I am working on a project that requires me to monitor the power consumption of wheelchairs across a spectrum of different activities. (electric wheelchairs batteries are 2x12V) To do that I am going to measure current flow using a split core hall effect sensor (link: http://www.amazon.com/Loulensy-Current-Transmitter-Transformer-Output/dp/B00LMPP01Q/ref=sr_1_72?s=industrial&ie=UTF8&qid=1432039609&sr=1-72) luckily I have spoke with the company and they will be able to provide me with a sensor suitable for a 12V power supply, and the output voltage is o-5V so I can interface it with an adrduino. To measure the voltage I was thinking to simply use a voltage divider to drop the voltage from 12V to 5V so I can measure it directly with the arduino. To power my sensor and my arduino I was thinking to use a Li-ion battery pack i found on ebay (http://www.ebay.co.uk/itm/Super-Rechargeable-Protable-Li-ion-Battery-DC-12V-6800mAh-Lithium-ion-EU-Plug-/351353498255). As data logger I was thinking to ust the Arduino ethernet board with the slot for the SD card. The question is can I connect my battery directly to the arduino power jack and then use the Vin pin to wire the arduino to the breadbord so I can get the right voltage for my sensor? Considering that I am not an expert in electronics I am not sure my components are the right one and if someone could point out if I am making a mistake it would be really appreciated. Thanks!
You should use a voltage regulator or dc-dc converter to use the 12V batteries. Regulator will regulate the voltage from 12 to 5V. You can use it to power both the arduino and the sensor or you can use the arduino to power your sensor.
I am not familiar with Arduino, but i do know something that might be very important to your application. You must sample the voltage and the current at the very same moment, otherwise the power is not V*I. And you must filter both of them to leave any PWM currents higher than your bandwidth is. Otherwise you will get almost random power readings