Laptop Power Supply: How to Charge Battery Externally

[tdougherty08]

Greetings,

Does anyone know how to use a laptop's charger to externally charge its battery? I am using a battery from an old DELL laptop to power my handheld radio. I wish to use the charger that came with the computer to rejuvenate the battery, however, the charger circuitry is smarter than I. Only two wires come from the supply (+,-). The batteries are fully discharged and the chargers green light is on, but there is no charging current or voltage. Some how the computer must signal the supply to turn on or something. Can anyone help me to figure out how to use this supply?

Thanks

 

[Bob S]

The charger voltage regulator has a built-in current limit , and it may of the fold-back variety to limit power dissipation. I don't understand why the charger light is green- maybe it just means that it is plugged in. You might put a series resistor between the charger and battery. The resistance should be roughly the nominal (nameplate) charger output voltage divided by half the current limit on the charger. Be sure to get a high enough wattage resistor. Measure the voltage drop across the resistor. If it is the nameplate voltage, the battery is being charged at half the current limit.
Bob S

 

[tdougherty08]

Thanks for the quick reply Bob, I'll give it shot this afternoon and if it works I will let you know.
-Tad

 

[tdougherty08]

Bob,

Here's what I did. I hooked up (for power dissipation reasons) a brake light bulb 25w I had laying around and now I have a charging current of around 1.38A and the charging voltage is right where it needs to be 19.-- v . Thanks for the heads up.

-Tad
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