The basic theory is that most sensors run on 5 Vref. If there is not enough juice to keep the ref to EXACTLY 5 V the sensors will not work. The computers need 9 V to even think about booting, and they draw more amps while booting. A weak battery will draw down a lot with a little load the 5 Vref circuit is designed to deliver exactly 5 V if it is supplied with X Volts; resistors are stupid; they just drop the voltage/current they get by whatever value they are.

Most of them also use Lowref, instead of ground, that can be off because of the battery.

 

Replaced the battery. Its was dated 8/2015. So far so good, its been 2 days.