Altec Lansing Boombox shutting down on Battery

Altec Lansing Boombox shutting down on Battery
My Altec Lansing Imt810 shuts down when on battery power after 5 - 10 minutes. After that initial start/stop, it can generally be powered back on but will shut down soon afterward. It seems to me like something might be overheating. The batteries are new and from the same package and each has been tested, each carrying a near full charge. The symptoms are the same whether playing the radio or a device.

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