My iPhone 3GS keeps shutting down (or going dfu mode) at low battery

Old bootrom, 5.1.1, bb 5.13, jail broken
It doesn't shut down when battery is above 75% or so, but below it using heavy apps, calling creates problems…
It was ok a month ago, then suddenly it starts… Whacky behavior

Ok first you must light 4 candles and place the iphone in the center of the candles and pray to the god exhaberous god of battery and you must tell him to banish the battery demon if you can't banish the battery demon you will bring bad fortune to your family name.

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