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If the battery is not recognised by the system, it does no attempt to start from it.
What are the voltages of R5280 and R5281 with the battery plugged in?
Does it actually charge the battery with this motherboard? If not, try a known good battery because something is not going right on the SMC_5_G3 data lines. This could be caused by the SMC, pull-up resistor, battery or because of ugly U7000 (unlikely)