's
collector to +12 the other coil to the base, the base coil is terminated by
a 10 - 15nF capacitor to GND and a 470k to +12 if it doesn't oscillate first
go, try reversing one of the windings, then reducing the resistor.

For bench testing add an output reservoir electrolytic fed by a fast soft
recovery rectifier from the collector - the voltage will probably be several
times the supply voltage.

Now add a small signal transistor (2N2222 etc) emitter to GND alongside the
BO transistor, collector to base of the BO transistor, the base is connected
via a voltage reference and current limiting resistor (about 2k2 to 10k or
so) to the capacitor/diode junction.

If you can't find a zener diode for the voltage reference that gives the
required float charge voltage (about 13.6 - 13.8V) then use a TL431
adjustable zener + trimpot to set the output exactly where you want it.

Once you've bench tested it, discard the electrolytic and connect the
rectifier to your 12V SLA.

 

ian field Inscribed thus:


Surely just plugging into the cigarette lighter socket whilst the engine
is running will charge the battery just fine.

--
Best Regards:
                     Baron.

 



No need to worry about charging current, battery life, or even exploding
batteries then?

MrT.