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I am building a flashlight using a large(ish) 35W HID headlamp from a car, and I'm having troubles choosing a good battery technology.

The inverter draws about 3.5A - 4A at 12V, tested on a large power supply. The current range I have supplied is through a Pb battery's discharge curve, from 12.5V to 10.5V. I have looked at purchasing a small 4Ah 12V Pb battery, but I am hesitant, because I am unsure of what currents the battery may be able to supply. I know the battery is small, but I have a small budget (Got the HID bulbs for free off a friend), and I am OK with a 30 minute battery life, as that is the max time I will be using it.

Any suggestions on what kind of discharge rate I could get from such a small Pb battery?

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30 minutes is about what you will get. At 1C (the theoretical '1 hour' discharge rate) a typical Pb battery has about half its rated capacity, so a 4Ah battery can do 4A for 30 minutes.

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