Ask PYS: Power

Terry and Karen - Battery Monitor Wiring, on Dec 10, 2013 asked:

On your advice at the boat show seminar I bought a battery monitor. I have a couple questions on wiring it up. The diagram shows the operating power coming directly from the battery so does it have to be on all the time and cannot be wired past the isolation switch from a fuse panel? For the 2 wires coming from the shunt to monitor - can they be encased boat wire and not inter looped single wires as demonstrated? To use with an auxiliary battery, does the entire negative auxiliary load have to be connected to shunt (batteries are in 2 different locations)?

Remember the old adage: "there are no shortcuts". This rings especially true on boats. Here are the answers to your questions.

  1. Your battery monitor needs to be powered all the time. As such should have a fused connection at the battery. Voltage on a trunk line under load is not the same as voltage at the battery. Furthermore in the event of a short on the fuse of the trunk line, you want to be able to measure battery voltage at the battery.
  2. Wiring to shunt is measuring extremely small voltages; twisted pair wiring reduces the chance of interference. You definitely want to use twisted pair.
  3. Aux option only measures voltage. Connect positive at aux battery. Negative is generally common and is located at main battery.