# Laptop power supply for battery charger

Discussion in 'Electronic Basics' started by Harry, Sep 22, 2008.

1. ### HarryGuest

I have a 19 volt 3.5 amp laptop power supply that I want to use for
equalizing lead acid batteries (car batteries and 75ah rv batteries).
I know I can't leave it unattended because the voltage will fry the
battery.

If I cut the end off the cord, attach alligator clips, and attach to
battery until it boils will it work?

2. ### Sjouke BurryGuest

It is likely to fry your laptop supply, and if it doesnt,
and your lead battery is small , you will miss the important

3. ### ehsjrGuest

There's more to equalizing than just charging the battery
until the cells boil. Connecting as you mention might work,
but you stand the chance of burning out the supply and killing
the batteries.

To protect the supply and provide a proper equalizing
voltage and current, I would recommend this:

D2
+--------|<----------+
| |
D1 | ----- |
+--->|---+---Vin|LM350|Vout---+
| ----- |
[Supply] | |
| - +----------+---> To Batt +
|
+---------------------------------> To Batt -

D1,D2 are 1N5408's (3 amp diodes) and the .5R resistor
is 3 1.5 ohm 5 watt resistors in parallel. The LM350
must be installed on a heatsink. Allelectronics sells
the resistors at 3 for \$1.00, CAT# 1.5-5. They also have
the diodes, the LM350 and heatsink.

Equilization requires a limited current (about 3 to 7 percent
of battery capacity) and a voltage between 14.5 and 16 volts.
The circuit above limits the current to 2.5 amps. At 2.5 amps,
the 1N5408 drops about .9 volts, the .5 ohm resistance drops
another 1.25 volts, and the LM350 drops about 2.5 volts, so you
have about 3.9 volts dropped in the circuit. Thus, with your

The circuit does NOT protect the battery from being charged too
_long_. It provides a proper equilization charge current and
voltage, and protects the supply, but does not prevent the
battery from overheating.

Ed

4. ### neon

1,325
0
Oct 21, 2006
2.5 amps is not floating charge is a very good charging current for a car .5 amps is floating charge. And if you don't put a diode across the reg. you blow the reg.

Last edited: Sep 24, 2008