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Wrong math. The car's battery will be about 14V. Your battery to be charged won't be 2V for the entire charging time, will it? The bulb might start the charging by glowing, then as the charging battery's voltage rises the bulb will get dimmer as the current becomes reduced. You need a bulb with a much higher rating than only 12W.joecool85 said:Lets say I used a 12watt bulb (for easy math), wouldn't that be 1amp going through it, so 1amp charging my battery?
The car battery is charging at avbout 14V. The battery to be charged is about 10V. Then there is 4V across a light bulb and you want the average current to be 1/10th of its A/hr rating. So the current should average at 500mA and be about 1A at the beginning of charging with 4V across the light bulb. 4V at 1A is about 12V at 3A. The bulb should be 12V/3A= 36W.justDIY said:why do you recommend a higher rated bulb, and what's the math to calculate the rating?
audioguru said:The car battery is charging at avbout 14V. The battery to be charged is about 10V. Then there is 4V across a light bulb and you want the average current to be 1/10th of its A/hr rating. So the current should average at 500mA and be about 1A at the beginning of charging with 4V across the light bulb. 4V at 1A is about 12V at 3A. The bulb should be 12V/3A= 36W.
It doesn't matter because the battery's voltage will quickly rise, causing the current to quickly be reduced.Nigel Goodwin said:Except that the bulb will be pretty cold, so will have a far lower resistance than your calculations would suggest.
I agree, it is best to know.I would suspect the best method is to stick a bulb in circuit and measure the charging current - starting with the 12W bulb originally suggested.
The 12.75V for full charge is measured when the battery has been at rest for a while with no load and no charging current. Typically, to charge a 12V SLA you would apply a constant voltage of around 14.5V without exceeding the maximum allowable charging current. When the current going into the battery drops to a low value (say 100ma or so for your SLA battery) then charging is finished. Then the float voltage should be reduced to around 13.8V or the battery disconnected from the charging source. These numbers I quoted are as examples only. The manufacturer of your battery will have the proper specs.Thanks for all the replies guys. But to make the math correct, my car runs at 14.4 volts, and the SLA to be charged is a 12v battery, meaning it should be at 12.75volts @ full charge.