On Oct 13, 6:47*am, "RogerN" wrote:
"Jim Wilkins" wrote in message

...
On Oct 12, 8:35 pm, "RogerN" wrote:





"James Waldby" wrote in message

...

Smart chargers are based on a chip that have the lead acid charge/
decay curves telling it how to act. I suspect it monitors the voltage
of the battery and if that info is faulty then the charger works
erractically.
jiw

The 2A rating on a lead acid battery charger is more like 2A for a
discharged battery and lower current depending on the state of charge,
quite
different than a 2A constant current charger.

RogerN

/
/The "2A" range on my 2/10/50A Schumacher starts at 4 - 5A and
/decreases as the battery comes up; it passed smoothly through 2A but
/didn't stay there, IIRC at cutoff the current was less than 1A.
/
/An LM317 adjustable regulator IC is enough to make a decent trickle
/charger. Adjust the pot that sets it to the end voltage, the device
/limits the current to 1.25A max, then it falls off as the battery
/nears the set point. You could print the temperature compensation
/chart and tape it to the box.
/
/This is a commercial version with voltage and current meters:
/http://www.testequipmentdepot.com/lascar/dataloggers/psu130.htm#00034...
/The one-turn voltage adjustment is tricky but good enough for lead-
/acids, though you'll need a separate voltmeter with more resolution
/
/jsw

That would be interesting, I could send a higher DC voltage to the vehicle
and have the voltage regulator on board. *It would be neat if I could get
temperature compensation close with a PT100 RTD or similar, I guess I'd just
have to calculate the temperature coefficient of the platinum
versus -0.022V/deg C for the battery. Anyway, if I could get the circuit
right, I could power it a couple of hours a day and keep the battery topped
off. *I would think that would do at least as good as an automotive
alternator/regulator does.

RogerN

The LM317 regulates current instead of voltage if you tap off the ADJ
terminal. It tries to force the voltage between OUT and ADJ to be
1.25V, so a 12.5 ohm resistor between them will set a constant current
of 100mA, for example. Make the current a little higher than the
measured parasitic drain current of the car's electronics and don't
worry about voltage or temperature.

I don't do this to in-vehicle batteries because anything left
connected to the positive terminal is a possible short-circuit fire
hazard if it fails.

jsw