Quote:
Originally Posted by ryan
If you want constant amperage a light bulb is ideal for what you want to do.
Ryan
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Yes, but the devil is in the details.
A light bulb will act as a current limiter, until it blows out. The resistance of a light bulb varies with it's power consumption - so that the ratio of off to on resistance is about 15:1 Ben has some 50 watt 12 volt bulbs. The on resistance is about 3 ohms, the off resistance is about 0.2 ohms.
Ben:
I assume you want to charge the lithium cells at a high rate of charge, perhaps 50 amps initially, but also limit the voltage to 12 volts or so maximum, and when the lithium cells show 12 volts the charger output should reduce to a trickle of around 1 or 2 amps. A resistance of around 0.24 ohms initially, gradually increasing is what you want. In addition the initial charging current may be applied for several minutes, so the 0.24 ohm resistance will be dissipating 600 watts.
You will need a dozen (12) 50 watt 12 volt bulbs in parallel to limit the current to 50 amps. Assuming the initial lithium cell voltage is 0 and the supply is a 12 volt car battery. (Fewer light bulbs for a lower current limit.)
But as the lithium cell's voltage comes up and the charging current drops the light bulb resistance will fall (we'd like it to rise).
How many light bulbs do you have? What maximum charge rate are you aiming for?
-mort