Laptop power supply for battery charger
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Posted by Harry on September 22, 2008, 3:13 pm
Please log in for more thread options 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? | ||||||||||||||||
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Posted by Sjouke Burry on September 22, 2008, 3:34 pm
Please log in for more thread options Harry wrote: and your lead battery is small , you will miss the important moment and fry your battery. | ||||||||||||||||
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Posted by ehsjr on September 22, 2008, 11:02 pm
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Harry wrote: > 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? 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---+
| ----- |
| + Adj [.5R] [Supply] | | | - +----------+---> To Batt +
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+---------------------------------> 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 19 volt supply, the battery receives about 15.1 volts at 2.5 amps, which is about right for equalizing your 75AH batteries. 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 | ||||||||||||||||
> 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?